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GEOPHYSICAL RESEARCH LETTERS, VOL. 24, NO. 16, PAGES 2035-2038, AUGUST 15, 1997

Lagrangian flow at the foot of a shelfbreak front using a dye

tracer injected into the bottom boundary layer

RobertW. Houghton

Lamont Doherty Earth Observatory,Palisades,NY 10964

Abstract. Convergent low at the foot of the shelfbreak ront n

the Middle Atlantic Bight has beendetectedusinga dye tracer,

Rhodamine-WT, injected into the bottom boundary ayer. The

observations ubstantiatemodel simulationsby Chapman and

Lentz (1994) of convergentlow in the vicinity of the front which

would be very difficult if not impossible to detect with

conventionalmoored current measurements. y tollowing the

dispersal of the dye patch over a 4 day period Lagrangian

velocitiesof the order of 0.015 m/s with respect o the front were

resolved even as the frontal boundary was displaced 12 km

onshore. The water-following properties of the dye tracer

provides a useful technique for studying the small-scale

circulationand mixing at the frontal boundary.

Introduction

Understanding he processesesponsibleor the positionand

structureof the shelfbreak front, a continuous baturealong the

northeast ontinentalmargin from Nova Scotia o Cape Hatteras,

has been a focus of much research.Typical springtimestructure

of this front, separatingcold, fresh Shelf Water from denser

warm, salty Slope Water is shown n Fig. 1. Numerical model

calculations by Gawarkiewicz and Chapman [1992] and

Chapmanand Lentz [ 1994] highlight he dynamical ole of the

bottom Ekman layer (BBL) by which the offshore flow is

arrested at a convergent zone generated by the cross-shelf

buoyancy lux and then separatesrom the bottom o shoalalong

the frontal boundary.

This convergent low has not been detected. n fact all mean

Eulerian cross-shelf velocities, as measured by current meters

moored in the BBL on the outer shelf and upper slope, are

offshore anging n magnitude rom 0.01 to 0.04 m/s [Beardsley

et al., 1985; Aikman et al., 1988; Bum•an, 1988; Houghtonet al.,

1994]. A continuous offshore flow across he shelfbreak without

any convergences difficult to reconcilewith the persistence f a

narrow frontal boundary.The fact that the foot of the front often

undergoes ross-shelf xcursions reater han 20 km [Houghton

et al., 1994] makes observationof a small scaleconvergent low

at the frontal boundary by an array of current meters neither

feasiblenor cost effective. For these easons new technique,

dye tracer, was proposed as a means of observing the

convergencendperhaps ven he detachment f the BBL flow at

the frontal boundary.The resultsof a pilot cruisewhich not only

tested this technique but also produced some striking

observations re presented ere.

Experiment

Both injectionand detectionof the dye were accomplished

using a sled (0.5 m high, 2 m long, weighing -150 kg and

ballasted o remainhorizontal) owedat speeds anging rom 1 to

4 kts. The sled was fitted with a downward ooking altimeter,a

Sea Cat SBE-19 CTD, and two Chelsea MK II Aquatracka

fluorometerswith optical filters suited o detect Chlorophyll a

and Rhodamine-WTdye. The chlorophyllmeasurement as used

to remove spuriousbackground ignal n the dye channeldue to

in situ chlorophyll. hirty-five kilograms f Rhodamine-WT ye

in a 20% water solutionmixed with isopropylalcohol o achieve

in situ densitywas pumped nto the BBL producing n initial

streak1 km in lengthparallel o the 100 m isobath n 8.9øC water

on the shoreward side of the center of the front. The vertical

temperature radientat the top of the BBL at the Ibm of the front

wasmuchgreater han depicted n Fig. 1. Above a mixed ayer4-

7 m thick, the transition to cool, fresh water above was a 2.4øC

decreasen 3 m correspondingo a 0.4 kg/m density ecrease

and a Brunt-V•iis•il•i period of 2.9 minutes. Because of the

fluctuationsn sleddepthand he unexpected harpnessf the top

of the BBL approximately alf of the dye was actually njected

into the BBL. Within 24 hours all evidence of streakiness in the

dye patch disappearedand dye injected above the BBL was

advected out of the study area by the vertical shear of the

horizontalvelocity.

lOO

15o

200

•1 ' ß ß ß

4O 50 6O 70 8O 90 lOO

Distance kin)

250

i

30 140

10 120 130

Copyright 997by theAmericanGeophysical nion.

