7On Estuarine and Continental-Shelf We shall attempt in

7On Estuarine andContinental-ShelfCirculationin the MiddleAtlantic Bight

Robert C. BeardsleyWilliam C. Boicourt

7.1 Introduction

We shall attempt in this chapter to trace the develop-ment of ideas about circulation over the continentalshelf in the Middle Atlantic Bight and in the majorestuaries which drain into the Middle Atlantic Bight.The term Middle Atlantic Bight refers to the curvedsection of the continental shelf off the eastern UnitedStates stretching between Cape Hatteras to the southand Cape Cod and Nantucket Shoals to the northeast.The New York Bight is a subsection of the MiddleAtlantic Bight and refers to the shelf region stretchingbetween the New Jersey and Long Island coasts. Aschematic version of Uchupi's (1965) topographic mapis shown in figure 7.1, indicating both the general shapeof this shelf region plus the names and locations of themajor estuaries and key positions discussed in the text.

We have decided to focus this review of estuarineand shelf cirulation on the Middle Atlantic Bight andits estuaries for several reasons. The major Middle At-lantic Bight estuaries have been extensively examinedand have provided several important case studies inthe development of new ideas about circulation andturbulent-mixing processes in moderately stratifiedcoastal-plain-type estuaries. These estuaries and theadjacent continental shelf border on one of the world'slargest urban complexes; a better description and un-derstanding of the circulation and dominant mixingprocesses occurring in this particular region is clearlyneeded for a more effective management of the regionalestuarine and shelf resources in the face of man's manyconflicting uses of the water bodies. Fostered in partby increased environmental concerns, more adequateresearch funding, and the availability of new instru-mentation and observational techniques, many newcirculation and related physical studies have been un-dertaken in the last two decades, and a synthesis ofboth old and new material into a review of the regionalestuarine and shelf circulation seems particularly ap-propriate at this time. While a few scientists have madeimportant contributions in both fields and have thushelped to carry new ideas and techniques from onefield into the other, basic research on problems con-ceming estuarine and continental-shelf circulationhave evolved more or less independently in time, sothat we will present here separate reviews of the his-torical and the modem ideas about estuarine and shelfcirculation in the Middle Atlantic Bight. One impor-tant research objective in the 1980s will be to developa better kinematic and dynamic description of thephysical coupling between estuarine and shelf waters.Our present meager knowledge about the differentphysical processes that connect the shelf and estuarytogether prevent a more unified discussion.

i98R. C. Beardsley and W. C. Boicourt

_ ___·_I_ _�_·��I______

Figure 7.I A topographic map for the Middle Atlantic Bight 200-m isobaths are shown.and a western section of the Gulf of Maine. The 60-, 100-, and

7.2 Estuarine Circulation in the Middle AtlanticBight

We shall trace here the evolution of estuarine-circula-tion ideas. We will use the Middle Atlantic Bight es-tuaries as a focus because physical studies of thesewater bodies have played a major role not only in thedevelopment of the early concepts of the physics ofestuaries but also in the recent refinement and refor-mulation of these ideas. Narragansett Bay, the HudsonRiver, Delaware Bay, and especially the ChesapeakeBay system-all have provided case studies from whichsignificant advances have been made in our under-standing of "coastal bodies of water having free con-nection with the open sea and within which sea wateris measurably diluted by fresh water drainage" [Prit-chard (1967a)].

7.2.1 The Development of Estuarine-CirculationConceptsThe idea that the introduction of fresh water into thesea can produce an oppositely directed, two-layer cir-culation has existed for a long time. Some early ocean-ographers understood that the lighter, fresher waterspreads away from the source along the surface and theheavier, more saline water moves toward the sourceunderneath. The kinematic details and the dynamicsof this process have, however, remained elusive untilrecent years. On a scale as large as the MediterraneanSea, early oceanographers correctly held that the cir-culations were driven by density differences. Two-layerflows observed at the Kattegat in the Baltic (Ekman,

1876), the Strait of Gibraltar in the Mediterranean Sea(Douglas, 1930), and the Strait of Bab el-Mandeb at themouth of the Red Sea (Buchan, 1897) all reflect thebalance between evaporation, runoff, and precipitationin the enclosed seas. On a smaller estuarine scale, theformulation of clear circulation ideas has been hind-ered by the difficulty of unraveling the different cir-culation components in a variety of estuarine geome-tries. In addition to oscillatory tidal currents andcurrents driven by density differences, there has beenthe possibility of "reaction currents" as suggested byF. L. Ekman (1876). These were countercurrents andundercurrents associated with the entrainment of am-bient water by the discharge of a river into the sea.Although the idea of reaction currents seems to havebeen supported in Helland-Hansen and Nansen's (1909)discussion of the rivers entering the Norwegian Seaand in Buchanan's (1913) description of the flow off themouth of the Congo River, F. L. Ekman's son, V. W.Ekman, showed that significant reaction currents wereunlikely at the mouths of rivers (Ekman, 1899). Theterms "reaction current," "induction current" (Cor-nish, 1898), "undercurrent" (Dawson; 1897), and "com-pensatory bottom current" (Johnstone, 1923) wereoften employed without detailed discussion of thephysics. In some cases, these terms were used simplyto refer to low-salinity water flowing seaward overmore saline water flowing landward. In other cases,however, the usage harked back to F. L. Ekman's senseof a countercurrent associated with the river outflowjet.

I99Estuarine and Continental-Shelf Circulation

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Reaction currents were invoked to explain the two-layer circulation phenomena found in the early studiesof the Middle Atlantic Bight estuaries: Although Harris(1907) correctly described the distribution of pressuresurfaces in an estuarine situation, he interpretedMitchell's (1889) observations on the Hudson River asillustrative of F. L. Ekman's countercurrent. Mitchell,in contrast, had interpreted the observed "underrun"of salt water below the outflowing fresher water as aresponse to a decrease in fresh-water discharge in theHudson River. He calculated that "the surface of theriver water would have to stand 2-1/2 feet above theocean to prevent the salt water from running in alongthe bottom; and the sea-water would creep into thebasin as soon as the head fell below this." If there werenot mixing between the salt water and the fresh water,Mitchell's analysis would explain the movement of thedensity interface between the upper and lower layerand the position of the "neutral plane" where "inflowand outflow balance."

Although we now know there is significant mixingbetween the upper and lower layers in the Hudson,Mitchell's interpretation of his remarkably good cur-rent and density observations provided a reasonableexplanation for the seasonal variation in salt intrusion.R. P. Cowles (1930) in his monograph on the Bureau ofFisheries' studies of the Chesapeake Bay waters notedthat such behavior was not adequately explained byreaction currents. He wondered "why the undercurrent(as deduced by salt intrusion) moving in an ingoingdirection is so marked during the winter months, whenthe discharge from the rivers is not ordinarily at itsheight?" That the surface layers were moving seawardwas evident from the U.S. Coast and Geodetic Surveydata collected by Haight, Finnegan, and Anderson(1930). They report 320 days of current pole measure-ments at the Chesapeake Bay mouth (Tail of the Horse-shoe Lightship), where the mean current flowed out ofthe bay at 13 cm s - . At Thomas Point in the upperBay, a 27-day record showed an 8-cms-1 mean flowseaward. Cowles mentioned many possible mecha-nisms for moving water and salt in Chesapeake Bay,and remarked on the difficulty in analyzing the result-ant complexity. He did succeed in documenting thedistribution and seasonal progression of temperatureand salinity. Of particular note was the lateral gradientin salinity whereby the eastern side of the Bay wasmarkedly saltier than the western side. Cowles as-cribed this feature to the "fact that the deep-waterchannel which contains the most saline bottom waterlies on (the eastern) side throughout most of its extentand to the fact that a large volume of fresh water fromthe rivers of the western shore presses the more salinewater toward the eastern shore." He did not mentionthe possibility that the rotation of the earth played a

role. Wells, Bailey, and Henderson (1929), who titratedthe salinity samples for Cowles's surveys, did suggestthat although the lateral gradient in salinity "had beenascribed to the fact that the principal rivers enter theBay on its west side, the rotation of the earth may alsobe a factor."

Marmer (1925) thought that the greater nontidal sur-face flow and greater ebb duration) along the westernshore of the Hudson River was due to "the effect of thedeflecting force of the earth's rotation." He noted that,at mid-depth, the flood-current velocities were greateron the eastern side of the river, and that the ebb veloc-ities were greater on the western side. Manner in hisHudson River study, and Zeskind and LeLacheur (1926)in their study of Delaware Bay, pointed out the de-crease with depth in ebb-current duration in the estu-ary, and ascribed this decrease to the river discharge.Although they noted that the duration of flood may begreater than ebb near the bottom of the estuary, neitherMarmer nor Zeskind and LeLacheur conveyed thesense that there is an internal, nontidal circulationpresent and that there is net up-estuary motion in thelower layer.

Haight's (1938) review of current measurements inNarragansett Bay contains little discussion of the non-tidal flows. Although the U.S. Coast and Geodetic Sur-vey's interest was to define and predict the tidal cur-rents, Haight may have disregarded reporting the meanflows because the collected measurements were takenby a variety of methods under a variety of conditions,and because Narragansett Bay displayed such "irregu-larity of currents." Hicks (1959) later noted that Pills-bury's 1889 measurements [summarized by Haight(1938)] did show nontidal flow into the estuary in thelower layer.

Early current-measurement techniques did not alloweasy determination of flow direction at depth. Mitch-ell's vertical profiles and Pillsbury's 5.5-day time seriesappear as notable achievements. Mitchell (1859) devel-oped an apparatus to measure the "countercurrent" atdepth in the Hudson River by modifying a device usedto measure subsurface currents in European canals.Two floats (copper globes) were connected by a wire,one weighted to sink to a depth and act as a drogue. Inorder to reduce the errors resulting from the drag of thesurface float, Mitchell added a third float attached tothe surface float and weighted to have the same cross-sectional area. The attachment line of this additionalfloat was connected to a reel to allow the two surfacefloats to separate freely. Mitchell argued that the orig-inal pair of floats would move at the mean of thesurface and subsurface velocities and that the free floatwould move with the surface velocity. The Price cur-rent meter used by the U.S. Coast and Geodetic Surveyhad no provision for measuring direction. The commonpractice was to assume the current direction at depth

200R. C. Beardsley and W. C. Boicourt

corresponded to that indicated by the drift pole at thesurface, a practice that could create substantial errorsin nontidal-flow determinations in the estuary. Mar-mer's (1925) observations were successful because heemployed a "bifilar direction indicator" in conjunctionwith the Price meter. This direction sensor, developedby Otto Petterson (Witting, 1930), consisted of a set ofthree vanes that were positioned at various depths andthat transmitted their alignment to the surface bywires (Zeskind, 1926). Petterson also developed an in-ternally recording current meter that could recordspeed and direction at 30-minute intervals for 2 weeks.The Petterson meter became available to the U.S.Coast and Geodetic Survey in 1925. The majority ofcurrent measurements reported in the cited survey re-ports on the Middle Atlantic Bight estuaries were madeby current pole (often from anchored light ships) andPrice current meters. Although Haight, Finnegan, andAnderson (1930) reported measurements made in theearly 1920s by the U.S. Fisheries Commission employ-ing Ekman current meters, their use does not seem tohave been widespread.

The Coast and Geodetic Survey's collected currentmeasurements in Long Island Sound were reported byLeLacheur and Sammons (1932). Again, they were pri-marily interested in tidal currents and they did notreport the mean currents obtained from the long timeseries at the lightship. Prytherch (1929) released 500drift bottles with drogues in his study of oyster-larvaetransport and setting. With the 300 returns and a fewEkman and Price current-meter measurements, he de-duced a net outflow from the Long Island Sound on thesurface.

Important estuarine research was also being con-ducted elsewhere during the early decades of the twen-tieth century. Europeans and Canadians were active inthe coastal regions where river runoff affects the re-gional general circulation. Palman (1930) employedBjerknes' (1898) solenoid method in a study of thewind-driven circulation in the Gulf of Finland. Hedemonstrated an oppositely directed two-layer flowand determined a wind-stress coefficient.

Jacobsen (1930) provided further details of the two-layer flow near the Kattegat through an analysis ofcurrent time-series measurements made from twolightships. Jacobsen also examined current and densitymeasurements made in Randersfjord on the east coastof Jutland. From these data, which showed clearly theestuarine outflow and inflow (of the order of 10 cm s-l),he calculated the surface slope along the axis of thefjord and determined coefficients of viscosity and mix-ing. He also considered the problem wherein a concen-tration of plankton was placed at the level of no netmotion and allowed to disperse, illustrating the inter-action of vertical diffusion and horizontal advection insuch a two-layer system.

Following a suggestion by A. G. Huntsman that Wat-son's (1936) observations from Passamaquoddy Bay inthe Gulf of Maine could be interpreted as a three-layerflow driven by tidal mixing, Hachey (1934) conducteda series of tank experiments in which he produced bothtwo-layer and three-layer flows (figure 7.2). He con-cluded that

the mixing of stratified water sets up dynamic gra-dients causing the following differential movements:

(a) where, through the addition of fresh water at themixing point, the mixed water is of a density which isless than that of the waters otherwise available formixing, the mixed water is carried away from the mix-ing area in the upper layers, while a compensatingcurrent carries water to the mixing area in the lowerlayers; and

(b) where the mixed water is of a density which isintermediate between the densities of the surface andbottom waters available for mixing, the mixed wateris carried away from the mixing area at some inter-mediate level, and surface and bottom waters are car-ried to the mixing area to compensate for the watersentering into the mixing.

A steady wind blowing towards the area of mixing isresponsible for considerable modification of the abovesystems of currents. Such a wind seems to offer someresistance to the system outlined in (a), but consider-ably enhances a system of currents outlined in (b).

While the current measurements (in Digdeguash Har-bor off Passamaquoddy Bay) offered by Hachey as anexample of the three-layer flow may not be convincingbecause of their short duration and the uncertaintiesin density structure and in the strength of the wind-driven component, his conclusions from the tank ex-

B,,.:·. ~ ~ ~ wimr·(7 .2C) I(7.2C)

Uamm

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enored

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Figure 7.2 Diagram of Hachey's (1934) experimental approach(A), and resultant three-layer (B) and two-layer (C) circulationpatterns. Mixing was provided by rotor A and fresh water wasintroduced by pipe B.

20IEstuarine and Continental-Shelf Circulation

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periments were correct and later substantiated by ob-servations made in Baltimore Harbor. Hachey's workis especially significant because it was one of the firstexplanations of a density-driven circulation in strati-fied water generated by wind and tidal mixing.

In the years immediately following World War II,there was a marked increase in interest in the circu-lation of estuarine waters, caused in part by militaryneeds and a heightened sense that the resources of theestuary and coastal waters were threatened by thenearby activities of man and should be protected.'Many of the papers from this period address the flush-ing characteristics of the estuary rather than the cir-culation per se. In addition to the Office of Naval Re-search's interest in basic research in the oceans, theNavy recognized a need for shallow-water studies toaid in mine warfare, amphibious warfare, and subma-rine-detection problems (Solberg, 1950). The Navy wasalso concerned with the possible environmental threatfrom nuclear submarine activity in bays, harbors, andestuaries. An indication of the scientific interest andtalent dedicated to estuarine-circulation studies in thelate 1940s is given in the proceedings (Stommel, 1950b)of the Colloquium on the Flushing of Estuaries held atthe Massachusetts Institute of Technology in Septem-ber 1950 and sponsored by the Office of Naval Re-search. The papers and discussion show not only thatoceanographers were beginning to model the mixingprocesses in the estuary, but also that they were begin-ning to understand the possibility of an internal es-tuarine circulation.

