Upper-ocean transport mechanisms from the Gulf of Maine to Georges Bank, with implications for Calanus supply

Pergamon Contim, ntalShe(fResearch, Vol. 17, No. 15, pp. 1887 191 I. 1998 ~i 1998 Elsevier Science Lid

All rights reserved. Printed in Great Britain PII: S0278"4343(97)00048--4 0278 4343/98 519.00+0.00

Upper-ocean transport mechanisms from the Gulf of Maine to Georges Bank, with implications for Calanus supply

C. G. HANNAH,t* C. E. NAIMIE,:~ J. W. LODERt and F. E. WERNER¶

(Received 24 July 1996; accepted 9 January 1997)

Abstract--Potential upper-ocean pathways for the supply of biota from the Gulf of Maine to Georges Bank are investigated by numerically tracking particles in realistic 3-d seasonal-mean and tidal flow fields. The flow fields, obtained from a prognostic model forced by observed M2 tides and seasonal- mean wind stress and density fields, include the major known observational features of the circulation regime in winter, spring and summer - - a wind-driven surface layer (in winter and early spring) overlying seasonally-evolving baroclinic and tidally-rectified topographic gyres. The surface layer in winter and early spring, with generally southward drift for typical northwesterly wind stress, can act as a conveyor belt for the transport of biota to Georges Bank, provided that the biota can spend a substantial fraction of time in the surface Ekman layer. The numerical experiments indicate that the upper-ocean drift pathways for biota in the southern Gulf of Maine are strongly sensitive to biological and/or physical processes affecting vertical position in relation to the surface Ekman layer and horizontal position in relation to topographic gyres. The seasonality and location of the identified pathways are generally consistent with observed distributional patterns of Calanusfimnarchicus based on the I 1-year MARMAP surveys. © 1998 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION

The copepod Calanus finmarchicus is the dominant contributor to annual zooplankton biomass in the Gulf of Maine (e.g. Bigelow, 1926; Davis, 1987) and on the Scotian Shelf(e.g. Sameoto and Herman, 1990), and an important potential prey source for early life stages of fish on Georges Bank (e.g. Sherman et al., 1987). A recent description of the seasonal variation in Calanusfinmarchicus abundance in the Gulf of Maine-Georges Bank region (Meise and O'Reilly, 1996), based on the 1977-87 MARMAP surveys (Sherman, 1980), indicates that there are spatial differences in the seasonal cycle that correspond to differences in hydrodynamic structure and which suggest particular supply pathways to Georges Bank. Specifically, there are high concentrations of adults and late-stage copepodites year-round in the Gulf, and lower concentrations with seasonally-varying patterns on Georges Bank. Over the central bank, concentrations are relatively low year-round, with a weak seasonal maximum in March/April, suggesting limited supply peaking in late winter. In contrast, the stratified Northeast Peak and southern flank of the bank have higher concentrations and a

* Author to whom correspondence should be addressed. t Fisheries and Oceans Canada, Bedford Institute of Oceanography, Dartmouth, NS, B2Y 4A2, Canada

Dartmouth College, Hanover, NH, 03755-8000, USA ¶ Marine Sciences Program, UNC, Chapel Hill, NC, 27599-3300, USA

1887

1888 c.G. Hannah et al.

maximum in May/June, suggesting increased supply peaking in late spring. Meise and O'Reilly (1996) point out that these distributional patterns are qualitatively consistent with known features of the hydrodynamic regime, such as wind-driven surface flow from the Gulf of Maine to Georges Bank in winter, inflow from the western Gulf to the seasonally-varying Georges Bank gyre, and a seasonal tidal front surrounding the central bank. They also note indications of a supply pathway from the Scotian Shelf to the bank's southern flank, but the data coverage of this alternative source is limited.