Papernumber 7GL02000.

0094-8534/97/97GL-02000505.00

Figure 1. Cross-shelfalonga PRIMER transect 0ø from the

cross-shelfaxis with CTD stations ndicated by crosses)

temperaturesection across he shelfbreak ront south of Martha's

Vineyard wo weeksbefore he pilot cruise,courtesy f Bob

Pickart.

2035



2036 HOUGHTON' LAGRANGIAN FLOW AT THE FOOT OF A SHELFBREAK FRONT

40.•

40.4

was derivedusing he temperature t the peak dye concentration

to represent he temperatureof the center of massof the patch

and constructing a mean cross-shelf front BBL temperature

profile from a compositeof all the cross-front ections.Over the

3-day periodof the surveys he dye patch,and hence he tagged

water parcel n the BBL, moved onshoreby 3.5 km with a mean

speedof 0.015 m/s. Thus the water moved n a directionopposite

to that expected.

The situation s clarified by considering he spatialvariations

ß ' of other variables n the BBL structure Table 1). Although the

i .' ' .......................... tandardeviationssociatedithndividualeasurementss

• '••.•,• "•;i :iii approximately0o0%ftheean,herendreel

•100, defined.ovingnshoreocolderemperatures,he ross-shelf

,q4o.a........ . ................'•It-• :•-fl"'h.•,a,......................... temperatureradient,x, increases,heverticalemperature

'-' ; ß ': R... • .......7-,. i

40.•....... 40

......._ ..............................5

. kiltieters ', '.

-70.7 -•ot -•o_s -70.4 -•o.a

Figure2.ruiserackduringtheinjectionanddetectionofC3•15[-i T?•/ 'l'i 1....

ye patch. Three surveysare identified as TR3, TR4, and TR5.

Bottombathymetry s indicated.Contoursof dye concentrationn

unitsf10- are , 12, nd 0 orTR3 nd .5 nd forTR5. 1

Note the onshoredisplacementof the dye patch and hence the

frontal boundary rom the 100m to 80m isobathbetweenTR3 and

TR4.

0 •,"•'• i i i ,"' i "• i \ I

6.5 7 7.5 8 8.5 9

Tempe atu e degC)

Theubsequentispersalf heyeatchn he BLas 45 : T

appedor henext daysFig. ).Measurableye

detection was a concentrationof 10 11b

concentrationlevelf .... y 40 .... ... .... .... ....... .... ........ ....

volume) was confined to the BBL. The sled altitude was '

maintainedetweento5 m aboveottomith rief ertical 35- '

excursions to measure the B BL thickness and stratification by

adjustingheowingpeedndengthf ableeployed.he

estwardriftf he atch-0.06 /s),oughlyarallelo he 30.... ........ ............... '...........

local isobaths,was expected, he sudden nshore isplacement f

approximately2mn 4ionrs-0.14/s)wasot.hereas i 25................/..................concurrentdisplacement f the frontal boundary nferred rom

theross-shelfositionf heøCsotherm.inceuringhis • 20 : ' : ß i '"'" '

eventross-shelfemperatureradientsithinheBBL idnot

change,t s nferredhat t eastheowerortionf herontal O• 5 ... .... .... .... ' 'Ti ' : .

boundaryasdisplacednshorendnot uststretched. >, : : ....

As the dye patch dispersedts mean temperature ecreased. 10 .... : ... : .... : .... : -' .... : ......... : ...

Thesalinity lsodecreasedithT-S values f thedyepatch /::l _

evolving along the mean T-S mixing curve for the BBL water

across the front. This cooling is illustrated by representative

sections cross he dye patch Fig. 3) measured uringsuccessive

surveysof the dye patch. The dye inventory derived from the

final survey indicates that 11.5 1 or approximately 70% of the

original dye injected into the BBL was still in the dye patch.