The task at hand during the late 1940s was to deter-mine the flushing mechanisms for estuaries. AfterTully's (1949) extensive work on Alberni Inlet in Brit-ish Columbia, much of the observational study wascarried out on Middle Atlantic Bight estuaries. Ket-chum (1950, 1951) sought to improve the tidal prismmodel whereby the sea water brought into the estuaryon flood tide is assumed to mix completely with thewater in the estuary. In addition, the water flushed outof the estuary on the following ebb is assumed lost tothe system and does not reenter on the subsequentflood. Estuaries, however, do not mix completely oneach tide. Ketchum therefore proposed to divide theestuary into successive volume segments the lengthsof which were determined by tidal excursions. Withineach segment complete mixing is assumed at high tide.Ketchum applied this concept to Tully's observationson Alberni Inlet and to his own study of Raritan Bay,New Jersey, and of Great Pond in Falmouth, Massa-chusetts, and achieved good agreement with the ob-served salinity distribution. Ketchum's successprompted Arons and Stommel (1951) to translate hissegmented model into a continuous-mixing-lengthmodel. They produced a family of curves that were also

successful in describing the salinity distribution in Al-berni Inlet and Raritan Bay. The constant of propor-tionality relating eddy diffusivity to the tidal excursionand the tidal current amplitude differed, however, byan order of magnitude between the two estuaries. Prit-chard (1965b) pointed out that these two treatmentswere applicable only to vertically homogeneous estu-aries in which tidal mixing was sufficiently intense toeliminate vertical stratification. Stommel (1953b) laterapplied both Ketchum's model and that of Arons andStommel to the Severn estuary, which has small ver-tical stratification. He showed that neither hypothesisworked for the Severn and mentioned that "it does notappear likely that any good purpose can be served atpresent by making a priori suppositions about the tur-bulent mixing process."

The obvious differences in the topography and sa-linity distributions in various estuaries led Stommel(1950b) to call for an estuarine-classification systememploying differences in morphology and mixing proc-esses as criteria. Stommel (1951) began the processwith a classification scheme based primarily upon the"predominant physical causes of movement and mix-ing of water in the estuary," identifying river flow andtidal and wind mixing as the important processes. Prit-chard (1952a, 1955, 1967b) and Cameron and Pritchard(1963) developed and refined Stommel's initial scheme.Hansen and Rattray (1966) later advanced the classifi-cation scheme by suggesting a two-parameter systemthat includes the stratification and the ratio of the netnontidal velocity at the surface to the river flow dividedby the cross-sectional area of the estuary. An attemptat further refinement of these classification schemeshas not been fruitful because of the difficulty in quan-tifying the parameter-selection process for a particularestuary. Many estuaries exhibit a variety of estuarinetypes.

In his discussion of estuarine classification, Pritchard(1952a, 1967a, 1967b) proposed the definition of anestuary quoted earlier as "a semi-enclosed coastal bodyof water which has a free connection with the open seaand within which sea water is measurably diluted withfresh water derived from land drainage." While thisdefinition excludes inverse estuaries such as LagunaMadre, Texas, and San Diego Harbor, which are drivenby evaporation, it is the most useful yet proffered be-cause it sets the scale and the important elementscontrolling the characteristic estuarine circulation-lateral boundaries, the transmission of tidal energy andsalt between the open sea and the estuary, and theintroduction of sufficient fresh water to provide densitygradients driving the currents. The Baltic Sea, for in-stance, would not be considered an estuary under thisdefinition because its large scale renders the lateral


_ � qll� _ � �

boundaries less important to the kinematics and dy-namics of water movement than they are in a trueestuary.

One of the notable aspects of the development ofestuarine-circulation concepts in the active decade fol-lowing World War II was the extensive (and successful)use of laboratory models in deciphering mixing andtransport processes. The first problems addressed withthese models involved the simplest of estuarinetypes-the highly stratified or salt-wedge estuary. Asphysical oceanographers began to exchange ideas inmeetings such as the 1950 colloquium at MIT, theybecame aware that the U.S. Army Corps of Engineershad been working with flumes and physical models forover 10 years at the U.S. Waterways Experiment Sta-tion in Vicksburg, Mississippi (Simmons, 1950).Among the earliest salt-intrusion studies were theArmy Corps of Engineers' investigations of water-sup-ply problems in the lower Mississippi River. The ArmyCorps of Engineers recognized a need for analytic helpand in 1945 requested the aid of hydrodynamicists atthe National Bureau of Standards "to investigate andestablish the basic laws of similitude for models in-volving a study of density currents and the mixing ofsalt water and fresh water." 2 Keulegan provided thishelp and addressed many problems concerning the lab-oratory modeling of salt-wedge circulation. Keulegan(1949) produced salt wedges in flumes in which therewas almost no mixing between the upper and lowerlayer. When he increased the flow of the upper layer,however, breaking internal waves formed on the fluidinterface. Keulegan noted that, in this entrainmentprocess, the waves only broke upward, carrying fluidfrom the lower layer to the upper layer.

Stommel and Farmer (1952) also examined the salt-wedge estuary with the aid of a laboratory flume. Theyshowed that an abrupt widening in a channel can pro-duce a stationary internal wave on the interface if theinternal Froude number equals a critical value. Thisinternal wave acts as a control on the outflow of theupper layer by restricting the thickness of the upperlayer. Stommel and Farmer (1953) later noticed that ifthey added mixing to their flume, there was a pointbeyond which increased mixing has no effect on theoutflow of the upper layer. Dyer (1973) explains thatthis "overmixing" mechanism is a result of the down-ward erosion of the density interface reaching the levelwhere it restricts the compensating inflow in the lowerlayer. Model analyses were also conducted for wider,well-mixed estuaries such as Delaware Bay. Pritchard(1954a) studied flushing in the Army Corps of Engi-neers' Delaware model at Vicksburg, Mississippi, byemploying dye as a tracer. Pritchard found that theeddy diffusivity was spatially scale dependent approx-imately in the proportion suggested by Stommel (1949).

The Chesapeake Bay Institute began a study of themoderately stratified James River in the summer of1950 to examine the influence of the salinity and cur-rents on the oyster seed-bed region in the middlereaches of the James River. The recent development oftechniques for rapid sampling of currents and salinityfrom an anchored vessel allowed for the first time thecollection of continuous detailed measurements forperiods of 3 days or more. Current velocity was meas-ured with a biplane drag (Pritchard and Burt, 1951), amodification of a method used by Jacobsen (1909) andapparently by Nansen {(Witting, 1930), and salinity pro-files were obtained with in situ conductivity and tem-perature sensors (Schiemer and Pritchard, 1957). Thesenew observational tools were used to collect a data setsufficiently extensive in both time and space thatmeaningful temporal and spatial averages could becomputed. This averaging procedure further minimizedthe (apparently low) variability due to local wind-driven currents and variations in the river flow thatoccurred during the sampling periods. Pritchard (1952b,1954b) used this data set to evaluate the terms in theaveraged salt-balance equation and to conclude thatthe horizontal advective flux and the vertical diffusiveflux of salt were the most important in maintainingthe balance. Pritchard (1956) also examined the mo-mentum balance in the James River, determining theunknown terms in the equation of motion from theobservations of the mean distribution of temperature,salinity, and current velocity. He evaluated the impor-tant Reynolds-stress terms and described the topogra-phy of the pressure surfaces, which sloped down towardthe sea in the upper layer and down toward the headof the estuary in the lower layer. He found, as didCameron (1951), that the cross-estuary pressure gra-dient and the Coriolis force were in approximate bal-ance.

Rattray and Hansen (1962) used the James River ob-servations and Pritchard's analysis to develop a theo-retical steady-state circulation model for a moderatelystratified estuary. Employing similarity transforma-tions, whereby functional forms were assumed for thedependence of the stream function and salinity defecton the longitudinal position in the estuary, Rattray andHansen reduced the two-dimensional partial differen-tial equations governing the stream function and sa-linity defect to a pair of simultaneous ordinary differ-ential equations. Under the conditions specified byPritchard for the James River (in which the field accel-erations and the vertical advective and horizontal dif-fusive fluxes were unimportant), and given the surfacesalinity distribution, Rattray and Hansen produced ver-tical profiles of salinity and velocity that matchedthose observed in the James River. Hansen and Rattray(1965) later relaxed some of the restrictive assumptionsto retain the river-forced component of circulation.

203Estuarine and Continental-Shelf Circulation

Their work also found good agreement with the JamesRiver observations.

While this matching between these two theoreticaltreatments and Pritchard's analysis of the James Riverdata has enhanced the attention paid to these studies,the value of these analytic models lies less in the agree-ment per se with observations than in the insight theyprovide into fundamental estuarine processes. Thesesolutions to Pritchard's (1956) dynamic equations werethe first to show clearly the interdependence of salinityand velocity fields in the estuary. While Agnew (1961)had considered two separate aspects of this interde-pendence in the free-convection part of the problem,Hansen and Rattray (1965) solved the coupled equa-tions, including both the free-convection and forced-convection modes. Hansen and Rattray (1965) not onlydelineated the effects of fresh-water discharge and windstress on the gravitational circulation in the JamesRiver, but also considered the interrelationships in anestuary such as the Mersey, which has a well-developedgravitational circulation despite the fact that tidal mix-ing nearly eliminates the vertical salinity gradient.

The concept and description of the internal circula-tion in a moderately stratified estuary evolved primar-ily from these studies of the James River data set. Thiscirculation differed from the salt-wedge circulation,not only because the lower layer moved strongly to-ward the head of a moderately stratified estuary, butalso because the transport in the individual layers wasmuch greater than in the salt wedge. Near the mouthof a moderately stratified estuary, the upper-layer net(nontidal) transport can be an order of magnitudegreater than the river flow entering the estuary. Indescribing the driving mechanism for this internal cir-culation, Pritchard (1967b) stated:

It has been attributed to the increased potential energyof the system which follows from increased exchangebetween the fresh-water and saltwater layers. Moreaccurately, tidal mixing produces horizontal densitygradients of increased strength, which in turn producehorizontal pressure gradients of sufficient magnitudeand extent to maintain the relatively higher velocitieseven in the face of increased eddy friction. Tidal mixingis responsible for both the increase in potential energyand the distribution of potential energy within the es-tuary.

7.2.2 Recent Developments in the Study ofEstuarine-Circulation ProcessesWhile the contributions of Pritchard, Rattray, and Han-sen represent significant advances in our understandingof estuarine-circulation processes, fundamental ques-tions remain as to the nature of the transport of saltand momentum, the role of the wind in the transportprocesses, and the effects of topography in producingboth order and disorder. In spite of an improved ability

to attain spatial coverage and resolution with moderninstrumentation, our ability to describe and model es-tuarine physics is still limited by inadequate parame-terizations of friction and turbulent mixing. Longercurrent-meter records are showing that the current var-iability due to wind forcing is more complex than pre-viously thought. As more detailed information on thecirculation becomes available, there is a growing con-viction on the part of estuarine investigators that thevariations in the lateral direction are significant in thedynamics, and that bottom topography can generateboth secondary flows and residual circulations.

Three observational methods have been employed toseparate and examine mixing processes in an estuary:(a) the evaluation of terms in the temporally and spa-tially averaged salt-balance equation; (b) the directmeasurement of turbulent fluctuations in velocity andsalinity; and (c) the observation of dispersion by anintroduced tracer. The first method is the analytictechnique employed by Pritchard (1952b, 1954b) on theJames River data set. His conclusion that the horizon-tal advective flux of salt and the vertical diffusive fluxare the dominant terms is based on an analysis thatassumes lateral homogeneity. For estuaries such asDelaware Bay, which can exhibit vertical homogeneitybut have lateral gradients in salinity and velocity, Prit-chard (1955) suggests that, by analogy with the JamesRiver, the dominant salt-flux terms are probably thelateral-diffusive and the longitudinal-advective terms.These analyses involved tidally and spatially averagedvalues of salinity and velocity, but the averaging proc-ess is not explicitly developed in the salt-balance equa-tion. Pritchard (1958) begins the rigorous averaging ofthe three-dimensional salt-balance equation, express-ing salt and velocity variables as sums of time-meanvalues and deviation terms. Bowden (1963) and Cam-eron and Pritchard (1963) further decompose the vari-ables into a time mean, a turbulent fluctuation, and asingle oscillatory term varying sinusoidally over thetidal cycle. Bowden employs this decomposition to ex-amine the effect of vertical shear on the longitudinaltransport of salt in a laterally homogeneous estuary.He finds that, for the Mersey River, there are occasionswhen the advective flux of salt out of the estuary(driven by the river discharge) is approximately bal-anced by the transport associated with the verticalshear in velocity and vertical variations in salinity. Onother occasions, Bowden finds that the up-estuarytransport is shared between the "shear effect" and thetransport arising from the correlation between the har-monically varying terms of the depth-mean velocityand salinity. There are also times when this tidal-cor-relation term dominates and times when the upstreamand downstream salt transports do not balance.


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Okubo (1964) has carefully examined the averagingprocess for an estuary with lateral as well as verticalvariations, specifying the assumptions under which hissalt-balance equation is appropriate. He uses the saltequation averaged over the cross section ip a successfulanalysis of measurements made at the Delaware estu-ary model at the U.S. Waterways Experiment Station.Hansen (1965) also considers variations over the crosssection of the estuary. He decomposes the cross-sec-tional mean variables into a mean, a harmonic tidalvariation, and a turbulent fluctuation. For the Co-lumbia estuary, which has a large river flow, a largetidal range, and a weak gravitational circulation, Han-sen finds that the advection of salt driven by the riverdischarge is balanced primarily by fluxes associatedwith the correlation of velocity and salinity fluctua-tions of the tidal period and with the shear effect.Fischer (1972) argues that for the Mersey, the salt fluxassociated with the lateral shear is not only larger thanthat associated with the vertical shear, but that it isdominant. He proposes decomposing the deviationsfrom the cross-sectional. mean into variations in thevertical and lateral directions. Fischer's conclusions forthe Mersey stand in contrast to the analysis by Bowdenand Gilligan (1971) of Mersey observations made in thereach where density currents are significant. Theremay be agreement for the reaches seaward of this re-gion. While Fischer's point that the lateral shear canmake a significant contribution to the longitudinaltransport of salt is well taken, his estimates of theterms in the salt-flux equation are based primarily onparameterizations of the dispersion coefficients and noton direct computations using the salinity and velocityobservations in the manner of Pritchard, Bowden,Okubo, or Hansen. Dyer (1973) states:

So far it is not possible to define precisely which arethe dominant factors since different investigators haveused slightly different methods of analysis; they splitup their components in a variety of ways with certainimplicit assumptions. Consequently, the results of dif-ferences in tidal response and topography between es-tuaries are not clear.

The approach of averaging the salt-balance equationdoes seem to offer a promising means of attaining anexplicit separation of the flux components. Dyer (1973)suggests combining Hansen's (1965) and Fischer's(1972) schemes, and applies (Dyer, 1977) the full set ofterms to a salt-wedge, a partially mixed, and a well-mixed estuary. He finds that for Southampton Water,a partially mixed estuary, the salt flux associated withthe vertical shear and the lateral shear are of the sameorder. It is clear that great care is required to avoiddependence on the observational scheme and methodof data handling. In light of the recently observed wind-driven variability in estuaries and the importance of

topographic effects, proper evaluation of this approachwill require long record lengths, good spatial coverageand resolution, and shrewdness in averaging proce-dures. Rattray (1977) calls for both better methods ofintegrating the governing equations in conjunctionwith field programs and more extensive and elaboratefield observations.