The physical oceanography of the Georges Bank region (Fig. 1) has been extensively studied, going back to the early part of this century (see Backus and Bourne, 1987 for a broad review, and Wiebe and Beardsley, 1996 for some recent advances). However, a bank- wide quantitative representation of the hydrography and circulation with realistic resolution of primary features such as the gyre, fronts and vertical structure has not been available. Through the US GLOBEC Georges Bank/Northwest Atlantic program, this has changed with the development of a progression of three-dimensional (3-d) numerical circulation models with increasingly sophisticated physics on realistic geometry (Lynch et

al. , 1992; Lynch and Naimie, 1993; Naimie et al. , 1994; Naimie, 1996; Lynch et al. , 1996). The recent availability of climatological bimonthly flow fields for the Georges Bank and Gulf of Maine region, based on observ ltional forcing data and prognostic model dynamics

Fig. 1. Major bathymetric features of the Gulf of Maine and Georges Bank region (39-44.5°N, 65- 71°W). The acronyms are GSC for Great South Channel; NEP for Northeast Peak; and NF and SF for the Northern and Southern Flanks of Georges Bank. The position of the velocity profile in Fig. 9

is indicated by the x.

Upper-ocean transport mechanisms from the Gulf of Maine to Georges Bank 1889

with advanced turbulence closure (e.g. Naimie, 1996), provides the opportunity to evaluate quantitatively issues regarding supply pathways and other physical oceanographic influences on organisms such as Calanusfinmarchicus.

In this paper we provide a first exploration of the importance of different flow features and pathways to the supply of Calanusfinmarchicus to Georges Bank. Our approach is to simulate individual organisms, through the calculation of particle trajectories in the Naimie (1995b, 1996) seasonal-mean and tidal circulation fields. The choice of seasons and initial release positions is based on the contemporary paradigm of the Calanusfinmarchicus life cycle in the Gulf of Maine and Scotian Shelf region (e.g. Davis, 1987; Sameoto and Herman, 1990; Meise and O'Reilly, 1996; Miller, 1995) - - in particular, that adults and late-stage copepodites ascend from the bottoms of the deep basins in winter and early spring (where they have spent the summer and fall in a diapause phase) and hence become susceptible to advection by currents near the surface. Thus, seasonal and other variations in the Gulf's near-surface circulation may be critical in determining whether various stages of Calanus finmarchicus are advected to fish spawning and nursery areas such as Georges Bank. Consequently, we focus our release positions on the deep Gulf (with depths greater than 100 m) including the northern Great South Channel region where large springtime concentrations of Calanus finmarchicus have been recently documented (Wishner et al., 1995). While a wide range of vertical distributions and migration behaviour has been reported for Calanusfinmarchieus in the Gulf of Maine region (e.g. Clarke, 1933, 1934; Durbin et al., 1995a,b), we use simplified representations of vertical movement in order to focus on the influences of the flow fields, as an initial step towards more realistic biophysical models.

In Section 2, we briefly describe the circulation model and the strategies used in the particle-tracking simulations. The results are summarized in Section 3, starting with the Eulerian circulation patterns as a context for interpreting the influence of particular flow features on the particle trajectories. The relationships among the trajectories, the major flow features and the physical processes governing the flow fields are examined in Section 4, together with the sensitivity of the results to unmodelled flow components and biological behaviour. The conclusions and implications for the major distributional features described by Meise and O'Reilly (1996) are summarized in Section 5.

2. METHODS

Our study is based on 3-d velocity fields representing the seasonal circulation for four bimonthly seasons: January/February, March/April, May/June, July/August. The fields provide realistic estimates of the mean and M2 tidal velocity fields for the Gulf of Maine and Georges Bank region; subject to forcing from the regionally-dominant M2 tide, mean wind stress, and baroclinic and barotropic pressure gradients. The calculation method and aspects of the resulting flow fields have been described by Lynch et al. (1996) and Naimie (1995a,b, 1996). Here we give a brief description of the circulation model and the particle- tracking methodology.