Therefore the dye patch is truly a Lagrangian ollower, and this

temperature hange epresents oolingof the waterparcel agged

by the dye. The dye patchposition elative o the front (Table 1)

o

-5 -4 -3 -2 -1 o 1 2 3 4 5

Distance(krn)

Figure 3. Representative cross-shelf sections of dye

concentration cross he dye patch during the three surveys

showingBBL dye concentration s a functionof temperature

(top) and cross-shelfdistance bottom).



HOUGHTON: LAGRANGIAN FLOW AT THE FOOT OF A SHELFBREAK FRONT 2037

Table 1. Evolution of dye patch

Time Time Temp x T x Tz ho Tt u

1•- øC km øC/km øC/m m 10-6øC/s rn/s

Injection 0 8.95 0

TR3 28 8.7 - 1.0 0.21 1.2 7

TR4 67 8.15 -3.45 0.28 0.9 5

TR5 90 7.85 -4.5 0.41 0.7 3.5

-3.9 -0.017

-3.6 -0.013

Dye patch ime from njection, emperature,ndpositionelative o the ront positive ffshore) efined

by thepeakdye concentrationor the hreesurveys: R3, TR4, andTR5. Temperatureradients ndBBL

thickness,o, aremeasuredt he ocationf thedye oncentrationeak. emperaturehangend ross-shelf

velocityare derived rom changes etweensuccessiveurveys.

gradient, t the opof theBBL, Tz, decreases,nd he hickness

of the BBL, ho, decreases. hese trends ndicate that the dye

patchhad been njectedon the seaward ideof the convergence

zone in the front instead of the shoreward side as intended.

These observations re used to construct schematicdiagram

(Fig. 4) illustrating he evolutionof the dye patchshown n the

BBL. Although he diagram s two-dimensionalt representshe

cross-shelf otionof a waterparcel hat s flowingnearly20 km

alongshore o the west. Thus, there is an implicit three-

dimensionality o the circulation.From its injectionpoint near

9øC the centerof massof the dye patchmovesonshore owards

the frontalconvergence oint as t broadens.Within the BBL the

dye was well mixed vertically. Traces of dye appeared o

penetratento hestratifiedayerabove heBBL as hr as he7øC

lO 8 6 4 2 o •

cross - shelf Distance (k•)

Figure 4. A schematic iagramof the evolutionof the dye patch

in the BBL at the foot of the shellbreak front. The cross-shelf

temperatureprofile is a mean derived from all sections aken

during he cruiseand s used o infer cross-shelf osition elative

to the front from temperaturemeasurements.he positionof the

dye concentrationpeak (dots) and approximateone standard

deviation width of the patch for each survey are shown along

with the time elapsed from the dye injection. The vertical

gradient above the BBL at x=0 is from the dye injection

measurements.he top of the BBL (dashedine) is definedby the

height of the maximum vertical temperaturegradient.Arrows

qualitatively epresenthe flow although nly the onshorelow is

actually measuredby the dye patch displacement.sothermsare

hand drawn to connect the mean BBL temperature with the

vertical stratification at x=0 and to represent the cross-shelf

variation in the vertical gradient. Between TR3 and TR4 the

entire rontalboundarywas displaced 2 to 15 km onshore.

isotherm. Here the decrease n dye concentrationwas abrupt

suggesting convergence n the center of the stratified layer

above he BBL as ndicatedby the arrows n Fig. 4. The thickness

of the BBL defined by the height of the maximum gradient

region diminished onshore so the dye patch thickness was

reduced as it moved onshore. The arrows pointing onshore

indicate aterlowdef:inedy thedyepatchmotion hile he

other arrows are a more speculativerepresentationof inferred

flow in the BBL at the fbot of the frontalboundary.