While averaging and evaluating the various terms inthe salt-balance equation provides insight into the spa-tially integrated mixing processes in the estuary, thedirect measurement of turbulent fluctuations providesa unique look at mixing on a small scale. This com-plementary method is particularly suited for the deter-mination of the source(s) of mixing, about which littleis now known. The various roles of wind stress, shearat the pycnocline, bottom stress, and surface and in-ternal waves in providing the turbulent mixing of salthave not yet been evaluated. Bowden (1977) reviewsturbulence measurements made in estuaries [includinga noteworthy early attempt by Francis, Stommel,Farmer, and Parson (1953)] and the subsequent at-tempts to parameterize the observed fluctuations forthe construction of models. The measurement of tur-bulent velocity fluctuations has required many inno-vative techniques. Bowden and Fairbairn (1952)mounted two Dodson-propeller current meters on arigid stand on the bottom of the Mersey estuary. Thisdevice employed a spring-loaded propeller, which en-abled a rapid response to the turbulence. Bowden andHowe (1963) were the first to employ an electromag-netic flow sensor [developed earlier by Bowden andFairbairn (1956)] to measure turbulent fluctuations inan estuary. Many of the subsequent turbulent-velocitymeasurements were made in estuaries tributary to theChesapeake Bay. Cannon (1971) used a biaxial currentmeter (Cannon and Pritchard, 1971) to measure inter-mediate-scale turbulence from a tower erected in thePatuxent River. Seitz (1973) also measured turbulencein the Patuxent River, using an acoustic Doppler-shiftcurrent meter Wiseman, Crosby, and Pritchard, 1972).He showed the approach to isotropy of the three Carte-sian velocity components at high wavenumber, andalso showed the spectral distribution of horizontalshearing stress, which reaches a peak at intermediatewavenumbers. Pronounced intermittency in the Rey-nolds stresses in the Choptank River was reported byC. M. Gordon (1974). J. D. Smith (1978) has developeda profiling system which is particularly suited formeasuring turbulent fluctuations of temperature, con-ductivity, and velocity in an estuary. With this instru-mentation, Gardner and Smith (1978) investigated mix-ing events in the Duwamish salt-wedge estuary inWashington that are apparently triggered by a hydraulicjump that occurs at a sharp change in river depth.

Tracking the dispersion of an introduced dye tracerprovides a third method for examining mixing proc-


esses in an estuary. Pritchard and Carpenter (1960) de-veloped a technique for detecting a concentration of0.04 parts per billion of Rhodamine dye. The three-layer circulation of Baltimore Harbor was discoveredthrough the use of this dye-tracer technique. An appli-cation of particular interest is the examination of theshear effect by Wilson and Okubo (1978) in the YorkRiver, off Chesapeake Bay. They employed Okubo's(1967, 1969) theoretical methods to analyze the disper-sion of a dye release in the lower layer of the Yorkestuary. They provide a model for separating the lon-gitudinal dispersion in a stratified estuary due to hor-izontal turbulence and to the interaction of verticalshear with vertical mixing. They also include the mod-ifications to the shear effect caused by the nontidalupward advection.

Although early investigators showed a keen aware-ness of the effects of strong wind forcing on estuarinecirculation, and they often invoked wind effects toexplain discrepancies that arise in interpretations thatignore wind driving, the significance of wind-drivencirculations in estuaries has only been recently discov-ered through the analysis of long current observations.Pickard and Rodgers (1959) show a wind-induced shiftin the mean velocity profile in Knight 'Inlet, BritishColumbia. Hansen and Rattray (1965) suggest that evena small wind stress could have a marked influence onthe gravitational circulation. Weisburg and Sturges(1976) were among the first, however, to examine thewind transport in a partially mixed estuary using long-term current-meter data. They show, using month-longcurrent measurements from Narragansett Bay, thatwind transients can easily dominate the longitudinalflux of water in an estuary and that a proper separationof the gravitational circulation is difficult with shortrecords (Weisberg, 1976a). Weisberg (1976b) has devel-oped a stochastic model for the wind-driven longitu-dinal flow at one position in the Providence River inNarragansett Bay. Farmer and Osborn (1976) describethe wind circulation in Alberni Inlet. Up-estuary windscan reverse the current in the upper low-salinity layerand cause a deepening of this layer near the head.Farmer (1976) presents a simple model for the fresh-water thickness in the upper reaches of the inlet, andproduces an accurate simulation of the wind-drivenbehavior.

A year-long series of current measurements made inthe Potomac River estuary led Elliott (1978) to thediscovery that the estuarine circulation was not onlyaffected by local wind forcing, but also by sea level inthe Chesapeake Bay proper. This nonlocal forcing isexamined by Wang and Elliott (1978), who find thatthe nonlocal forcing extends to the continental shelf.The dominant sea-level fluctuations in the Chesapeake

Bay have a period of 20 days and are the result of up-estuary propagation of coastal sea-level fluctuations.Local winds operate on a shorter time scale, drivingseiche oscillations in the bay at a period of 2.5 days.Wang (1979a,b) has examined this wind driving further,and finds that the predominant current fluctuations inthe lower Chesapeake Bay are barotropic.

The increasing evidence for wind control on timescales of 10 days or less leads to speculation on therole of wind mixing versus tidal mixing in providingthe energy source for the gravitational circulation.While we do not have sufficient evidence at present todecide this question, investigators are beginning to re-veal both the mode and details of the topographic ef-fects on the tidal mixing process. The oscillatory move-ment of the tides acting on shoreline irregularities andcomplex bottom topography is known to increase thelongitudinal dispersion in estuaries (Pritchard, 1953;Holley, Harleman, and Fischer, 1970; Okubo, 1973).Sugimoto (1975) and Zimmerman (1978) describe theproduction of residual vortices by propagation of thetidal wave over a complicated topography. Ianello(1977) and Zimmerman (1979) remind us that, fortransport processes, careful consideration of the Stokesdrift must be given. The inherent errors and logisticaldifficulty of Lagrangian current measurements as yetleave us with Eulerian measurements as the onlymeans of spatial and temporal coverage. Rattray's(1977) call for elaborate and extensive measurementprograms should be repeated if we are to consider themeasurement of the Stokes drift by Eulerian means.

The generation of secondary flows by bends in riversis well known to fluid dynamicists and geologists. Sec-ondary flows in estuaries that have stratification andtidal oscillation, however, are less well understood,partly because observational evidence is scanty. Dyer(1977) outlines the expected cross-estuary flow patternfor various degrees of stratification and shows, as doesStewart (1957), that the field-acceleration terms cannotbe neglected in the lateral dynamic equation whenthere is curvature in the estuary.

Episodic tidal-mixing events may occur not only ona time scale of the semidiurnal tide, but also on a scaleof the fortnightly variation in tidal range. Haas (1977)reports observations from the lower York River andRappahannock River on the Chesapeake Bay and sug-gests that, in these rivers, the increase in tidal-mixingenergy from neaps to springs provides sufficient in-crease in tidal mixing to eliminate the vertical strati-fication. Cannon and Ebbesmeyer (1978) and Cannonand Laird (19 78) describe fortnightly salinity intrusionsin the fjordlike Puget Sound estuary in Washington.These events are associated with the large spring tidesover the entrance sill.

Garvine (1977) shows that lateral fronts in estuariesmay be important to both vertical and lateral mixing.


�-^-- ·-- ·i-·-1--� -- �I _I_

-

O

E

0v

km

C 5 2. 0 2

Figure 7.3 Salinity and density (ort) distributions at two crosssections of the Chesapeake Bay. Top section is located nearAnnapolis, Maryland, approximately 220 km up the estuary

These fronts may be tidally time dependent or occurwhen the pycnocline breaks the surface, as in lowerChesapeake Bay. Figure 7'.3 illustrates salinity and den-sity distributions at two positions in Chesapeake Bay;one section (top) is near Annapolis, Maryland, approx-imately 220 km up the estuary from the mouth, andthe other section (bottom) is between the Virginiacapes at the mouth of the Bay. The cross-estuary tiltof the pycnocline is evident in the Annapolis section.If this cross-estuary tilt is approximately in geostrophicbalance, the increase in transport in the gravitationalcirculation toward the mouth of the bay requires acorresponding increase in tilt. In the lower ChesapeakeBay, the tilt increases to the point where the pycno-cline breaks the surface, often appearing as a series ofstrong lateral fronts. This observed increase in tilt isprobably the combined result of the increase in thegeostrophically balanced gravitational flow, the addi-tion of fresh water by rivers on the western side of thebay, and by the widening of the bay in the lowerreaches.

from the mouth, and bottom section is located at the mouthof the Bay between the Virginia Capes.

The salinity and density sections shown in figure 7.3serve to illustrate the inherent three-dimensionality ofthe flow near the mouths of estuaries. This three-di-mensionality and complexity near the mouth oftenmakes it difficult to formulate realistic boundary con-ditions for numerical circulation models of the estuary.While the estuary does not often strongly affect thecirculation on the adjacent continental shelf, the es-tuary often dominates the flow in the mouth and inthe nearshore regions.

7.3 Continental-Shelf Circulation

We shall discuss in this section some ideas and obser-vations about the general circulation over the conti-nental shelf in the Middle Atlantic Bight. Bumpus(1973) and Beardsley, Boicourt, and Hansen (1976) havepresented recent reviews on the circulation within theMiddle Atlantic Bight. Bumpus (1973) describes someof the historical ideas about the Middle Atlantic Bight


circulation and gives a summary interpretation of thelarge amount of surface-drift bottle and sea-bed-drifterdata acquired during the 1960s over the eastern UnitedStates continental shelf. In the 1970s, moored arrays ofself-contained current meters and other in situ instru-mentation have been deployed in the Middle AtlanticBight, and Beardsley, Boicourt, and Hansen (1976) pres-ent some of the preliminary results from these newfield programs. In the 4 years since that review, longercurrent-meter records have been obtained and otherdescriptive and theoretical advances have occurred,making it seem both worthwhile and appropriate forus to attempt here to update the preliminary physicalpicture presented in 1976.

We shall begin with a brief physiographic descriptionof the Middle Atlantic Bight and then present a reviewof the early observational work and ideas about waterstructure and the general circulation in the MiddleAtlantic Bight. This review is presented both for com-pleteness and to give the reader a sense of the originand evolution of key ideas and observational methodsused to study the shelf circulation. We shall next de-scribe the nature and structure of atmospheric forcingover the Middle Atlantic Bight because the earlymoored-array work demonstrated that much of thesubtidal current variability observed in the Middle At-lantic Bight is directly wind driven. We shall next de-scribe what is known about the temporal and spatialstructure of the wind-driven subtidal transient circu-lation on both the synoptic (2-to-10-day) time scale andthe longer monthly time scale. The observed meancurrent field and ideas about how it is driven and main-tained will be discussed at the end.

7.3.1 Physiographic SettingUchupi's (1965) bathymetric map shows that the shelftopography within the Middle Atlantic Bight is rela-tively simple and smooth in comparison to the morecomplex topography within the Gulf of Maine and Sco-tian Shelf region. The depth within the Middle AtlanticBight generally increases in a monotonic fashion fromshore out to the shelf break. The depth of. the shelfbreak decreases from about 150 m south of GeorgesBank to about 50 m off Cape Hatteras. The width ofthe shelf from shore to shelf break is generally about100 km except near Cape Hatteras, where the shelfbecomes very narrow (about 50 km), and near NewYork, where the New Jersey and Long Island coastsform a comer region making the shelf there about150 km wide. Both the mean depth and cross-sectionalarea of the shelf decrease roughly by a factor of twofrom the New England shelf to off Cape Henry. Thecontinental slope is indented by many submarine can-yons, but only a few penetrate up onto the outer shelf.Several drowned river channels partially cross the

shelf, the most notable being the Hudson River Chan-nel off New York (see figure 7.1).

Milliman, Pilkey, and Ross (1972) have mapped thesuperficial sediments over the eastern United Statescontinental margin and find the Middle Atlantic Bightto be covered mostly with medium-sized sand. Finer-grained sediments are found in a large region southwestof Nantucket, near the major estuaries, and generallyseaward of the shelf break. A wide spectrum of small-scale morphological features exists over much of theshelf, ranging from wave-formed ripples 10 to 15 cmlong and 1 to 10 cm high up to large-scale ridges 2 to4 km long and up to 10 m high. Intermediate-scale fea-tures such as sand waves of varying size are frequentlysuperimposed on the larger-scale features. The topog-raphy in the transition region between estuary andinner shelf is complex and most estuaries within theMiddle Atlantic Bight have at least one relatively deepchannel connecting the estuary and shelf. These larger-scale topographic features can influence currentsthrough both topographic steering and generation ofhorizontal eddies, while the smaller-scale features canexert a significant form drag on the flow. More detaileddescriptions of the Middle Atlantic Bight bottom to-pography, superficial-sediment distribution, and ideasabout the formation of these features are given byEmery and Uchupi (1972), Swift, Duane, and McKinney(1973), Swift et al. (1976), Freeland, Swift, Stubblefield,and Cok (1976), and Freeland and Swift (1979).

7.3.2 Early Development of Ideas about the ShelfCirculationIt was considered accepted knowledge before 1915 thata rather sharp transition zone existed near the shelfbreak between the generally cooler and fresher"coastal" water found over the shelf in the MiddleAtlantic Bight and the generally warmer and more sa-line "Gulf Stream" water found offshore.3 The text-books and ocean atlases of this early period [e.g., Find-lay (1853), Maury (1855), and the current chart of theU.S. Navy published by Soley (1911)] showed thecoastal water to be generally moving slowly toward thesouthwest along the shelf from Nova Scotia to CapeHatteras. The low temperature and salinity of thecoastal water suggested a northern origin, and Verrill(1873), among others, emphasized that the coastal cur-rents supported a boreal littoral fauna rather than thewarm-water fauna characteristic of the Gulf Stream.The cold coastal water had been mapped as far northas Newfoundland and most oceanographers like Libbey(1891, 1895) and Sumner, Osbum, and Cole (1913)believed that the Labrador Current flowed along thecoast from the Grand Banks past Nova Scotia and theGulf of Maine into the Middle Atlantic Bight and per-haps even as far south as Florida. This belief was mod-ified when Schott (1897) and Dawson (1913) showed,


using direct-current as well as temperature and salinitymeasurements, that the outflow of the Gulf of St. Law-rence via the Cabot Straits is the primary source ofcoastal water on the Scotian Shelf. The British Admi-ralty (1903) charts show this coastal water flowing to-ward the southwest into the Gulf of Maine at CapeSable, where the current either turned northward to-ward the Bay of Fundy, or became too diffuse to deter-mine from the mariner reports.

In 1912, H. B. Bigelow began a remarkable series ofcruises that provided the first comprehensive descrip-tion of the hydrography, circulation, and biology of theMiddle Atlantic Bight and Gulf of Maine region. Bige-low had first gone to sea as a college undergraduatewith Alexander Agassiz in 1902 (Schlee, 1973), andafter finishing his doctorate at Harvard in 1906, hejoined Agassiz as a research assistant at the Museumof Comparative Zoology, where he spent much of histime describing and classifying jellyfish collected onAgassiz's expeditions. In 1908, an ailing Agassiz di-rected Bigelow to conduct a short cruise of his ownacross the continental shelf to collect animals from theGulf Stream, which Bigelow did aboard the Bureau ofFisheries' 90-foot schooner Grampus. Agassiz died inthe summer of 1910, and Bigelow spent the next yearworking on jellyfish at the Museum and reading aboutthe research being conducted in the eastern North At-lantic by Scandinavian scientists under the guidance ofJ. Hjort, the Director of the Norwegian Board of SeaFisheries. Then Sir John Murray visited Harvard in1911 and convinced Bigelow to leave the laboratory fora time and launch his own expedition, which he eagerlydid the following summer aboard the Grampus (Schlee,1973).

It seems clear that Bigelow, in developing his ownfield program, was strongly influenced by both Hjort'ssystematic approach to oceanographic research (seeSchlee, 1973) and several key technological advancesmade by the Scandinavians in the period 1900-1910(see chapter 14). Knudsen (1901) had prepared tables forconveniently calculating salinity and density at at-mospheric pressure (at) from values of temperature andchlorinity, Ekman had developed a mechanically re-cording propeller-type current meter [see von Arx(1962) for a description] that could be used from ananchored ship, and Nansen had perfected a practicablereversing water sampler with an attached thermome-ter. Equipped with these new tools, plus a variety ofimproved biological and geological sampling gear, andsponsored by the Bureau of Fisheries and the Museumof Comparative Zoology, Bigelow and his coworkersset sail on the Grampus in July 1912 to study thehydrography, currents, and biology of the Gulf ofMaine. Bigelow conducted a similar research cruise inthe next summer.