2.1. Circulation model

The velocity fields were computed using the prognostic Dartmouth Circulation Model (Lynch et aL, 1996), which uses a Galerkin finite element method with nodal quadrature to


solve the 3-d shallow water equations under Boussinesq and hydrostatic assumptions. The version used here was prognostic in density and employed a level 2.5 turbulence closure scheme to parameterize vertical mixing (Mellor and Yamada, 1982; Galperin et al., 1988; Blumberg et al., 1992). Variable grid spacing was employed in all spatial dimensions, providing vertical resolution of order 1 m in the surface and bottom boundary layers and horizontal resolution of order 0.5 km over the steep flanks of Georges Bank. Throughout the simulation, the 3-d mesh remained fixed in horizontal extent but adjusted vertically to track the movement of the free surface. All aspects of the numerical simulations, including the time-step of approximately 45 s, were identical to those reported by Naimie (1996).

For each season, the initial density field was derived from the historical temperature and salinity data base maintained at the Bedford Institute of Oceanography (see Naimie et al.,

1994, hereafter NLL94, and Hannah et al., 1996 for the estimation procedure). The initial conditions for velocity were taken from the corresponding NLL94 diagnostic (fixed density) bimonthly solutions. Each solution used a constant wind stress taken from the estimates of NLL94 (Table 1). The other boundary conditions are as described by Naimie (1995a,b, 1996) and NLL94.

The duration of the prognostic model simulations was 8 M2 tidal periods, which Naimie (1996) found to be sufficient for adjustment of the hydrodynamic variables to dynamic equilibrium. The short integration time means that the baroclinic forcing is dominated by the initial conditions rather than the surface and lateral boundary conditions. The 3-d structure of the mean and M2 variations of the hydrodynamic variables was extracted from the last tidal period for use in the particle tracking.

The near-surface response to wind stress is controlled by the vertical eddy viscosity (from the turbulence closure) and Ekman dynamics, with no explicit representation of a surface shear layer such as described by Csanady (1984). With the inclusion of only the seasonal- mean component of wind-stress (i.e. contributions of storm-band wind stress fluctuations to circulation and turbulence are not included), the depth of the surface Ekman layer is generally underestimated. Thus, we focus the interpretation of the particle-tracking experiments on vertical position relative to the Ekman layer, rather than absolute vertical position.

2.2. Part ic le t rack ing

The particles were tracked in the seasonal-mean and M2 velocity fields, with no interpolation between seasons. The particle-tracking methodology is described by Werner

Table 1. Bi-monthly mean wind stress for a two-degree square over Georges Bank, estimated from the Comprehensive Ocean-Atmosphere Data Set ( Woodruff et al., 1987). The wind stress direction is measured

clockwise from true north

Season Magnitude (Pa) Direction towards (degrees)

January/February 0.096 119 March/April 0.047 121 May/June 0.014 49 July/August 0.014 51


et al. (1993, 1995) and Blanton (1995) with small extensions to the vertical and horizontal displacements as described below.

Observations from the Gulf of Maine indicate that Calanus finmarchicus can undergo substantial vertical migration after their emergence from diapause at depth in the Gull's basins (e.g. Clarke, 1933; Durbin et al., 1995a,b). However, there are differences among the patterns observed in different studies, apparently related to variations in prey, predators (Bollens and Frost, 1989) and the physical environment. To explore the influences of different flow features and vertical behaviour assumptions, we consider five different representations of the vertical movement:

(1) Zero vertical velocity, with the particle depths fixed at prescribed values chosen as 5 and 25 m. This case can be viewed as a simplification to explore the horizontal velocity structure at different depths, or as an extreme degree of biological control with behaviour dominating physical influences.

(2) A diel migration pattern between the 5-m level and the 25- or 150-m level (or the bottom), with particle ascent and descent at a speed of 1 cm s - J and equal periods at each level. In this case, biological influences are again assumed to be dominant.

(3) The particles move passively with the mean and/or M2 components of vertical velocity taken from the hydrodynamic model. This is the extreme case of no biological behaviour.

(4) A vertical random walk or "turbulent kicks" based on the model's turbulence fields, which can be added to the zero vertical velocity and passive scenarios. We use this to explore the sensitivity to simulated turbulent mixing. When used with the vertical velocity field from the model, this represents the most realistic representation of physical influences without biological behaviour.