Discussion

There are several aspectsof theseobservationswhich at first

appearcontradictory.n particular,onshore nd westward low in

the BBL is not consistentwith Ekman dynamics.This apparent

contradictions resolved n model calculations y Chapmanand

Lentz [ 1994] where they consider frontogenesis on the shelf

driven by the BBL advection of buoyancy from an inshore

source.The offshoremigration of the front is arrestedwhen n the

BBL offshore flow of buoyant Shelf Water convergeswith an

onshore flow underneath the frontal boundary. This BBL

convergence zone is on the shoreward side of the frontal

boundary. The thermal wind vertical shear in response o the

increased cross-shelf density gradient reverses he alongshore

flow at the foot of the front. However, when an alongshore

pressure gradient is added to simulate the observed mean

southwestwardlow along the northeast ontinentalmargin there

is no reversal n the alongshore low in the BBL under the front

(see heir Fig. 13). Although he model s applied o an uniformly

slopingshelf while the front is actually situatedat the shellbreak,

the presentobservations ubstantiate everal mportant eaturesof

this frontogenesismodel.

Since the dye-tagged water was tbllowed in a Lagrangian

sense,heobservedooling, T/dt 4x10 6 øC/s,mustbe the

result of diffusive mixing. From the cross-shelf preadingof the

dyepatch cross-shelfiffusivityf K x - 10m2/s s estimated.

The cross-shelf arianceof the dye patch ncreases pproximately

as time squared indicating a major contribution from shear

dispersion.From measuredcross-shelf emperature radientsan

upper oundor Txx s - 0.1xl0-6øC/m, hencehecross-shelf

diffusive eat luxKxTxx lx10-6øC/s. incehis s onlyone

forthof the rateof the observed ye patchcooling, heremustbe

significant vertical heat flux through the top of the BBL to

achieveheat balance within the dye patch. An estimateof the

vertical diflhsivity Kz across he highly stratified op of the BBL



2038 HOUGHTON: LAGRANGIAN FLOW AT THE FOOT OF A SHELFBREAK FRONT

isproblematical;oweveralue f Kz - 10 m2/s ields zTzz-

4x10-6øC/s, hich s approximatelyhe magnitudeequiredo

account for the cooling of the dye patch. Using the same

diffusivity results n a loss of approximately3 1 of dye which

when ubtractedrom he 16 1 nitially njectednto heBBL s

consistentwith the final dye inventory of 11.5 1. There is no

evidence hat double-diffusivemixing is a factor here. First, the

density atio,R = azXT/I3ASs approximately.4 andnot near

unity where doublediffusiveprocessesre more active.Second,

the T-S valuesof the water parcelevolve along he meanmixing

curve between Shelf and Slope Water properties with no

evidenceof a counterclockwise otation n T-S spaceas modeled

by Schmitt 1981 . When warranted y betterdyepatchsampling

in futureexperiments,hese lux calculations ill be repeated y

properly ntegrating ver he entirepatch ather hanusingmean

patch values.However, even thesecrude estimates mposean

upperboundon the diffusive lux through he highly stratified

boundary f the BBL near he foot of the front.

If to someextent the structuredepicted n Fig. 4 represents

steady-stateondition n the coastal egimewith no alongshore

variation, then both mass and heat balance must apply. Mass

balance s probablyachieved y an offshore low confined o the

stratified ayer at the top of the BBL. Houghtonet al. [1982]

estimate that during the summer the 'cold pool', shelf water

beneath he warm surfacemixed layer, warms at approximately

1øC/month. Approximating he cold pool as a wedge 100 km

wide and 60 m thick at the shelfbreak front this warming is

equivalento a heat luxof 4.9x10 W permeter f alongshelf

distance. When mixed to 6øC the onshore flow of 9øC water in

the BBL 6 m thick at the measured peedof 0.015 m/s represents

a flux of 1.1x106 W. Thus the heat flux associatedwith the

observed onshore flow in the BBL at the foot of the front could

contributesignificantly o the Shelf Water heat balanceand to

exchangeof Shelf and Slope Water propertiesvia diapycnal

mixing across he top of the BBL.