In 1915, Bigelow published his first tentative chart(shown here in figure 7.4) of the summer surface cir-culation for the Gulf of Maine and the Middle AtlanticBight region, as inferred from the July 1913 cruise.Bigelow stated that "the combined evidence of the var-ious records of ocean currents, our own included,points to the conclusion that the dominant drift overthe continental shelf south of New York is to thesouthwest; and this is certainly the prevalent opinionof practical navigators and hydrographers" [p. 232]. Hischart suggested the importance of runoff from the ma-jor estuaries within the Middle Atlantic Bight in boththe surface salinity and current patterns. Bigelow alsospeculated that the cold bottom water found in theMiddle Atlantic Bight was formed locally in the pre-vious winter and was essentially static and not ad-vected into the Middle Atlantic Bight from the east.He (1922) found further support for this idea in theAugust 1916 data. While the idea of a mean near-sur-face drift toward the southwest in the Middle AtlanticBight has been confirmed by more modem measure-ments, the concept of the cold bottom water as staticwas clearly refuted when direct-current measurementsbegan in the 1970s.

Bigelow was primarily interested in the Gulf ofMaine during this period, however, and after a briefinterruption due to World War I, he resumed his fieldwork and began to focus more on the circulation there.He began to release surface drift bottles along strategicsections within the Gulf of Maine and also experi-mented with E. Smith with the Scandinavian methodfor geostrophic-current computation. 4 In 1927 Bige-low's monograph on the physical oceanography of theGulf of Maine was published by the Bureau of Fisheries.Using hydrographic data and geostrophic computationsas well as current information inferred from the move-ment of fish eggs and larvae and drift bottles, Bigelowdeveloped a rather accurate conceptual model of thegeneral circulation of the Gulf of Maine on a seasonaltime scale, which has become the foundation for allsubsequent work in this region. He described thespringtime formation of a counterclockwise circulationaround the basin (called the Gulf of Maine gyre) and aclockwise circulation around Georges Bank (theGeorges Bank gyre). He recognized that slope watercharacterized by S 35%bo penetrated through theNortheast Channel into the deeper basins of the Gulfof Maine and that this water mixed with very freshshelf water from the Scotian Shelf and farther north toform the intermediate salinity water found in the Gulf.Bigelow's schematic near-surface circulation diagram(1927, p. 973) showed that at least during the summer(when drift-bottle returns were highest), some shelfwater flowed westward past Nantucket Shoals into theMiddle Atlantic Bight.


C.

Figure 7.4 Surface circulation map for July 1913 published byBigelow (1915). Surface salinities are shown and dots havebeen added to show hydrographic station locations.


1 _____ _ _� ___�_

J

III

Summarizing their past work and incorporatingsome new measurements made aboard the Atlantis, 5

Bigelow (1933) and Bigelow and Sears (1935) producedthe first complete description of the seasonal temper-ature and salinity fields within the Middle AtlanticBight. They found that vernal warming and fresh-waterrunoff built a strong stratification during the late springand summer months, which was subsequently de-stroyed in the fall and early winter by surface coolingand winter storms. Bigelow and Sears recognized thatshelf water represented a mixture of continental runoffand the more saline slope water and documented thebasic structure of the transition zone between thesetwo water masses. The transition from shelf to slopewater often occurred as a sharp outward-sloping frontlocated near the shelf break during winter, while thefront was less distinct in summer because of the de-velopment of a seasonal thermocline in the adjacentslope water. Large temperature and salinity gradientsstill persisted in the offshore direction below the sea-sonal thermocline on account of a band of cold, low-salinity shelf water that was located near the bottomon the outer shelf and was described by Bigelow (1915,1922, 1933) as a remnant from the previous wintercooling. Bigelow incorrectly visualized an essentiallystatic pool of cold bottom water extending from southof Long Island to Cape Henry that was entirely sur-rounded by warmer water and persisted without re-plenishment through the summer.

In the late 1930s, Bigelow's personal research re-turned to fish and he did not write further about coastalcirculation per se. Bigelow and C. Iselin did encouragea 3-year interdisciplinary field study of the GeorgesBank region and its high biological productivity(Schlee, 1978), and, although it was stopped early in1941 by World War II, this study did produce a numberof biological and ecological papers, including one onecosystem modeling (Riley, Stommel, and Bumpus,1949). Based on his own work on slope water and theBigelow-Sears picture of the Middle Atlantic Bight hy-drography, Iselin (1939b) stated without discussion that"the coastal waters, because of their relative freshness,are at most times of the year less dense than the cor-responding layer offshore and consequently a currentis maintained which for some reason not clearly under-stood, tends to have its greatest strength just outsidethe 100-fathom curve." The idea that the geostrophicbalance represented a driving mechanism was appar-ently a common misconception. Iselin clearly believedthat the density distribution over the shelf and slopewas the principal driving mechanism of the shelf cir-culation. The maximum horizontal density gradientsoccurred in the frontal zone near the shelf break, sowith an assumed level-of-no-motion near the bottom,as suggested by Bigelow (1915, 1922, 1933), the surface

geostrophic current would be a maximum, and directedtoward the southwest along the shelf break. Iselin(1939b, 1940b) did correctly point out that the generallyobserved increase of salinity with depth over the shelfin the Middle Atlantic Bight implies an offshore mo-tion near the surface and an onshore flow at depth.

The first dynamic model for the Middle AtlanticBight circulation was published in The Oceans by Sver-drup, Johnson, and Fleming (1942). The circulationscheme shown in figure 7.5 is taken from The Oceansand indicates a drift of coastal (meaning shelf) and slopewater along the continental margin towards the south-west in the correct sense. According to Sverdrup, John-son, and Fleming (1942, pp. 677-680), precise geodeticleveling experiments conducted in the early 1930s in-dicated that mean coastal sea level rose between CapeHatteras and Cape Cod by some 10 cm. The north-south gradient of mean atmospheric pressure wasknown to be small enough that oceanographers be-lieved that these measurements indicated a real north-ward rise in the absolute sea-surface topography. Sincethe Gulf Stream presumably did not run uphill, andDietrich (1937) had "showed" that the northward sur-face slope was not caused by a northward decrease inmean density along the slope, Sverdrup inferred thatthe sea surface had the profile labeled 2 in figure 7.5,which would imply a southwestward geostrophic cur-rent over the shelf with a maximum near the shelfbreak as argued by Iselin (1936, 1939b). Sverdrup statedthat "a current to the south must also flow over the

Figure 7.5 Schematic representation of the character of theGulf Stream, taking results of precise leveling into account.Inset: Profiles of the sea surface along the line A-B. Profile 1derived from oceanographic data only; Profile 2, from thesedata and the results of precise leveling. [Circulation schemegiven by Sverdrup, Johnson, and Fleming (1942).i


- -

shallow portion of the shelf where it flows downhilland where the balance of forces is maintained by theeffect of friction" [Sverdrup, Johnson, and Fleming(1942, p. 678)]. Sverdrup speculated that this surfacetopography pattern was caused by large-scale windforcing but his reasoning was vague. Even though theaccuracy of the geodetic leveling has been disputed bySturges (1968) and others, recent circulation modelsalso invoke a mean alongshore pressure gradient. Thispoint will be discussed again below (and see chapter 4).

Haight (1942) published the first long-term surface-current observations made in the Middle Atlantic Bightand Gulf of Maine region. The U.S. Coast and GeodeticSurvey had a 30-year-long cooperative program withthe Lighthouse Service and the Coast Guard to measuresurface currents using the current drift-pole techniqueat lightships and other stations on the shelf. Haightpresented quite accurate charts for the tidal currentsand summary charts for the nontidal or mean andwind-driven currents. This tidal-current informationand other direct measurements made in the estuariesand harbors form the basis for the current roses foundon today's navigation charts.

World War II stopped active research on the MiddleAtlantic Bight and Gulf of Maine, and although a num-ber of useful instruments like the bathythermograph(BT) and Loran A were developed and perfected, andmany BT profiles were taken over the shelf, mostoceanographers were busy with defense-related re-search, and active work on the Middle Atlantic Bightdid not resume until the late 1940s. Spilhaus and Miller(1948) modified the BT to obtain discrete water sampleswhile ascending, and Spilhaus, Ehrlich, and Miller(1950) and Miller (1950) then used this new instrumentto examine the shelf-slope water front south of NewEngland. Miller (1950) found evidence for significantmixing across the deeper ro,-surfaces near the shelfbreak, which he attributed to internal wave breakingin the frontal zone. Ford, Longard, and Banks (1952)found narrow filaments of relatively cold and freshwater along the shoreward edge of the Gulf Streamnorth of Cape Hatteras, and Ford and Miller (1952)correctly surmised that this water was, in fact, shelfwater from the Middle Atlantic Bight entrained alongthe edge of the Gulf Stream near Cape Hatteras.

In 1950, the National Lead Company began to dumpacid-iron waste from barges in the New York Bight anda number of oceanographers were asked by the Na-tional Research Council to examine the environmentaleffects and estimate the flushing time for the NewYork Bight. The results of this work were reported byRedfield and Walford (1951) and Ketchum, Redfield,and Ayers (1951). A concerted effect was also made inthe early 1950s to understand and model the circula-tion and mixing within estuaries. This effort was in

part stimulated by concern over the environmental im-pact of waste disposal within rivers and estuaries, andit produced a number of key ideas [e.g., Ketchum's(1950) tidal prism method to compute flushing times,and Stommel's (1953b) method for estimating the lon-gitudinal diffusion coefficient in a well-mixed river orestuary from the observed salinity field]. Ketchum andKeen (1955) segmented the Middle Atlantic Bight fromCape Hatteras to Cape Cod and computed the flushingtimes for each segment, assuming only cross-shelf mix-ing and advection. They concluded that a considerableamount of cross-shelf transport of salt and river watermust occur in both winter and summer to account forthe observed mean salinity field. Ketchum and Corwin(1964) later examined the water structure south of LongIsland over the period 1956-1959 and incorrectly con-cluded that the cool bottom water was formed only bylocal winter cooling, and then was warmed up by mix-ing with either warmer surface water or warmer slopewater.

Both Bigelow and Iselin had long been aware of eddy-like features in the near-surface water structure overthe shelf and slope region. In his interpretation of drift-bottle data obtained in the Middle Atlantic Bight inthe spring of 1951, Miller (1952) suggested that a seriesof distinct current branches or eddies was superim-posed on the general southwest alongshore drift. Thiswork, plus the growing evidence of current variabilityin the Gulf Stream obtained by Fuglister and Worthing-ton (1951) in Operation Cabot, led Iselin (1955) to urgethat new observational methods be developed to studythe Middle Atlantic Bight circulation. Iselin suggesteda several-year program of continuous measurements ofmeteorological and oceanographic variables using newinstruments deployed in moored arrays. Although oth-ers besides Iselin had also considered the potential oflong-term moored-array measurement programs, theinstrumentation and mooring technology required forsuch a program were simply not available yet. In 1954,D. Bumpus, C. Day, and J. Chase did start a cooperativeprogram (Bumpus, 1955) with the U.S. Coast Guard tocollect daily temperature and salinity measurementsas well as meteorological observations at lightships andlight stations in the Middle Atlantic Bight and Gulf ofMaine. This collection program ran through the 1960sand provided the data used by Chase (1959), Howe(1962), Chase (1969), and Bumpus (1969) to examinethe influence of runoff and wind on nearshore salinitiesand surface currents.

Bumpus, Miller, and others had frequently releaseddrift bottles during hydrographic and other cruises inthe Middle Atlantic Bight, and Bumpus and Lauzier(1965) summarized the results of both American andCanadian drift-bottle work conducted from 1948 to1962. Frustrated by the fact that a standard drift bottleprovides only a "birth-and-death" notice and little hard


information about the drift in between, Bumpus (1956)and Bumpus et al. (1957) experimented with radio-tracked surface drifters (the "talking drift bottle") inthe mid-1950s, but this effort was dropped by Bumpusas too expensive and inefficient ({Bumpus, personalcommunication). Howe (1962) did use radio-trackedbuoys with parachute drogues to make some short-term current measurements over the middle and outershelf. In 1960, Bumpus launched a massive 10-yearsurface drift-bottle program some 150,000 bottles re-leased) over the eastern United States shelf, and in1961 he began an equally massive 10-year bottom-drifter program (75,000 drifters released), using thenewly developed Woodhead seabed drifter described byLee, Bumpus, and Lauzier (1965). The essential ideabehind this work was to seed the shelf water with anetwork of drifters at least monthly over a 10-yearperiod in order to determine the annual cycle of surfaceand bottom drift from the inferred trajectories of thefield of drifters. At about the same time, Lauzier begana separate, more modest long-term drifter program overthe eastern Canadian shelf, and he and Bumpus decidedto share their data collected in the Gulf of Maine re-gion. The results of this work were presented in stagesby Bumpus (1965, 1969), and in his final sunimaryreport (Bumpus, 1973).

Bumpus found that throughout the year there was astrong nearshore movement of bottom drifters into orat least toward the mouths of the major estuarieswithin the Middle Atlantic Bight. He also found thatbottom drifters over the mid-shelf region in the MiddleAtlantic Bight primarily moved southwestward with amean speed of a few centimeters per second, with nosignificant seasonal variation in either pattern or in-ferred speed. Because very few bottom drifters deployedat depths greater than about 60 to 80 m over the outershelf were recovered, Bumpus concluded that a line ofdivergence existed in the bottom flow. The recoveryrate in the surface-drifter program was more meagerbecause offshore winds in the fall through early springdramatically reduced the percentage of drifters re-turned except from very near shore. The observed sum-mer surface drift was southwest all along the coastexcept during prolonged periods of strong northwardwinds and low runoff, when the nearshore surface flowwas reversed and became northeastward.

This drifter work summarized by Bumpus (1973) pro-vides the first general picture of the mean and seasonalsurface and bottom circulation in the Middle AtlanticBight. Bumpus (1973) concluded that a mean longshoreflow of order 5 cms - 1 occurs between Cape Cod andCape Hatteras, and that Nantucket Shoals and CapeHatteras appear to be oceanographic barriers that limitin some sense the alongshore flow. Near Cape Hatterasthe alongshore flow turns seaward and becomes en-trained in the Gulf Stream, a process discussed first by

Ford and Miller (1952) and more recently by Fisher(1972) and Kupferman and Garfield (1977).

7.3.3 Recent DevelopmentsThe drifter work undertaken by Bumpus and others inthe 1960s provided the first quantitative description ofthe mean and seasonal surface and bottom circulationin the Middle Atlantic Bight. In the 1970s, mooredarrays of self-contained current meters and other insitu instrumentation have been deployed as part ofseveral new field programs, and these new direct-cur-rent measurements are providing the first detailed de-scription of the regional circulation in the Middle At-lantic Bight. It is important to note that much of thenew in situ instrumentation used in the 1970s evolvedfrom development efforts begun in the 1950s and1960s, and it was not until the late 1960s and early1970s that U.S. oceanographers began to use instru-mented moored arrays as a routine and reliable "tool"to measure current, temperature, and other physicalvariables in both the deep ocean and the shallowercontinental shelf and lakes.6 The development of solid-state electronics, digital computers, and time-seriesanalysis methods in the late 1950s and 1960s especiallyhelped make the current-meter development effortssuccessful. This instrumentation revolution in the1950s and 1960s has had a tremendous impact on thefield of physical oceanography in the 1970s, for it hasallowed in many cases a first direct look at a widespectrum of oceanic motions and phenomena [seeGould (1976) and chapter 14].

The availability of these new observational tools plusan increased public concern over environmental issueshelped motivate much of the new field work begun inthe Middle Atlantic Bight in the 1970s. National con-cern over the environmental impact of marine wastedisposal in the Middle Atlantic Bight became para-mount in the late 1960s and early 1970s. The publicmedia characterized the New York Bight-the coastalocean between Long Island and New Jersey-as a "deadsea" caused by decades of sewage discharge and marinedumping of dredge spoils, building rubble, sewagesludge, industrial wastes, and other materials (Gross,Swanson, and Stanford, 1976). The NOAA MarineEcoSystems Analysis (MESA) Program was formed in1972 to help focus both government and nongovern-ment research on regional problems caused by man'suse of marine and estuarine resources, and, in 1973,the MESA New York Bight Project was started to de-velop a comprehensive research program to understandthe New York Bight as a productive marine ecosystem.Other environmental concerns also helped initiate newfield programs within the Middle Atlantic Bight. TheNew Jersey Public Service and Electric and Gas Com-pany and the Long Island Electric Power Company both

2I3Estuarine and Continental-Shelf Circulation

_ _

began to consider building floating nearshore nuclear-power plants, and New Jersey Public Service sponsoreda 5-year field study of the physical oceanography offLittle Egg Inlet, New Jersey, a potential power-plantsite, while the Brookhaven National Laboratory beganin 1973 a long-term program to study the nearshorecurrents, pollutant dispersion, and primary productiv-ity off the southern coast of Long Island. The Bureauof Land Management of the Department of Interior alsosponsored new field programs to identify and study thedominant sediment-transport processes in the Balti-more Canyon and Georges Bank regions in order toassess better the possible physical hazards and envi-ronmental impact of potential offshore petroleum de-velopment there.