(5) An additional "behavioural" component together with the vertical velocity from the model and the turbulent kicks. In particular, we explore the sensitivity of selected results to a surface-seeking behaviour (additional upward movement).

2.2.1. Vertical random walk. To include the effect of turbulent kicks, we follow the approach described by Legg and Raupach (1982) wherein the Langevin equation is used to derive a Markov equation for the vertical velocity (w') of a particle in inhomogeneous turbulence:

dw' - w'l~j + , ~ ( t ) + F (1)

dt

Here z~ is the Lagrangian integral time scale (or auto-correlation time scale) estimated from Nq =aw2Zl, where Nq is the turbulent exchange coefficient (see Galperin et al., 1988), aw 2 (= 0.3q2/2) is the Lagrangian velocity variance and q2/2 is the turbulent kinetic energy; 2 = a,,. 2 ~ ; ~(t) is Gaussian noise of zero mean and unit variance; and F = O(aw2)/Oz is a term involving the gradient in the turbulent velocity variance.

The Markov chain for wn + t the turbulent vertical velocity at time step n + 1, becomes ! !

Wn+ 1 = anW n q- bnawn~n + Cn (2)

where an = exp( - At/zl ,) , bn = [1 - exp( - 2At/~l,)] 1/2 and Cn = Fzln[l -- exp( -- At/zan)], and At < zl is required. In the simulations that include the turbulent vertical velocity component we set the time step At = 1 min. The dispersal of particles in inhomogeneous turbulent fields

1892 C.G. Hannah et al.

can lead to aggregations that are not realistic if the dispersal process is not treated in the above manner (e.g. Legg and Raupach, 1982; Thomson, 1987; Holloway, 1994).

2.2.2. H o r i z o n t a l r a n d o m w a l k . Estimates ofthe influence ofunmodeled horizontal motions were obtained assuming a random walk process where additional displacements (fix, 6y) were obtained from:

= 6y = ( 3 )

Here ~ is a random deviate from a Gaussian distribution of zero mean and unit variance, and K~, K~" are constant (externally specified) eddy diffusivities in the x, y directions respectively (e.g. Berg, 1993).

3. RESULTS

3.1. M o d e l f low. f i e lds

The spatial structure of the (upper-ocean) bimonthly mean velocity fields is illustrated in Figs 2 and 3, using the conventional Eulerian perspective. In all four periods there is strong anticyclonic flow around the edge of most of Georges Bank, and a weak cyclonic circulation pattern with topographic-scale structure in the Gulf of Maine. In January/February (Fig. 2), there are similarities in the velocity patterns at 5 and 25 m, particularly in the anticyclonic gyre-like flow over Georges Bank. However, there are differences in the patterns such as a broad southward flow of 5-10 cm s- i over the central Gulf at 5 m, in contrast to the weaker and more variable flow at 25 m. This vertical structure is qualitatively consistent with observational descriptions of an offshore near-surface drift in the region (e.g. Bumpus and Lauzier, 1965; Flagg et al., 1982) associated with the relatively-strong northwesterly mean wind stress in winter. The differences between the 5- and 25-m velocity patterns are smaller in the March/April fields (Fig. 2) and largely disappear in the May/June and July/August fields (Fig. 3, the 25-m velocity fields are not shown). This is associated with the intermediate wind stress magnitude in March/April and the weak wind stress and shallow Ekman layer in May/June and July/August (see Section 4.2 for further discussion of the Ekman layer structure). The spatial patterns of the 100-m and 150-m velocity fields are similar to the 25-m patterns but the magnitudes are much smaller.