Conclusions

The results of this pilot cruise demonstratedhe utility of a

dye tracer o investigatemixing and circulationat the tbot of the

shelfbreak front in the Middle Atlantic Bight. It resolved water

displacementsf 3 to 4 km relative o the frontalboundary ver 3

days even when during the courseof the observation he front

was displaced12 to 15 km onshore. t has revealeda heretofore

undetectedonshore low responsible br convergencewithin the

frontal boundary. The reason that this onshore flow was not

detectedby mooredcurrentmeters s now clear. n the Chapman

and Lentz [ 1994] model the onshore low is weak and confined o

an approximately to 6 km interval at the tbot of the front while

onshore nd offshore not shown n Fig. 4) of this region he BBL

flow is offshore.Since the front undergoes ross-shelf xcursions

that exceed its width, any moored current meter will

predominantly ample he offshore low regime.

Details of the flow field suggestedn Fig. 4 requires urther

confirmation.A subsequent ruise s scheduledn 1997 nvolving

more controlleddye injection nto the BBL on both sidesof the

convergence one to refine the flow patterns nferred from this

pilot cruise.

These observations nd their apparentconfirmationof model

simulations by Chapman and Lentz [1994] indicate the

importanceof the BBL to both frontal dynamicsand to shelf-

slopeexchange rocesses.t is in the BBL at the foot of the front

where large T-S gradients mposedby the frontal boundaryand

turbulent nergyderived rom bottomboundary riction combine

to inducestrong'diapycnalmixing.

Acknowledgments. The success f this pilot cruise s due to

the skill and patienceof Captain Tyler and crew, especially he

winch operators, f the R/V ENDEAVOR. The dye injectionand

detection system was designed and constructed by Miguel

Maccio and Marcela Stern.Cheng Ho and JarvisBelinne assisted

in the data analysisand figure preparation.The technicaladvice

and encouragementby Jim Ledwell is especially appreciated.

This project is supportedby NSF grant OCE94-16074. Lamont

Doherty Earth ObservatorycontributionNo. 5693.

References

Aikman III, F., H.W. Ou, and R.W. Houghton,Currentvariability

across he New England Continental Shelf Break and Slope.

ContinentalShelfRes., 8, 625-651, 1988.

BeardsIcy,R.C., D.C. Chapman, K.H. Brink, S.R. Ramp and R.

Schlitz, The Nantucket Shoals Flux Experiment (NSFE 79).

Part I: A basic description of the current and temperature

variability.J. Phys.Oceanogr., 15, 713-748, 1985.

Butman,B., DownslopeEulerian mean flow associatedwith high

frequency current fluctuations observed on the outer

continental shelf and upper slope along the northeastern

United States continental margin: implications tbr sediment

transport.ContinentalShelfRes., 8, 811-840, 1988.

Chapman,D.C. and S.J. Lentz, Trappingof a coastaldensity ront

by the bottom boundary ayer. J. Phys. Oceanogr.,24, 1465-

1479, 1994.

Gawarkiewicz,G. and D.C. Chapman,The role of stratificationn

the tbrmation and maintenance f shelf break ronts. J. Phys.

Oceanogr.,22, 753-772, 1992.

Houghton, R.W., C.N. Flagg and L.J. Pietrafesa, Shelf slope

water frontal structure, motion, and eddy heat flux in the

Southern Middle Atlantic Bight. Deep Sea Res. part II:

Topical Studiesn Oceanography, ontinental helfRes..,41,

273-306, 1994.

Houghton,R.W., R.Schlitz, R.C. BeardsIcy,B. Butman and J.L.

Chamberlin,The Middle-Atlantic Bight cold pool: evolution

of the temperaturestructureduring summer 1979. J. Phys.

Oceanogr., 12, 1019-1029, 1982.

Schmitt, R. W., Form of the temperature-salinity elationship n

the Central Water: evidence for double diffusive mixing. J.

Phys.Oceanogr., 11, 1015-1026, 1981.

R. W. Houghton, Lamont Doherty Earth Observatory of

Columbia University, Palisades, NY 10964 (e-mail:

houghton@1deo. o umbia.edu)

(ReceivedApril 28, 1997; revisedJune 19, 1997;

accepted uly 8, 1997.)

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