These specific field programs plus others undertakenwith National Science Foundation, Office of Naval Re-search, and other federal support have provided mostof our new knowledge about the mean circulation andlow-frequency current variability in the Middle Atlan-tic Bight. Using moored instrumentation, Boicourt(1973) and Beardsley and Butman (1974) showed thatstrong winter storms could produce alongshore cur-rents of order 20 to 50 cm s- l in the mid-shelf region,and Boicourt and Hacker (1976) demonstrated that thelow-frequency alongshore-current fluctuations werespatially coherent over a 200-km separation along the38-m isobath in the southern Middle Atlantic Bight.Some preliminary results from several of these fieldprograms were then presented by Beardsley, Boicourt,and Hansen (1976), who demonstrated that much ofthe subtidal current variability over the shallower por-tion of the Middle Atlantic Bight was directly winddriven in the synoptic (2- to-10-day) band. Their mapof the directly observed mean subsurface currents dem-onstrated that at least during the 1-to-3-month dura-tion of the initial moored-array experiments, the sub-surface currents did flow alongshore toward thesouthwest, and that the average currents generally in-creased in magnitude offshore and decreased withcloseness to the bottom. At most sites, the mean cur-rent veered toward shore with increasing depth. Thesummer current measurements made off New Yorkand Cape Henry showed that the alongshore currentsin the near-bottom cold pool equaled or exceeded thealongshore currents in the surrounding warmer water,clearly indicating that the summer cold pool is not thestatic feature originally envisaged by Bigelow and oth-ers.

Beardsley et al. (1976) also used the mean-currentobservations to estimate crudely the alongshore vol-ume transport of shelf water through three transectsacross the Middle Atlantic Bight to the 100-m isobath.They found that the three estimates were surprisinglyuniform considering the different time periods of themeasurements and the different instrumentation used,

and they suggested a value of 250 x 103 m3 s - 1 for theaverage alongshore transport of shelf water within the100-m isobath through the Middle Atlantic Bight. Thismean value, when divided into the volume of the Mid-dle Atlantic Bight, implied a mean residence time ofthe order of 0.75 years. This value, based on alongshoreadvection, is somewhat less than the value of 1.3 yearsbased solely on cross-shelf mixing given by Ketchumand Keen (1955). Beardsley et al. (1976) also speculatedthat most of the shelf water observed flowing westwardsouth of New England must, by continuity, flow intothe Middle Atlantic Bight around Nantucket Shoalsfrom the southern flank of Georges Bank and the Gulfof Maine region.

Longer current records have now been obtained inthe Middle Atlantic Bight, and, in the rest of this sec-tion, we shall reexamine the preliminary circulationpicture described in 1976. Since much of the subtidalcurrent variability over the shelf is wind driven, weshall first describe the nature and structure of the at-mospheric forcing found over the Middle Atlantic Bightin some detail, and then describe what is known aboutthe wind-driven shelf response on three time scales:the synoptic 2-to-10-day time scale, the monthly-meantime scale, and the long-term mean time scale.

Atmospheric Forcing The temporal and spatial struc-ture of the surface-wind-stress and pressure fieldsfound over the Middle Atlantic Bight will be describednext. Atmospheric motions are commonly classifiedinto micro-, meso-, or synoptic-scale motions, depend-ing on their characteristic time and space scales. Whilethe exact limits of these scales are somewhat ill defined(Fiedler and Panofsky, 1970; Orlanski, 1975), synoptic-scale motions are generally characterized by periods inexcess of 2 days and horizontal-length scales in excessof 500 km. The synoptic-scale weather disturbancesare generally caused by baroclinic instability, while theprincipal mesoscale phenomena in the Middle AtlanticBight-the summer seabreeze and cellular convec-tion-are primarily associated with mechanical andhydrostatic instabilities (Mooers, Fernandez-Partagas,and Price, 1976; Mayer, Hansen, and Ortman, 1979).Because the early direct oceanic wind measurementsby Millard (1971) and more recent work demonstratethat the synoptic-scale disturbances cause most of theobserved surface-wind variance over the shelf and openocean, we shall focus this discussion on the synoptic-scale surface weather over the Middle Atlantic Bight.

Synoptic-scale surface forcing: Mooers, Fernandez-Partagas, and Price (1976) have examined in great detaila variety of atmospheric-data sets to construct the firstcomprehensive picture of synoptic-scale atmosphericforcing over the Middle Atlantic Bight. The Atlanticcoastal region between North Carolina and New Eng-


____ _�__ __·__. __�__

land is well known for intense cyclogenesis. The com-bination of cool continental air over the eastern UnitedStates and warm moist maritime air offshore causesthe synoptic-scale low-pressure disturbances or cy-clones both to intensify with time and to tend to prop-agate toward the northeast along the coast.' The tem-perature contrast between continental and maritimeair masses is greatest in winter, so that winter cyclonesare usually the most intense and frequent (averaging 5per month), and cause most of the synoptic-scale var-iability. The summer cyclones are generally muchweaker and less frequent (averaging 2.5 per month),though strong storms can occasionally occur in sum-mer see Mather, Adams, and Yoshioka, 1964). Hurri-cane-strength winds occur in the Middle Atlantic Bighton average once in 6 years.

Mooers et al. (1976) have constructed a model wintercyclone based on a case study of 34 synoptic disturb-ances occurring in the Middle Atlantic Bight duringthe winters of 1972-1974. While quite simplified, theirmodel cyclone exhibits a number of features that ap-pear to characterize many winter cyclones. The modelcyclone forms near Cape Hatteras and propagates to-ward the east and north as shown in figure 7.6. Thecentral pressure deepens, and both the spatial scale ofthe cyclone and the strenth of the surface wind stressincrease with time. The surface wind stress and thesurface heat flux into the atmosphere are a maximumin the western sector of the storm, where cold conti-nental air flows out across the shelf and pronouncedmesoscale cellular convection occurs (Burt and Agee,1977). Pronounced warm-front-type precipitation oc-curs in the northern sector and causes the net precip-itation to exceed evaporation along the storm track.The model cyclone develops a surface wind-stress pat-

Figure 7.6 The evolution of the model winter cyclone devel-oped by Mooers, Femandez-Partagas, and Price (1976). The1008-mb contour outlining the cyclone is shown at 0, 12, and24 hours after the storm center has left the coast. The deep-ening of the central pressure is also shown along the stormtrack.

tern which is coherent in both time and space over theextent of the Middle Atlantic Bight, and the maximumwind stresses are larger than the winter mean stress inthe Middle Atlantic Bight by a factor of 2 to 4. Whilereal storms are more complex and follow a variety ofdifferent paths both north and south of the MiddleAtlantic Bight, the model cyclone provides a usefulconceptual framework of the spatial and temporal ev-olution of the surface wind stress and wind-stress curlfields from an event point of view.

Mooers et al. (1976) also examined surface pressureand wind data collected at coastal stations along theMiddle Atlantic Bight and the Gulf of Maine region,and found, using coherence and correlation analysis,that (a) the surface atmospheric pressure (Pa) field wasspatially coherent over the entire Middle Atlantic Bightfor time scales greater than 2 days, (b) the Pa and eastand north wind-stress components T

E and TN have (zero-time lag) correlation scales of about 1600, 600, and800 km, respectively, and (c) the Pa-, rE-, and TN-fieldsgenerally propagate toward the northeast with speedsof 15, 5, and 10 m s-. They argue that the stress fieldis not "frozen" into the cyclone as defined by the sur-face pressure field, and that the difference in phasespeeds between Pa and the stress components is asso-ciated with the tendency for winter storms to intensifyin time, as illustrated in the model cyclone.

Two NOAA environmental buoys designated EB34and EB41 were deployed in the central Middle AtlanticBight in late 1975. EB34 (40.1°N, 73.0°W) was located50 km southeast of New York City and EB41 (38.70N,73.6°W) was located 100 km off the New Jersey coast(see figure 7.7). Both buoys were equipped with vortex-shedding anemometers at a height of 5 m, and rou-tinely transmitted 8.5-minute averages of wind speedand direction to shore. The initial wind data collectedwith these buoys have been studied by Mooers et al.(1976), Williams and Godshell (1977), Overland andGemmill (1977), Noble and Butman (1979), and Mayeret al. (1979); Wang (1979c) has examined wind varia-bility at the Chesapeake Lighttower. These studies col-lectively demonstrate three key features of the synop-tic-scale surface pressure and wind-stress fields: (a) thewinter-buoy data are highly coherent with coastal data,a result which is consistent with the correlation-scaleinformation obtained by Mooers et al. (1976), and im-plies that their results apply to the surface pressureand wind-stress fields over the shelf; (b) the synoptic-scale surface wind stress increases in magnitude by afactor of 2 to 3 across the shelf; and (c) a significantcyclonic verring of the surface stress field by as muchas 30° may occur across the shelf.

We show in the upper half of figure 7.8 wind-stressspectra obtained at Atlantic City, EB41, and at OceanWeathership C (52.5°N, 35.5°W) located east of theGrand Banks in the western North Atlantic. The wind


____._

Figure 7.7 Map of the Middle Atlantic Bight showing the 200- current-meter ([) stations discussed in the text.m depth contour (dashed) and the wind (), sea-level (A), and

stress has been computed using the quadratic drag law,the observed or adjusted 10-m-high wind vector, and aconstant drag coefficient of 1.5 x 10- a. The AtlanticCity and EB41 spectra are computed by Noble andButman (1979) from 6-month-long time series obtainedfrom December 1975 through May 1976, while theWeathership C spectrum reported by Willebrand (1978)is based on a 25-year-long time series. See table 7.1 formore details. We have included the Weathership Cspectrum since the weathership was located some2500 km northeast of the Middle Atlantic Bight alongthe principal storm track followed by winter cyclonesthat develop along the Atlantic coastal region, and itrepresents the closest station (known to us) for whicha well-resolved wind-stress spectrum has been pub-lished. The spectra have been smoothed within theestimated statistical uncertainty to simplify the graph-ical presentation. (See the additional spectra in chapter11.)

The three spectra demonstrate that most of the wind-stress variability is caused by rather broadband syn-optic-scale atmospheric transients with characteristicperiods between 2 and 10 days. The spectra are red(with increasing power densities at decreasing frequen-cies), and show an approximate -2 power-frequency de-cay at frequencies above about 0.5 cpd. The two wintershelf spectra do not exhibit a diurnal peak since thesea breeze is a summer phenomenon (Mayer et al.,1979). While the Atlantic City and EB41 time seriesare too short to determine clearly the very low-fre-quency dependence, the Weathership C spectrum fol-lows a well-defined -0.4 power-frequency dependence

between 0.2 cpd and the annual frequency 0.003 cpd.The two Middle Atlantic Bight spectra demonstrate

the marked increase in wind-stress magnitude fromnearshore toward the shelf break. The EB41 power den-sity is about a factor of 8 larger than the Atlantic Citydensity over most of the frequency bands resolved. TheWeathership C power density is even a factor of 5 largerthan the EB41 power density. Willebrand (1978) hasexamined surface pressure and wind-stress data ob-tained at both Weathership C and Ocean WeathershipD (44°N, 41°W), located roughly about half-way be-tween the Middle Atlantic Bight and Weathership C.He finds that synoptic-scale disturbances in the 2-to-10-day band do propagate toward the northeast in thewestern North Atlantic but that at periods greater thanabout 10 days, atmospheric pressure fluctuations seemto have no preference for east-west phase propagation.Between the two weatherships, TN was coherent at allperiods greater than 1 day, while TE was incoherent atperiods greater than 10 days. These results suggest thatthe larger synoptic-scale transients continue to inten-sify as they move northeastward toward the GrandBanks region.

We have focused on the traveling winter cyclone asthe predominant synoptic-scale disturbance that influ-ences the Middle Atlantic Bight, and have describedthe frequency structure and correlation spatial-scaleinformation presently known in some detail. The sur-face pressure and wind-stress-correlation space scalesare sufficiently large in comparison to the cross-shelfand alongshelf dimensions of the Middle Atlantic Bight


106

105

11

I~J

LuKt

Lu 103

102

10I

In110

FREOUENCY (cpd).001 .01 .1

-5 iO-,4 10- 0o-

FREQUENCY (cph)

I o 10

)1

\o

01Q

10

10'- 10

Figure 7.8 Wind-stress and current spectra. The three uppercurves represent the wind-stress spectra for Ocean Weather-ship C, environmental buoy EB41, and Atlantic City, NewJersey. The two lower curves are the kinetic-energy spectraobtained at a nearshore site (site 1) off New Jersey and a deepersite (site 2) located near the shelf break south of New England(see table 7.1). The vertical brackets indicate the 95% confi-dence limits.

that the synoptic-scale surface forcing should be spa-tially coherent over much of the shelf.7 Frankignouland Miller (1979) have recently reviewed the meagerinformation available on the wavenumber structure ofthe surface wind-stress field over the ocean, and sug-gest several forms for the wavenumber spectra for thesynoptic-scale surface pressure, wind-stress, and wind-stress-curl fields. Their model wind-stress spectrum isessentially white for wavenumbers k << kb =

27r/5000 km-' (which is a wavenumber magnitudecharacteristic of mid-latitude baroclinic instability),and decays at smaller scales like k- 2 for k > kb. Thesmaller-scale surface wind-stress fluctuations appear tobe horizontally isotropic at higher wavenumbers k >2rr/200 km-'. If we assume that the shape of the fre-quency spectrum obtained at Weathership C and themodel wavenumber spectrum suggested by Frankig-noul and Miller (1979) are both applicable to the Mid-dle Atlantic Bight region, then we find that about 50%of the total wind-stress variance in the 1-day-to-i-yearband is caused by synoptic-scale transients concen-trated in the 2-to-10-day band, and some 70% of the

wind-stress variance in the 2/200-to-2r/10,000-km -1

wavenumber band is caused by atmospheric motionswith scales larger than 1600 km, which is twice thealongshelf length of the Middle Atlantic Bight.

Seasonal and mean surface forcing: Saunders (1977)has computed the seasonal surface wind-stress patternwith a 1-square resolution over the eastern continentalshelf of North America using 32 years of ship windreports. His annual mean and three-monthly meanwind-stress maps show that, except in summer, themean and seasonal wind stresses in the Middle AtlanticBight are generally to the east and southeast and ex-hibit a significant increase in magnitude and some cy-clonic veering with increasing distance from the coast.The relatively weak summer wind stress is directedtoward the northeast and exhibits some anticyclonicveering relative to the coast. The standard deviation ofthe stress in nearshore and offshore regions is 1.5 and2.5 dyncm -2 , respectively, in winter, and 0.5 and 1.5dyncm-2 in summer. The offshore increase in thestress is most obvious in winter and spring seasons,and, since these periods dominate the mean stress, theycause the pronounced offshore increase observed in themean stress pattern. The mean and seasonal wind-stress pattern over the adjacent western North Atlanticis described by Leetmaa and Bunker (1978) and will bediscussed below. The annual air-sea interaction cyclesand continental runoff are described in the appendixfor completeness.

The Synoptic-Scale Shelf Circulation We shall nowfocus on the response of the Middle Atlantic Bight tosynoptic-scale (2-to-10-day) atmospheric forcing. Weshall first examine what is known about the temporaland spatial structure of the transient wind-driven shelfcirculation, and then describe a conceptual model sug-gested by the existing current and sea-level data.