The Georges Bank circulation in these and precursor diagnostic solutions has been compared with moored current measurements and analyzed for major forcing mechanisms, and found to be generally consistent with present-day knowledge and understanding of a seasonally-varying gyre driven by tidal rectification and the baroclinic pressure field (Naimie et al., 1994; Naimie, 1995a,b). Below the surface Ekman layer, the Gulf of Maine circulation in these fields is mainly baroclinic, generally comprising cyclonic gyres over the Gulf's three primary basins - - Georges, Jordan and Wilkinson - - embedded in the overall cyclonic flow (Hannah et al., 1996; Lynch et al., 1996). This structure has many qualitative similarities to the available observational descriptions (e.g. Brooks, 1985; Brown and Irish, 1992), but its representativeness of the oceanic seasonal-mean flow is less clear than for the Georges Bank circulation. Therefore the present solutions should be viewed as plausible and dynamically consistent candidates for, rather than precise representations of, the Gulf's circulation.

Upper-ocean t ranspor t mechanisms from the Gul f of Maine to Georges Bank 1893

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , . . . 3 , • • - 7 ] Jan/Feb 25 m ~ - ~ . ~ , ~ ' . ~ .

~ _ , . . . ~ ' . : . . . . ~ r ' ,

~ 7 ~. ' . ' , : . ' ~ ..~7.':.'~ "* 'K ' ' " ", . ' " "~ , ",',~

- - - k?' K'-" "C .- 'V" " . ~ t ~ , ' + - . ~ "~' ~,~ ~"~ ",' , : V . ' - V. ~ ; , ~ ~.

- - > . . ~'.~ ; ~ - , , ~ s e m / s l : ' : ; r ~ ~ '

Mar/Apr 25 rn ~2;.,

b_zZ - .z . . . . ,: , : ' j~ ,

-d. -° -%-".," ~'~¢,".,ie~,.

1,1 ". ~ ' J " ~ ~ "

, . . , ; , . . . . . ~ , ~ .

- ,

~ ' ~ . , : :.-. - ;

- -> I:"-".' z5 crrv,, I: • ::t

. + ° o i ~,

~ , .

Fig. 2. Mean velocity field at 5 m and 25 m for January /February and March/Apri l with 50-m, 100- m and 200-m isobaths. The velocity vectors have been extensively thinned for presentation. Border

ticks are at 100-km intervals.

,,'~".~.+,. " "+L ';.'~-,~-:,~-~ il. io.-j".,..+.'..,".- ~,. .~r~,,, ,xa~

" ~ " ~ / . " ~ ~ ' ~ " ,,~ • o

".-3~'.~..".., , ~ - ° L . ~ ; , , ~ ' - ~ , a ~

~ ¢ ~ . , ¢ . " - Z. .~,'~-# / . , . ] [ d n

/ d • ,pr~ '" ~ ]

m

Sr ; ' ; . ~ - - - ~ . " : ~."'~ ". '\ ." . '" "~ +' , . . ¢ . ~ , , , + ~ + , . ~ , , • . . 2 ; . # o % , k /( o ,~ " . / g . , t + . d ° " ~" - .J" " k l

i , , +I++o'o. ¢_j.." .~'A~....." - g l ~

," i"..I' ~%. + ~ ~ .~ ."- "~".,'- . ,~t i. ~"'~ ' ~,±1. , ,a , . 4 . . ~ ¢ ' . . ~ , , ~ ~ ~ • .~ "'""'-g, ' ~ : , ' ~ . ' : 4 ~ +, " " ' . a F~,;/.~

~ ~./~.~,~ ..d..? .~ . ' ~a"~i' "~.'1[._~ ~ • ~ 3 + . = ~ , .~,, ' , . ~ . . - . ~

Fig. 3. Velocity field at 5 m for May/June and July/August; otherwise, same as Fig. 2.


~t) Jan /Feb .. ~ " .Y".. / ¢ ' - , ~ / .." : : \

,, - . - .... , . . /

~ ...... .~- -..

• .... ..

/ . . '

I ] . . ;

I

t )) Mar/Apr" J..-"" ...... ",I ) i ' ,~ - , . . : ( _

- -,_

c) May /Jun . . ~ - ' ....... " I ~ l ~r --,¢ / ." ~ \ '~I

1 1 • . ~ I

, i . •

Fig. 4. Particle trajectories (solid lines) at 5 m: (a) January/February; (b) March/April; (c) May/ June; (d) July/August. The trajectories have been detided by sampling at tidal cycle intervals. Initial positions are marked with O, tracking period is 120 days, and standard isobaths (50, 100, 200 m) are

in the background.