Temporal structure: We show in the bottom half offigure 7.8 kinetic-energy spectra computed from cur-rent records 6 months or longer obtained at two rep-resentative sites in the Middle Atlantic Bight. The twosites are labeled 1 and 2 and the locations and otherpertinent information for each site are given in table7.1. Site 1 is located 4.5 km off Little Egg Inlet, NewJersey, and site 2 is located on the outer New Englandshelf about 50 km east of the Hudson Canyon (seefigure 7.7). The kinetic-energy spectra for sites 1 and2 are from EG&G (1978) and Ou (1979) respectively,and have been smoothed within the estimated uncer-tainties to simplify the graphical presentation. It isimportant to remember that a significant spectral gapexists between energetic high-frequency motions char-acterized by periods from several seconds to minutes(associated with surface and internal gravity waves,and related wave and turbulent phenomena) and ener-getic lower-frequency motions characterized by periods


Io 1000 o lb ; .;PERIOD (days)

WEATHERSHI

SITE I

SITE ISITE 2

TIDAL FREQUENCIESSD-SEMIDIURNAL = 1/1.4h =.081 ph

= DIURNAL 1/24h .042 cph

INERTIAL FREQUENCY AT 40.5N I SDI/1.5 h .054c ph D I SDI= /18.5 h =.054 cph r

I , A TT T

-------- -_

Table 7.1 Location and Other Pertinent Information for the Three Wind-Stress and Two Current-Kinetic-Energy SpectraShown in Figure 7.8

Inst.height- Water

Site Location Time span depth (m) depth (m) Data source

Wind stressAtlantic City 39027'N, 74°34'W Dec. 75-May 76 6 - Noble & Butman (1979)EB41 38°42'N, 73°36'W Dec. 75-May 76 5 - Noble & Butman (1979)Weathership C 52°30'N, 35°30'W Jan. 45-Dec. 71 10 - Willebrand (1978)

Current1 39°28'N, 74°15'W Apr. 73-Dec. 76 4.5 12 EG&G (1978)2 39°59'N, 71°54'W Feb. 76-Aug. 76 38 83 Ou (1979)

greater than several hours. We do not show here thehigher-frequency end of the kinetic-energy spectra, butsimply note that significant kinetic energy can exist athigh frequencies, primarily associated with gravity-wave phenomena. While the importance of surfacewaves on beach erosion and nearshore sediment trans-port is generally appreciated (Lavelle et al., 1976), therecent field observations by Lavelle, Young, Swift, andClarke (1978) and Butman, Noble, and Folger (1979)demonstrate sediment resuspension by storm-gener-ated surface waves in depths out to 85 m in the MiddleAtlantic Bight, and Grant and Madsen (1979) describehow the combined motion of waves and a lower-fre-quency current over a rough bottom can lead to anincreased bottom drag on the current. While the exist-ence of the spectral gap allows the high-frequency cur-rent signal to be removed by vector averaging, the 5-to-15-second oscillatory currents and the mooring mo-tion associated with surface gravity waves are the pri-mary source of contamination in the measurement oflower-frequency currents with existing mechanicalcurrent meters and standard mooring techniques.

The two kinetic-energy spectra shown in figure 7.8illustrate several fundamental features of the transient-current variability observed in the Middle AtlanticBight. Both spectra are inherently red, with the kinetic-energy density generally increasing with decreasing fre-quency below 0.5 cpd. The marked similarity in shapebetween the three wind-stress spectra and the kinetic-energy spectrum at site 1 (especially the -0.4 fre-quency dependence below the break near 0.3 cpd) sug-gests that the subtidal current fluctuations over theshallow inner shelf are directly wind driven over a verywide frequency band, from 0.01 to 1.0 cpd. Approxi-mately 50% of the subtidal current variance observedat site 1 occurs in the synoptic-scale (2-to-10-day) band.At site 2 near the shelf break, the subtidal kinetic-energy density is smaller because of the increasedwater depth and measurement level, and the spectralshape is different in that the transition or break in thesubtidal portion of the spectrum occurs at a lower fre-quency, near 0.1 cpd. This indicates that the subtidal

current variability observed over the outer shelf iscaused by both direct wind forcing and by the trans-mission or leakage of lower-frequency motion onto theouter shelf from the deeper ocean. The propagation oftopographic Rossby waves up the continental rise andslope, the meandering of the Gulf Stream, and the pas-sage of anticyclonic warm-core eddies near the shelfbreak can conceivably generate strong low-frequencycurrents over the outer shelf. These three differentmechanisms and some supporting observations are de-scribed by P. C. Smith (1978), Ou (1979), Halliwell(1978), and Scarlet and Flagg (1979).

Flagg (1977), EG&G (1978), Bennett and Magnell(1979), Mayer et al. (1979), Butman et al. (1979), andChuang, Wang, and Boicourt (1979) have examined thecoherence between the local wind stress and the along-shelf and cross-shelf current components and all findthat alongshelf currents at all observed levels are sig-nificantly coherent with the local alongshelf windstress over the synoptic-scale band. The reported co-herence squared generally varies from about 0.5 to 0.8,with a poorly defined time lag of 4 to 10 hours betweenthe alongshelf wind-stress and current components. Toa lesser extent the cross-shelf current component isalso significantly coherent with the local alongshelfwind-stress component. Neither current component issignificantly coherent with the local cross-shelf windstress except perhaps very near the surface (within afew meters) and near the coast in the New York Bight(EG&G, 1978; Csanady, 1980), where cross-shelfwinds can set up trapped pressure fields that drivealongshore currents.

To illustrate the strong coherence between along-shore wind and currents, we show in figure 7.9 thecoherence and phase between the alongshelf wind orwind stress and the alongshelf current observed at thetwo shelf sites just discussed. The coherences shownare significant and relatively high over the synoptic (2-to-10-day) band at both sites, and the phases indicatethat the alongshelf currents lag the alongshelf wind orwind stress by about 4 hours at site 1 and about 10


---------- -- - --- -- -·----

hours at site 2. While the coherence at site 1 exhibitssome seasonal change [with a tendency for higher co-herence in the synoptic-scale band during less stratifiedwinter periods (EG&G, 1978)], the mean coherencesquared is relatively constant over a wide frequencyband between 0.03 and 0.5 cpd, indicating the along-shore currents just off New Jersey are directly winddriven at very low frequencies down to the monthlytime scale. The current record at site 2 is too short toresolve accurately the lower-frequency coherence cut-off.

Spatial structure: We have pictured next in figures7.10 and 7.11 composite vector diagrams to illustratethe vertical and cross-shelf structure of the synoptic-scale current fluctuations in the Middle Atlantic Bight.In figure 7.10 the adjusted sea level at KiptopeakeBeach near Cape Charles, the Norfolk wind stress, andthe subtidal currents observed on a cross-shelf transectoff Cape Henry are shown for the summer period 24July to 22 August 1974. In figure 7.11 the local windstress (computed from surface pressure charts) and thesubtidal currents observed on a cross-shelf transectsouth of New England are shown for the winter period1 March to 31 March 1974. The locations of the twotransects are shown in figure 7.7. The mooring numberand local water depth are given on the right of eachfigure and the depth of the current measurements isgiven on the left. The local alongshelf direction is ver-tical in both figures and the current time series havebeen low-pass filtered to remove the tidal and higher-frequency components above about 0.7 cpd. The meanshave not been removed from each time series.

SITE

1.0

0.8

0.6

0.4

0.2

0.0 tO-4

Zu

Cl)

LU

Ru

0o-3 0o- 2 o10-' io 10'

These two vector diagrams illustrate the followingcommon features of the synoptic-scale current fluctua-tions. There is a clear coherence between synoptic-scale wind-stress events and current fluctuations, withboth strong up- and downshelf currents being drivenby up- and downshelf wind stresses.8 The alongshelfcurrent fluctuations are themselves visually coherentand roughly in phase in both the vertical and cross-shelf directions. The amplitudes of the alongshelf cur-rent fluctuations do not vary significantly in the cross-shelf plane except within the bottom boundary layer,and the empirical orthogonal-function computations ofFlagg (1977) and Chuang et al. (1979) indicate that asingle barotropic vertical mode can account for approx-imately 90% or more of the subtidal alongshelf-currentvariance in both winter and summer. The observedcross-shelf currents have a more complicated verticalstructure, with strong upshelf wind stress drivingoffshelf flow near the surface and onshelf flow near thebottom, and strong downshelf wind stress driving on-shelf flow near the surface and offshelf flow near thebottom. This tendency for the profile of the cross-shelf-current fluctuation to reverse with depth has beennoted by Boicourt and Hacker (1976), Scott and Csan-ady (1976), Flagg (1977), EG&G (1978), Mayer et al.(1979), and Chuang et al. (1979). Flagg (1977) andChuang et al. (1979) find that the lowest baroclinicempirical orthogonal mode and the barotropic modemust both be used to account for more than 90% ofthe cross-shelf-current variance. Mayer et al. (1979)and Chuang et al. (1979) find that the depth of thecross-shelf-velocity node varies with stratification over

SITE 2

1.0

0.8

0.6

0.2

0.0 , .- . ..... 0 c1t4 10-3 2 1 0-1 0O IiC).4 i

FREQUENCY (cycles/dy)

Figure 7.9 Coherence and phase computed between localalongshelf wind and current components at site 1 and betweenlocal alongshelf wind-stress and current components at site2. Locations of the two shelf sites are given in Table 7.1 and

FREQUENCY (cycles/day)

the corresponding kinetic energy spectra at both sites areshown in figure 7.8. The horizontal lines indicate the 95%confidence limit for zero true coherence, and a negative phaseindicates that the current lags the wind or wind-stress.


C0)

lu11

Lu

- |

- - -

^.^ A~Al

I I I I l, I I I I IIII

I I , . . I ·. I I..., .

Water Level Kiptopeake Beach

Wind Stress- ,,; .1/ . // ,,II 'llll///\/ , .. 'I/ //'

MOORING 408AWATERDEPTH18m

IOm

MOORING 413AWATERDEPTH38m

20 m.,. , / 1 1 W._ .,~31 rn

|11fllll¥///I|/,li' - l ---'"5' 11 MOORING 415A30 rn ./ , WATER

DEPT DEPTH 70 m50 m

78 m

WATER

104 rn

25 30 IJuly 1974

5 10 15August 1974

Figure 7.I0 Summer vector time series of adjusted sea levelat Kiptopeake Beach, Norfolk wind stress, and subtidal cur-rents measured along the cross-shelf transect I located offCape Henry. Current-measurement depths shown to the left

and mooring numbers and local-water depths shown to theright. The transect mooring locations are shown in figure 7.7.North is upward, approximately parallel with the local along-shelf direction.

o20

R. C. Beardsley and W. C. Boicourt

25Q5

N

U

0C 0

-Q520

-20

2020

-2020 -0

-20-20

0

-20

E0 20

0

-2020

0

-2020

-20201

0

-20.

-20

20

16 M 16m ~~~~ ~_~~_~_~ll~~h~lllj~j~ ·Illll_\~~lA_ 1111111111L

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n

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EAST (LONGSHORE)

NORTH (ONSHORE)

' ' ' 07 ' ' ' ' ' ' 12' I?.' 17 22 27 t

WIND STRESS

,. (2 m)

\ (62 m)..

= (50 m)

i (70 m)

Z (08)

..........\\\\ .....,\\\-\ / // ..il l...

. . . . . . .. .. ... . . . . .2.6.. . . . . . .. .3 "IA 6 11 16 21 26 31

MAR74

Figure 7.11 Winter vector time series of local wind-stress andsubtidal currents measured along the cross-shelf transect IIlocated approximately 100 km west of Nantucket Shoals. Cur-rent-measurement depths shown to left and mooring numbers

MOORING I

WATER DEPTH 58 m

MOORING 2

WATER DEPTH 72 m

MOORING 3

WATER DEPTH IlO m

and local-water depths shown to right. The transect and themooring locations are shown in figure 7.7. East is orientedupward, approximately parallel with the local alongshelf di-rection.


NEWENGLAND

COASTLINE

4\ 2

-220

0-1020100

-1020100

-1020100

-1020

\ 100O

-10

20100

-1020100

-1020100

-1020100

-10-20

----

� r

cir

the mid-shelf. The node is relatively deep during win-ter, and is significantly shallower in the summer whenthe seasonal pycnocline is well established.

Much less is directly known about the alongshelfstructure of the synoptic-scale current fluctuationswithin the Middle Atlantic Bight. Boicourt and Hacker(1976), Butman et al. (1979), and Ou (personal com-munication) find that along an isobath over the middleand outer shelf, the alongshelf current component ishighly coherent over alongshore separations up to 235km, the largest separation examined, while the cross-shelf current component is incoherent over a 70-kmseparation, the smallest separation examined.

In summary, the synoptic-scale alongshelf-currentfluctuations are generally coherent with the localalongshelf wind stress, and appear to have a relativelysimple spatial structure throughout the year. Thealongshelf-current fluctuations are essentially baro-tropic and spatially coherent in the cross-shelf plane.The alongshelf currents are also coherent over along-shelf separations up to 235 km, although the structureof the synoptic-scale atmospheric forcing suggests thatsignificant coherence should be found over much largerseparations. The generally weaker cross-shelf currentcomponent, although spatially coherent in the cross-shelf plane, appears to have a much smaller alongshelfcoherence, of order 50 km or less.

A conceptual model: These current observationscan be interpreted within the conceptual frameworkprovided by continental shelf-wave theory. It is nowrecognized from a number of theoretical and experi-mental studies [see reviews by Mysak (1980) and Allen(1980) and chapters 10 and 11] that continental marginscan act as effective waveguides for the alongshelf prop-agation of subinertial current fluctuations. The slopingtopography of the continental margin and the densitystratification over the shelf and slope are two basicconditions that lead to coastally trapped wave motions.The offshore increase in depth can support barotropicvorticity waves known as continental shelf waves ina homogeneous fluid, while internal Kelvin waves canpropagate along a vertical boundary in a stratified fluid.Since the internal Rossby radius of deformation overthe inner shelf in the Middle Atlantic Bight is of order10 km or less, and is thus considerably smaller thanthe width of the shelf, the internal Kelvin wave activ-ity, if present, should be trapped in a relatively thincoastal boundary layer. The theoretical and numericalcalculations made by A. J. Clarke (1977) and Wang andMooers (1977) imply that even with realistic stratifi-cation, the alongshelf currents for both forced and freecontinental shelf waves in the Middle Atlantic Bightshould be essentially barotropic over the shelf, andcoherent with the local coastal sea level.9 The cross-shelf momentum balance is essentially geostrophic in

both free and forced shelf waves, so that in theory thesubsurface pressure fluctuations should have a simplemonotonic cross-shelf structure, with a maximum am-plitude at the coast and a vanishing amplitude off theshelf. Beardsley et al. (1977) have examined the spatialstructure of the synoptic-scale subsurface pressurefluctuations observed over the northern half of theMiddle Atlantic Bight, and they found that (a) the sub-surface pressure fluctuations observed over the shelfwere coherent and in phase with coastal sea level, and(b) the subsurface pressure fluctuations had a mono-tonic cross-shelf structure, with very small amplitudesobserved near the shelf break. This last result has beenfurther substantiated with bottom pressure measure-ments made by Brown (personal communication) overthe outer New England shelf and upper slope. Flagg(1977) found significant coherence (coherence squared- 0.7-0.8) between local subsurface pressure and along-shelf-current fluctuations at a mid-shelf site on theNew England shelf, and Chuang et al. (1979) also foundhigh coherence (coherence squared - 0.7-0.8) betweensynoptic-scale alongshelf currents and coastal sea leveloff Cape Henry.

These direct-current and pressure observations areconsistent with the conceptual model of coastallytrapped continental shelf waves. In view of the clearrelation observed between the essentially barotropicalongshelf-current and coastal-sea-level fluctuations,the coastal-sea-level studies of Wang (1979c) and Nobleand Butman (1979) can be used to infer the alongshelfstructure of the synoptic-scale transient shelf circula-tion over larger spatial scales than have been examinedyet through direct-current measurements. Wang(1979c) and Noble and Butman (1979) have investigatedthe relation between local wind stress and sea level inthe Middle Atlantic Bight; they find that north of CapeMay coastal sea level and the alongshelf wind stressare highly coherent over the synoptic-scale band, andcoastal sea level lags the local alongshelf wind stressby 8 to 12 hours, indicating that the alongshelf-currentand coastal-sea-level fluctuations are in phase towithin a few hours. Both coastal wind stress andcoastal sea level move slowly upshelf toward the north-east, and the very slow spatial decay in coastal-sea-level coherence suggests that the synoptic-scale along-shelf-current fluctuations are spatially coherent overthe entire northern section of the Middle AtlanticBight.

The observed coastal-sea-level response south ofCape May is more complex. Wang (1979c) finds thatlocal alongshore-wind-stress and coastal-sea-level co-herence is only high at frequencies above 0.3 cpd, whilethe lower-frequency coastal-sea-level fluctuations ap-pear to propagate downshelf toward the south (like free


r

shelf waves) with a phase speed of order 600 km day-1 .Downshelf propagation of the band-averaged (0.04 to0.4 cpd) coastal-sea-level fluctuations is also reportedby Noble and Butman (1979).

These results suggest the following physical inter-pretation. Free continental shelf waves have a down-shelf or southward phase velocity in the Middle Atlan-tic Bight, while forced shelf waves driven bypropagating atmospheric disturbances tend to move ineither alongshelf direction, in phase with the forcing.The nature of the forced shelf-wave response dependscritically on several factors: the shelf topography andcoastline geometry, the spatial and temporal structureand propagation characteristics of the wind-stress pat-tern, and the effective frictional-adjustment time scaleof the shelf. In the limit of no friction, both free andforced shelf waves are generated by wind-stress pat-terns moving along the shelf, and the alongshelf cur-rent generally lags the local wind stress by 90° (Gilland Schumann, 1974). In the limit of steady forcingand/or rapid frictional adjustment only the forced waveexists [called an arrested topographic wave in thislimit by Csanady (1978)], and the phase lag betweenthe alongshelf current and wind-stress components issignificantly reduced (Gill and Schumann, 1974; Hsuehand Peng, 1978; Brink and Allen, 1978). The frictionaltime scale for the shelf north of Cape May appears tobe sufficiently short that a quasi-steady-state responseto synoptic-scale atmospheric forcing is observed. Theanalytic and numerical-model calculations of Csanady(1974) and Beardsley and Haidvogel (1980) suggest thatthe frictional-adjustment time scale for much of theMiddle Atlantic Bight is roughly Tf = 10 hours. Thusthe ratio of Tf to the characteristic time scale Ta of theforcing (Ta = Llc where L and c are the alongshelflength scale and phase speed of the forcing) is smallenough (s0.2) for most storms that the wind-drivenresponse has much of the character of a heavily dampedor arrested shelf wave, slowly moving along the shelfin phase with the forcing. The shelf south of Cape Mayappears to exhibit both this quasi-steady-state responseand a southward-moving shelf-wave response at lowerfrequencies due to upshelf generation. Why this occursover the southern region of the Middle Atlantic Bightis not yet clear, although it must be related to thethree-dimensional character of the shelf topographynorth of Cape Hatteras, as well as to the spatial struc-ture of the atmospheric forcing. The scattering of ki-netic energy from longer to shorter shelf waves bytopographic perturbations like canyons and capes mayexplain the small alongshelf coherence scale for thecross-shelf current field (Wang, 1980).

We have focused in this section on the large-scaleresponse of the Middle Atlantic Bight to synoptic-scaleatmospheric forcing and have attempted to interpretrecent current and pressure observations in light of

continental shelf-wave theory (with its implicit three-dimensionality). It seems clear that more observationaland theoretical work is needed before the synoptic-scale response is thoroughly understood. It does seem.feasible, however, that the alongshelf current fluctua-tions within the Middle Atlantic Bight may be pre-dictable from the atmospheric forcing and coastal sealevel once the relative roles of free and forced shelfwaves and their generation and dissipation mecha-nisms in this particular shelf domain are clarified.Coastally trapped pressure fields and the transient cir-culations associated with them also occur on shorterspace and time scales. These fields are generally createdby either sharp spatial or temporal variations in thewind-stress pattern or apparent spatial changes in thecoastal wind-stress pattern due to changes in the coast-line orientation. The almost 90° change in the coastlineorientation at New York causes a smaller-scale trappedpressure and current field to exist within the New YorkBight (Wang, 1979c; Csanady, 1980).

The Monthly-Mean Circulation We focus in this sec-tion on the monthly-mean current field in the MiddleAtlantic Bight. We have chosen to use one month asa convenient averaging period for the following reason.In the previous section, we noted that the local along-shore wind-stress and current components over muchof the shelf are coherent and nearly in phase over thesynoptic-scale frequency band from 0.1 to 0.5 cpd. Atlower frequencies, the alongshore wind-stress and cur-rent components generally become incoherent al-though off New Jersey the nearshore wind stress andcurrents remain significantly coherent at lower fre-quencies down to at least 0.03 cpd (a period of 1month). From this we should expect to see some co-herence between the monthly-mean wind stress andcurrents over the middle and inner shelf and we shouldbe able, perhaps, to attribute some of the observedmonthly-mean current variability to the monthly-mean wind-stress fluctuations.

Mayer et al. (1979) have discussed the monthly-mean current variability observed at the MESA long-term mooring site in the New York Bight. We showhere in figure 7.12 a composite vector diagram incor-porating both the data given by Mayer et al. (1979)and other current data obtained during the same timespan at a long-term site located off Cape Henry byBoicourt and Chuang (personal communication). Theadditional wind-stress data shown for the two environ-mental buoys and Cape Hatteras have been suppliedby Halliwell (personal communication) and Noble andButman (personal communication). The wind-stressand current vectors at the different measurement levelsare plotted with true north directed upward, and the


��___

11975 1976 1977N J F M A M J J A S N D J F MA M J J A S O N D J F M A

IO I I I I I . . . I I I I I I i I I I I 197 I I I F I I

N

JFK

FI dn /cm 2(VECTORS)

EB-34

EB-41

ruI

ax-~i-~L~

_- _ '- [E8 34

M RINGj.*'1P/ SITE I

EB4

M OORINGSITE 2

-V.,

WINDSTRESS

7

,0 . # ' -- -11, . , * I- % I ), N . - "

ORI ENTATIONOF MAJORBATHYMETRY

N

45°

E

0

!0

20I

SPEED SCALE

0 5 0cm/s 30

40

N

7,E

E

U

20

30

'ISURFACE

7 V_ " -

Tr.T -- 17

~ ~' j

7 77

P-- 7-- -

' -- · .- .Z /-. BOTTOMSURFACE

I

!

.~--- BOTTOM

I I IIF II I I M IAIM J IAI1 ID I 1F I A I 9 r I I0 N D J F M A M J J A S O N D J F M A

1975 1976 1977

Figure 7.12 Vector time series of monthly-mean wind-stressand subsurface currents observed within the Middle AtlanticBight during the period October 1974 to April 1977. The fneas-urement locations are shown in the inset map. The monthly-mean wind-stress at two coastal and two mid-shelf sites are

shown in the top four time series. The currents shown at site1 are taken from Mayer, Hansen, and Ortman (1979). Boicourtand Chuang (personal communication) supplied the currentdata shown at site 2. The depth scale at left is shown inmeters.


CURRENTSat

SITE I

CURRENTSat

SITE 2

1

_ L _- _ \*"-

r

_

i.

j

n . _ __ __

L

g _

> I i -4

I.

)-

orientation of the regional topography at each mooringsite is indicated on the left of the figure. The locationsof the four meteorological stations and the two currentmeasurement sites are also shown in figure 7.12.

This composite figure illustrates several key featuresof the lower-frequency current variability of the MiddleAtlantic Bight. The monthly-mean wind-stress datashow both a definite offshore increase in strength anda clear tendency for cyclonic veering in most months.The wind-stress vectors are generally aligned to within20° although the wind-stress vectors at EB41 and JFKdiffer in orientation by 40° in February 1976. Themonthly-mean currents are generally directed down-shelf at all measurement levels except during a fewperiods when the alongshore flow is reversed for 1 to3 months. While the magnitude of the possible errorsin these current measurements is not clear [see Mayeret al. (1979)], the alongshore current components atboth sites exhibit a vertical shear consistent with anoffshore increase in the mean density field, so that theupshelf flow during the reversals appears to be strong-est near the bottom. While strong upshelf flows occuron shorter time scales, the submonthly current fluc-tuations are generally comparable to or less than thelong-term mean currents at both sites. Since a similarpicture is presented for the nearshore monthly-meancurrents off New Jersey and Long Island by EG&G(1978) and Scott (personal communication), we con-clude that the monthly-mean subsurface currents overmost of the Middle Atlantic Bight are directed down-shelf except for relatively infrequent reversals of oneto several months duration. The vertical and horizontalstructure of these current reversals is clearly complexalthough the preliminary data shown in figure 7.12suggest that the monthly-mean current fluctuationsover the middle and outer shelf may have a large along-shelf coherence scale.

We shall now focus on the monthly-mean windstress and currents observed during two specific pe-riods-the spring and summer of 1976 and the winterof 1976-1977. Winds for the period May through July1976 were more persistent and stronger than normal,and the monthly mean wind-stress vectors were di-rected toward the north and northeast (Diaz, 1980).This wind stress drove a definite upshelf and onshoresurface flow over the nearshore region from New Yorkto Cape Cod (Frey, 1978), which, coupled with a higher-than-average river discharge in May and June, led di-rectly to the severe pollution of the western Long Is-land beaches (Swanson, Stanford, and O'Conner, 1978).This wind stress pattern also apparently caused thereduced downshelf flow observed at both mid-shelfsites (figure 7.12), and the weak upshelf near surfacecurrent in June at site 1, and the strong reversal in Julyat site 2. The dissolved-oxygen concentration in thenear-bottom water over the shallower half of the New

Jersey shelf reached very low values (less than 2 ml 1- ')during June through August, resulting in an extensivemortality of shellfish valued at $60 million. This majoranoxic event and the environmental conditions thatmay have caused the severely depleted oxygen levelsoff New Jersey and not elsewhere in the Middle Atlan-tic Bight are examined in the comprehensive reportedited by Swanson and Sinderman (1980). We note herethat Armstrong (1980), Walsh, Falkowski, and Hopkins(1980), and others attribute the 1976 anoxic event to:(a) the early development of the seasonal pycnocline[this is clearly shown in the temperature time seriesshown for site 1 by Mayer et al. (1979)], which reducedthe initial dissolved-oxygen concentration in thedeeper water and inhibited later oxygen replenishment;and (b) an excessive local oxygen (respiration) demandcreated by an unusual abundance of the dinoflagellateCeratium tripos advected onto the New Jersey shelf bythe weak upshelf and onshore deep flow. The monthly-mean current measurements for this period presentedhere and by Mayer, Hansen, and Minton (1980), Han,Hansen, and Cantillo (1980), EG&G (1978), Butman etal. (1979), and Ou (1979) suggest that the persistentnorthward wind stress caused an upshelf flow over theshallower New Jersey shelf with perhaps an offshelfand enhanced downshelf flow of shelf water over theouter New Jersey shelf, producing in essence a persist-ent mesoscale clockwise gyre during June. Han et al.(1980) have used a diagnostic numerical model togetherwith observed density and current data to predict thequasi-steady current field over the New York Bightduring the 1976 anoxic event. The computed deeptransport fields indicate both a net convergence of deepwater over the inner New Jersey shelf and strong cross-shelf flow occurring in the Hudson shelf valley.

A more dramatic case for atmospheric forcing of shelfcirculation on the monthly time scale occurred duringthe winter period, November 1976 through January1977. Winds during this 3-month period were bothmore persistent and stronger than normal, and themonthly-mean wind-stress vectors were generally di-rected toward the east-southeast (Wagner, 1977). Thiswind-stress pattern produced strong upshelf currentsnear the bottom at both sites (thus providing an effec-tive bottom stress in opposition to the upshelf wind-stress component), and strong upshelf and offshore flowin the upper 20-30 m at site 1. The monthly-meandownshelf subsurface flow on the southern side ofGeorges Bank was less than average in November andDecember 1976, and the mean alongshelf current wasessentially zero in January 1977 (Folger, Butman, Kne-bel, and Sylvester, 1978). The downshelf mean flowwas observed over the shelf in February. These limitedobservations strongly imply that a sufficiently strongand persistent adverse wind-stress pattern can effec-tively stop and reverse the normally downshelf


- -

monthly-mean flow over much of the Middle AtlanticBight. On December 15, 1976, the tanker Argo Mer-chant ran aground on Nantucket Shoals, and laterbroke apart on December 22 during a major storm,causing one of the largest oil spills off the east coast ofthe United States. It was indeed fortunate that almostall of the oil remained on the surface and was blownoff the continental shelf by the unusually strong andpersistent winds occurring during late December andearly January (Grose and Mattson, 1977; Mattson,1978).

In summary, the sparse current observations dis-cussed here suggest that the monthly-mean currentsover most of the Middle Atlantic Bight are primarilydownshelf except during infrequent periods of strongand persistent adverse wind forcing. The spatial struc-ture of the monthly-mean current field is quite com-plex, especially during periods of upshelf flow.

The Annual Mean Circulation We have plotted in fig-ure 7.13 the long-term mean currents observed at fourshelf sites in recent moored-array programs. Only rec-ords of duration 1 year or longer have been used, andinformation about the individual current measure-ments is listed by site number in table 7.2. The meancurrents are plotted as vectors with the magnitudeequal to the average speed. The rectangle shown aroundthe head of each current vector represents the statis-tical uncertainty in the computed mean. Each side ofthe rectangle corresponds to the standard error E forthe current component parallel to that side, where Ehas been computed using the formula E = o(LFrN/TV

where C,,L is the low-frequency (subtidal) standard de-viation of that current component, T the length of therecord in days, and c, the correlation time scale in days.The quantity T is an estimate of the number ofindependent observations in the current time series,based on the assumption that the time series is sta-tionary. We have used here c = 5 days for both thealongshore and cross-shore components. This is a con-servative estimate since Mayer et al. (1979) and Flagg(1977) found somewhat shorter correlation time scales.However, the use of a more accurately determined cor-relation time scale would not substantially change thesize of the standard errors shown in figure 7.13. Twowind-stress vectors taken from Saunders (1977) havealso been plotted in figure 7.13 to illustrate the orien-tation of the mean wind stress with respect to the shelfbreak over the southern and northern sections of theMiddle Atlantic Bight.

These measurements of the annual mean currentfield on the continental shelf clearly demonstrate thatthe mean subsurface alongshelf flow is directed towardthe southwest through the Middle Atlantic Bight.While there are not enough long-term measurementsto define the spatial structure of the mean circulationin detail, this picture when combined with other cur-rent information and hydrographic evidence substan-tiates the previous concept that over much of the shelf(except perhaps very near the major estuaries), the long-term subsurface currents are downshelf towards thesouthwest. The mean vectors at sites 1, 2, and 3 showa definite tendency for onshore veering of the meanvelocity vector with increasing depth that is consistent

Table 7.2 Tabulation of Long-Term Mean Currents, Standard Errors, and Subtidal Standard Deviations Obtained at Four Sitesin the Middle Atlantic Bight and Georges Bank Region"

Orienta- Nominal E' (cm s- ') N' (cm s- )Site tion of Coordinate Measure- Record Water instrumentnum- topography orientation ment length depth depth St. St. St. St.ber Location (0T) (0e) time span (days) (m) (m) Mean error dev. Mean error dev.