3.2. N e a r - s u r f a c e r e l e a s e s

The broad-scale features of the drift patterns associated with the near-surface flows are illustrated in Fig. 4 using particles seeded along two line segments which cut across the major inflows to the central and southern Gulf. The particles are fixed at 5 m depth, which is within the (model) surface Ekman layer in the Gulf in January/February and March/April , but below it in May/June and July/August. In the first three seasons, the drifter trajectories indicate throughflow across most of the domain of interest. In July/August the throughflow is weak and no particles reach the 100-m isobath along the northern flank of Georges Bank (where an additional particle is released to illustrate the summer gyre).

The January/February release (Fig. 4a) is dominated by the generally southward drift in the Gulf which moves the particles into position to be advected onto Georges Bank. The supply pathway is a combination of the surface Ekman layer, the cyclonic circulation around Georges Basin and a baroclinic flow feature extending from Wilkinson Basin to western Georges Bank. The influence of the jet-like mean flow along the northern flank of Georges Bank can be seen in the eastward turn in the particle trajectories as they cross the


flank onto the bank. Once on the bank, the particles move southward and onto the outer southern flank.

In March/April (Fig. 4b) the winds are weaker and the northern flank jet is stronger than in January/February, but there is still an on-bank flow component at 5 m which feeds particles onto the bank. The weaker winds lead to smaller velocities on the bank and the particles move slowly across the bank. The most direct supply pathway is the cyclonic circulation around Georges Basin which feeds particles onto the northern flank. There is also a slow pathway westward across the central Gulf and then southward off Cape Cod, but most of the particles exit the Gulf across Nantucket Shoals. These features together with the horizontal shear and convergences in the 5-m flow field, are illustrated by snapshots (Fig. 5) from a Gulf-wide release.

In May/June (Fig. 4c) the wind is weak, the (modelled) Ekman layer is shallow and there is little direct wind influence at 5 m. The flow across the Gulf is much slower and more localized than in January/February with three primary pathways: a coastal current which exits the Gulf over Nantucket Shoals; a central Gulf current which describes an "S" shape as

.... j . . .

, / o ' - o - ' o ~ o * - * : ~ * ~, g ' : o o o o ~ / - d o o o o o o {, * ~ ' * * * * ' ~ ' ~ o o /

~ o o o oo o o o **~ * **.~- UI~oa" o o o o o o o o o * * i ~ . * * ~ :~ ; . . a ' 6 o d o o o o o o o o o ~* , {* ~ "& "~ .~ .n .o a a ~ - _ - 0 O' .q 0 0 0 0 0 0 • • • • e ,~ • O~o OOO ~ 00 "000000 • m • • 4 • • 0 ~O O O f - - "~

o o o ~ . . . . . ~ . . . : , . o ~ o o \

c~ 0 o o o o d ' o "e . . : * - ~'.~* * ° a a o o o " " . . \ - l ~ * . O O 0 0 . . o . o ' * o e e e . e r ' e s e O 0 0 0 O O "7.

,~ & o o.~ o o o • . . £ . . , r ~ ' : ~ " i i ~ 3 ; @ ' ~ " . . . . " \o"~ o "~ o o o • ¢ *.-'~.~_..---~ - \ ":

~ 1 ~.o o o o o o * /< - . - - ; ' . : ~oooo/( } I : .

_ ' \oooo/~, ~ , / . . . : -

o o ~ i " ¢." "~ ~,o / / ...} ~"

(..j.. - ~ <..."

i ~ ~ - ~.,q.'.."" I

~. . . . . . ~ ..... ~ - V ... ~ o'"'0: ......... . ....