1 36°52'N, 7° 0° Nov. 74- 469 38 10 2.1 +1.3 13.0 -5.8 ±2.5 24.7

75°03'W April 77 573 20 0.3 +0.9 9.2 -4.4 +2.1 22.3

668 30 0.5 -0.6 6.9 -2.6 +1.5 17.5

2 39°28'N, 36 ° 36 ° May 72- 1089 12 4.5 1.6 ±0.2 2.7 -3.9 +0.8 12.2

74°15'W Dec. 76 1082 10 -1.1 ±0.2 3.0 -2.1 +0.6 8.2

3 40°07'N, 45° 45 ° June 74- 482 47 10 1.3 +0.7 7.0 -5.2 ±0.9 9.0

72°54'W March 77 575 20 0.5 ±0.7 7.0 -3.9 ±1.1 12.0

528 41 0.2 +0.2 2.0 -0.9 ±0.5 5.0

4 40°51'N, 58° 58° May 75- 742 85 45 -0.8 +0.3 3.9 -8.8 +0.6 7.6

67"24'W Dec. 78 692 75 0.4 +0.3 3.4 -3.6 ±0.5 5.5

a. Each velocity has been decomposed into an offshore component E' and an orthogonal alongshore component N' where theorientation of the alongshore direction relative to north is indicated by c,. The orientation of the regional topography relativeto north is denoted by T. Except at site 1, 0, and OT are equal. [Boicourt and Chuang (personal communication), EG&G(1978), Mayer et al. (1979), and Butman and Noble (1978, 1979) provided the data for sites 1-4, respectively.]


,��, �_ _____

Figure 7.I3 Map of long-term mean currents computed from1-year or longer current time series with moored current me-ters in the Middle Atlantic Bight and Georges Bank region.The individual sites are circled and numbered according totable 7.2; the measurement depth in meters is shown next tothe mean-current vector. The standard error for each mean-

with the mean surface- and bottom-drifter results re-ported by Bumpus (1973). This mean-current veeringis most pronounced at site 2 off the New Jersey coastwhere nearshore upwelling must occur on average.

Beardsley, Boicourt, and Hansen (1976) speculatedthat the mean alongshore transport of shelf waterthrough the Middle Atlantic Bight might be approxi-mately constant throughout the year, so that the meanvelocities should increase over the southern section ofthe Middle Atlantic Bight where the cross-sectionalarea of the shelf is smaller. This mechanism may ac-count for the somewhat larger mean currents found atsite 1 off Cape Henry. The observed low-frequencycurrent variances are also largest at site 1, where thetypical standard deviation of the alongshelf currentcomponent is about 20 cm s -', or a factor of two largerthan the typical values found at the other sites. Al-though more measurements are needed to determineaccurately the spatial distribution of the subtidal cur-rent variance over the Middle Atlantic Bight, the fewvalues given here do suggest that the wind-driven-cur-rent variability is significantly larger over the shal-lower southern section of the Middle Atlantic Bight.

In summary, these long-term direct measurementsdemonstrate that the alongshelf flow, if averaged overa year or longer, is directed toward the southwest atall levels throughout the water column except perhapsnear the surface, and even there the shorter-term meas-urements at 3 m reported by Mayer et al. (1979) sug-

current computation is indicated by the rectangle around thehead of the current vector. Two representative mean wind-stress vectors taken from Saunders (1977) are also shown toillustrate the relative orientation of the mean wind-stress nearthe shelf break.

gest that the long-term mean surface flow is also di-rected towards the southwest with an offshorecomponent. The long-term current measurementsmade on the southern flank of Georges Bank help sub-stantiate the notion from hydrographic evidence thatthe southern side of Georges Bank is the immediateupstream source region for much of the shelf waterfound in the Middle Atlantic Bight. We shall now dis-cuss the dynamic ideas that have been put forth toexplain the observed mean circulation over the MiddleAtlantic Bight.

A dynamical discussion: As mentioned above in sec-tion 7.3.3, Sverdrup, Johnson, and Fleming (1942) sug-gested at a very early date that the southwestward flowover the shelf was driven against friction by an along-shore pressure gradient, but this dynamic explanationwas ignored by Bumpus (1973) and others as too vagueand speculative, especially after the accuracy of theoriginal coastal geodetic leveling results was ques-tioned by Sturges (1968) and Montgomery (1969).

Some 30 years later, Stommel and Leetmaa (1972)constructed the first theoretical model for the meanwinter circulation on a flat two-dimensional continen-tal shelf driven by a steady, uniform wind stress and adistributed fresh-water source located at the coast. Themodel incorporated linear Ekman dynamics with con-stant vertical friction and diffusion coefficients, whilethe cross-shelf salt balance was maintained by an ad-vective-diffusive Taylor shear-dispersion mechanism.


_-_- I

Stommel and Leetmaa (1972) applied their model tothe Middle Atlantic Bight and concluded that an along-shore pressure gradient, or surface slope of order 10 - 7,

must exist to drive the mean alongshore flow towardthe southwest against the mean wind stress, which intheir model computations) had an alongshore compo-nent toward the northeast.

Using a similar dynamic model for the steady flowover a sloping two-dimensional shelf, Csanady (1976)also concluded that an alongshore surface slope mustexist to account for the observed circulation within theMiddle Atlantic Bight. The essential physical argumentfor the alongshore pressure gradient can be found inthe alongshore momentum balance. The net Coriolisforce on the onshore flow is negligible since the depth-averaged onshore flow is very small, and the divergenceof the onshore flux of alongshore momentum (or on-shore Reynolds stress) is also negligible. Since themean windstress and the bottom stress have along-shore components directed toward the northeast, theboundary stresses must then be balanced by an along-shore pressure-gradient force directed toward thesouthwest in the direction of the mean alongshoreflow. Csanady (1976) noted that if the alongshore sur-face slope remained approximately constant across theshelf, then the total pressure-gradient force associatedwith this surface slope, the alongshore bottom stressrequired to partially balance this total pressure-gra-dient force, and the alongshore velocity should all in-crease as the local water depth increases offshore. Theearly mean-current data summarized by Beardsley etal. (1976) clearly show such an offshore increase in thealongshore currents. Csanady (1976) also pointed outthat the alongshore pressure gradient is required toaccount for the line of bottom-drift divergence reportedby Bumpus (1973) to occur near the 60-m isobath. Scottand Csanady (1976) then offered some supporting in-direct evidence for the existence of the alongshore pres-sure gradient off Long Island, although their estimatedvalue of the gradient has been questioned by Wang(1979c) and Beardsley and Winant (1979).

Csanady (1978) then presented a simple linear modelfor the frictionally damped steady depth-averaged flowdriven over a sloping shelf by a uniform alongshorepressure gradient externally imposed at the shelf break.He found that the surface elevation over the shelf isgoverned by a parabolic differential equation, and thatdownstream from the initial (spatial) transient behav-ior, the alongshore surface slope or pressure gradientremained constant across the shelf. Since in this modelthe total alongshelf pressure gradient is balanced solelyby bottom stress, the alongshore velocity increasedwith increasing depth. Csanady (1978) concluded thatthe open ocean was not "inert," but instead imposedthe mean pressure gradient along the outer edge of the

Middle Atlantic Bight that was required to drive theobserved mean southwestward flow on the shelf.

This suggestion has been strengthened by Beardsleyand Winant (1979), who found supporting evidence inthe numerical simulations by Semtner and Mintz(1977) of the circulation in the western North Atlantic.Semtner and Mintz (1977) constructed a multilayerednumerical model, using a rectangular basin orientedwith the long side of the basin adjacent to the eastcoast of the United States. The model basin had a flatbottom except along the western boundary where asimple continental shelf and slope were located. Themodel ocean is driven by steady zonal surface temper-ature and wind-stress patterns, and Leetmaa andBunker (1978) indicate that the model wind-stress curlis reasonably realistic over the northern region of themodel. The model topography and wind-stress distri-bution are shown in figure 7.14. Semtner and Mintz(1977) first conducted an initial highly damped spin-upexperiment and then two shorter experiments withreduced dissipation in which the model Gulf Streambecame unstable and mesoscale eddies developed. Themean volume transport, surface pressure, and temper-ature fields obtained in the three experiments are quitesimilar in the northern shelf and slope region (themodel equivalent to the Middle Atlantic Bight) and soonly the mass transport and surface elevation patternsfound in the spin-up experiment are shown in figure7.14. In all three experiments, a mean alongshore pres-sure gradient corresponding to a surface slope of order10-7 was imposed over the northern shelf and upperslope by the large wind-driven cyclonic gyre formednorth of the Gulf Stream. Because continental runoffwas not included in the model, Beardsley and Winant(1979) concluded that these model results demon-strated that the shelf circulation in the Middle AtlanticBight could be considered a boundary-layer componentof the offshore large-scale ocean circulation.

Beardsley and Winant (1979) also discussed (withoutreaching any conclusions) the possibility that a majorfresh water source like the St. Lawrence system couldproduce a coastally trapped pressure-gradient field thatmight extend downstream through the Middle AtlanticBight. This other possible driving mechanism has beenrecently ruled out by Csanady (1979),.who concludedfrom the steric set-up distribution over the continentalmargin from Cape Hatteras to the Grand Banks thatthe effects of the St. Lawrence outflow are primarilyconfined to the Scotian shelf and the northern sectionof the Gulf of Maine.

The mean southwestward flow of shelf waterthrough the Middle Atlantic Bight thus appears to bedriven primarily by an alongshore pressure gradientimposed at the shelf break by the deep-water cyclonicgyre found between the continental shelf and the GulfStream. The magnitude of this mean alongshore pres-


_�___ __ _�_ _�__��___ � __._ __ _I_

IC BIGHT

DEPTH (km)

l l I4 3 2 10

20 N 30 N

Figure 7.I4 Steady mass transport (top) and surface elevation(bottom) fields obtained by Semtner and Mintz 11977) in theirinitial spin-up experiment. The contour intervals for top andbottom are 20 x 106 m3 s- and 10 cm. The zonal wind-stress

sure gradient can be expected to vary along the shelfbreak due to changes in both the strength of the re-gional wind stress over the shelf and in the relativeorientation of the wind stress with respect to the re-gional shelf topography. The calculations of Bush andKupferman (1980) suggest that the relative magnitudeof the alongshore pressure gradient may be quite smalloff the southern section of the Middle Atlantic Bightwhere the regional mean wind stress is directed pri-marily offshore with a weak alongshore componenttoward the southwest. While local wind stress and run-off definitely influence the mean nearshore circulation,the model results of Semtner and Mintz (1977) indicatethat the mean circulation over much of the MiddleAtlantic Bight may be viewed as a boundary-layercomponent of the large-scale general circulation of thewestern North Atlantic. Continental runoff injectsfresh water into the boundary layer, which is then

pattern and the model topography is shown to the upper leftand right of top. The model equivalent of the Middle AtlanticBight is indicated at top.

mixed with an onshore flux of upper slope water toproduce shelf water of intermediate salinity, while en-trainment by the Gulf Stream provides the principaldownstream sink of this shelf water.

What does the dynamic model presented here implyabout the seasonality of currents in the Middle AtlanticBight? Both Saunders (1977) and Leetmaa and Bunker(1978) indicate that the wind-stress and wind-stress-curl distributions vary significantly over the westernNorth Atlantic on a seasonal basis, yet the phase ofthese seasonal fluctuations is not stable enough fromyear to year to produce a very significant annual peakin the surface wind-stress spectra at Weathership Creported by Willebrand (1978). The volume transportof shelf water through the Middle Atlantic Bight cor-responds to just a few percent of the mean volume-transport estimates for the slope water gyre given byWorthington (1976) and Leetmaa and Bunker (1978).The numerical ocean-model computations made with


____ _ ___

realistic time-dependent winds by Willebrand, Philan-der, and Pacanowski (1980) suggest that the volumetransport of the slope water gyre should not vary sig-nificantly over the year, and, in fact, Thompson (1977)found no evidence of an annual signal in the long-termdeep-current measurements made near site D on theNew England continental rise. These two results implythat the magnitude of the alongshore pressure gradientimposed at the shelf break should remain nearly con-stant over the year. Since the long-term current meas-urements made just off the New Jersey coast by EG&G(1978) also exhibit no significant annual cycle, we sug-gest that the very low-frequency currents over most ofthe Middle Atlantic Bight will reflect broadband forc-ing and not exhibit a significant annual variation.

7.3.4 Summary and Some Remaining ProblemsWe have attempted here to describe the shelf responseto wind forcing on three time scales. Synoptic-scaleatmospheric disturbances and in particular winter cy-clones can drive strong transient current fields thattend to move along the shelf in phase with the forcing.The cross-shelf momentum balance is approximatelygeostrophic and both the current and subsurface pres-sure fluctuations are generally coherent over much ofthe Middle Atlantic Bight, reflecting the relativelysmall size of this shelf region with respect to the at-mospheric forcing. The synoptic-scale shelf responseappears to be consistent with continental shelf-wavetheory. The direct effect of wind forcing is also evidentin the monthly-mean shelf circulation, although theobserved currents have a complex spatial structure andother processes like runoff and offshelf forcing contrib-ute to the variability on this time scale. The mean flowover the Middle Atlantic Bight is primarily driven notby local runoff and wind stress but by the large-scalewind stress and heat-flux patterns over the westernNorth Atlantic. Thus the observed currents can be de-composed into a mean component driven by a steadyoffshelf forcing and a fluctuating component driven bythe regional wind stress field acting over the shelf.

This review has focused on the wind-driven-circu-lation components in the Middle Atlantic Bight. Theactual influence of density stratification on the differ-ent components of the general circulation is still notclear, and only a crude estimate of the flushing rate ofthe shelf is available. The processes controlling thelocal position and movement of the shelf-slope-waterfront are poorly known. As noted by Fofonoff (chapter4, this volume), satellite infrared mapping of the seasurface temperature has greatly added to our perceptionof the spatial variability and complexity of the near-surface current and thermal fields. The satellite in-frared photograph shown here in figure 7.15 illustratesthermal structures or fronts on a wide range of scales

throughout the Middle Atlantic Bight. The most pro-nounced thermal front is the shelf-slope-water front,and while it crudely follows the shelf break, some ofthe colder shelf water does extend far offshelf into theslope water or along the northern side of the GulfStream. Several anticyclonic Gulf Stream eddies arealso shown in the slope water north of the Gulf Stream.It remains to be determined whether Gulf Stream ed-dies and other very low-frequency phenomena in theslope water have much real influence on the flow ofshelf water through the Middle Atlantic Bight.

Appendix: Annual Air-Sea Interaction Cycles andMean Runoff for the Middle Atlantic Bight

The mean and average monthly heat-flux cycles overthe southern section of the Middle Atlantic Bight havebeen described by Bunker (1976). We show here hisresults, plus additional data kindly supplied by Bunker(personal communication) for the northern section, togive a complete picture of the seasonal heat flux cyclesfor the entire Middle Atlantic Bight. The monthly airand sea surface-temperature and heat-flux cycles forboth sections shown in figure 7.16 and the annualmean values are listed in table 7.3. We note that thesea surface-temperature cycle lags the net heat-fluxcycle by about 90° , and that the approximately 17°Cincrease in sea surface-temperature from March to Au-gust is consistent with a uniform mixing of the netinternal-energy gain by the shelf water during that pe-riod to a mean depth of 30 m.

Average precipitation and evaporation data for theMiddle Atlantic Bight are also shown in figure 7.16 andin table 7.3. The evaporation rates are computed fromBunker's heat-flux data. Although few precipitationdata are available over the Middle Atlantic Bight, pre-cipitation along the coast of the Middle Atlantic Bightis roughly uniform and exhibits relatively little season-ality [see Geraghty et al. (1973) and Lettau, Brower,and Quayle (1976)]. We thus have used data from NewYork City as an estimate for the precipitation cycleover the entire Middle Atlantic Bight. Mean steamflowof fresh water entering the Middle Atlantic Bight alongthe coast via the major estuaries is also given in table7.4. Most of the fresh water is contributed by a fewmajor sources, especially the Chesapeake Bay. Themean precipitation crudely balances evaporation overthe Middle Atlantic Bight and the net input of freshwater via local precipitation minus evaporation intothe Middle Atlantic Bight is minor in comparison torunoff at the coast. Advection of fresh water (meaninghere zero salinity) from the Gulf of Maine and GeorgesBank region accounts for most of the total fresh waterflux into the Middle Atlantic Bight.


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7On Estuarine and Continental-Shelf We shall attempt in