, . ; ) o ;.....'-

I o o " ~ - ~,, ' .~"':" .... I

, / v ..: - - ~ a". / I , " o o~ . . . . ., . . - , " e . p ~ , ~ ~ 4

- o 8 .: . " . " ' : ~ .~ '" . . . - " - - -I • 0.0% 0 0 el eo ~Q3.." O ",

'Lo" " . . ~ o o o o , . ~ . . . , , . a . O : r---- ,J

o .--'-...-,I ~ . . o o .~......-. ~. s¢ ~ '"..-1 ~'. ,.~. ~ .:~--'...~.-.~.'~ .......... .,

"'.~*", • • o ~ m'.T ..*." .~. n = a ~ r "- -- ..

.~-o o o~=r~ / - ~ ) ~.. . . . . o o ~ "~ / . .... i~"

o ~ w I _ / " -<.*'

/ " \ \ ~ . /

• : J" ~'"'i~: ~ : . . . . . . . . . : , . . . * ' / -

- ) i "'"-, d" r - - - , . I t • , ,'" • "

~,,.=... . . '..,e.~o ..:[~ ....... ? , e~ \ . o ..a....~ o o...": . . . . . . . . . . . ~ . . . . . . ...

" a .v-. ~ " g . - " ' ~ - - ~ "..

~, '~. . . o.. ~ =. . . . , ~ - \ . . .

~-..~ " , \ ~ / | . * * ' , t o ~ ~ .... ~,~/ ., Do ~..

0 ~_ ~.I" D 0 .... /" o oO~o ~ ,,c...<.

I I ~ -- ~,...I..-" I

Fig. 5. Snapsho t s o f particle positions for a 5om fixed depth release in the March/April flow field at 0, 15.5, 31 and 45.5 days: The initial grid contains 316 particles on a 15-km grid which fills most of the Gulf with depth greater than 100 m. Different symbols are used to indicate different longitude bands

in release position, and standard (Fig. 4) isobaths are included.


it flows from Jordan Basin to Wilkinson Basin and then clockwise around Georges Bank; and a cyclonic circulation around Georges Basin. No particles reach the central bank, but several reach the northern flank, Northeast Peak and southern flank.

In July/August (Fig. 4d) the Gulf flows are again dominated by baroclinic structure, but the pattern differs from that in May/June. There are no large-scale flows moving particles across the western and central Gulf to Georges Bank, except to the bank's eastern end. The gyre around the bank has intensified and, at 5 m, most of the bank is isolated from the Gulf.

Estimates of the transit time from different locations in the Gulf to Georges Bank are summarized in Fig. 6, using the release grid shown in Fig. 5 and particles fixed at 5 m depth. In January/February most of the particles from the southern and north-central Gulf reach the bank in 50 days or less. In March/April and May/June the region inside the 50-day isochron shrinks to a corridor across Georges Basin to Jordan Basin (plus a supply region in the southwestern Gulf in May/June). In July/August the region shrinks further to the western end of Georges Basin and the near field along the northern flank.

A measure of the supply of particles to the bank is the number reaching the bank as a function of time (Fig. 7). Using the (fixed-depth) particle trajectories and bank definition from the transit time calculations, we compute the cumulative number of particles that reach the bank (Fig. 7a; Table 2). We also compute the instantaneous number on the "outer bank" (the portion deeper than 50 m; Fig. 7b), and the instantaneous number on the "central bank" (the portion shallower than 50 m; Fig. 7c). The number of 5-m particles which reach the bank is greatest in January/February and lowest in July/August. The

................................ ~ r ~ / L ~

J a n / F e b . - . ~ .."'-.] , -,, , " ( ..... ', ..-- / / ~ ...... s

/ .: ." .:" f

• "".. r ....'~ ." " ~ ' ~ ~ ! ' . ...... .:.-. ..... . , . , , _ " • ~ - ) ....: .... "..,._

- - ~ l t / \ ; l . . . . • i i : ...

"~ I ~J //I ) ~." . *" i

M a r / A p r .- -" ...... . ~ - , , _ _ . - :..-' t. ' , ~ 1 7 - " / ~ . J k' i Jr ...... \ i ~ " . - - . . ' - q - I t ! ........ , ('".. :f:,,,. . . . . . . ....-;7.--.

t - , ! ": " " : ~ " $ / ~ ~ 1 ~ " . ", ..... : r .'m I ,r . . . . . . . . . . . .

...... I ~ " , , in,</- I /." L ~ ", i x , ~ /...:

"~ ~ i \ , / ' l ) IJ .J..':]

i~ M a y / d u n . - --"; , Ic~

f ~ . . - " _, . "'"": /

/~ ~ ........ ~ . . " " ~ , , t ~J . . - - , I : "~. ". f ". ~, " .~ .........

[~ Jul/Aug ........................... '"~ . . . . . , " ..I ~ ' l ~ / . . " " I' •

' " ":" " L -1 ... : ....... 7:1.1::::i. - - - /

\~ "......... . - /z \ ',

I '~p x, I / I ) i; ~."i /

Fig. 6. Isochrons of transit time to Georges Bank for particles fixed at 5 m depth and released on the grid in Fig. 5. The isochrons are 50 days (solid) and 25 days (dashed). The "bank" was defined as that portion of Georges Bank shoalward of the 90-m isobath and east of the western extent of the 60-m isobath thereby excluding the center of Great South Channel. Standard isobaths are shown.


Fig. 7.

200

150 . .E l

E 100 Z

50

100

E = 50 Z

100

. .O

E

- ~Cumula[ive nurr{ber reaching th$ bank i i ~ JanFeb

. . . . . . . . . . . . . ........... ~ ~ . • ~A ,-,vu,nu~

t a) I I I I I I I

_ Number on outer bank

~ JanFeb - J . _.~.,~ ~ ~.~....._... MayJun ~ . ~ Mai'Apr

.... ...~".7 ~" J u l A q g ' " " " ~-'" " b)

I I I I I I I Number on central bank

= 50 ~ JanEeb Z ~ .................................................... MarApr

. , . . . . . . _ , . . , . , , , . . . . . . . . . . . .

0 ~ ~ l I I L f j C) 0 10 20 30 40 50 60 70 80

Time (days)

Number of particles on Georges Bank in the 5-m fixed-depth experiments. The total number of particles released was 316.

"central b a n k " is isolated in May/June and July/August , and supply to the "ou te r b a n k " is also largely cut off in July/August . In January /February , the number on the bank starts to decrease after 40 days because there are no particles left in the Gul f to supply the bank and particles on the bank cont inue to leave.

When the (5-m release) particles are allowed to move passively with the horizontal and vertical velocity components , the supply pathways and the numbers that reach the bank do

Table 2. Cumulative numbers of particles reaching Georges Bank after 60 days.for the release grid shown in Fig. 5. The total number of particles is 316

Cumulative number

Release depth (m) Verticalscheme Jan/Feb Mar/Apr May/Jun Jul/Aug

5 Fixed 195 94 99 46 5 Passive 188 75 106 75 25 Fixed 16 19 77 20 25 Passive 46 21 44 18 25 Mean w only 48 - - 52 - - 25 M2 w only 19 - - 41 - - 100 Passive 19 11 11 14

5/25 Diel migr. 135 59 96 25 5/150 Diel migr. 119 49 35 27


not change very much (Table 2). The largest changes are in July/August when the cumulative number reaching the bank increases by 60% and some particles even reach the central bank (not shown). The latter particles are upwelled into the upper few metres along the northern flank and then advected onto the bank in a thin Ekman layer (Section 4.2).

The seasonal and spatial variations in the number of particles on the bank in these simulations show qualitative similarities to the bimonthly time series of Calanus finmarchicus abundance on Georges Bank derived by Meise and O'Reilly (1996, their Fig. 6) from the MARMAP data. On the central bank (or Mixed GB) they showed that the abundance increases rapidly in January/February, peaks in March/April and then declines in May/June. On the outer bank (combination of their Tidal Front GB and Stratified GB) the abundance increases in January/February, is constant or slightly increases in March/ April and May/June, and decreases in July/August. The fact that our results show a peak supply in the January/February flow field is consistent with the observed peak abundance on Georges Bank in March/April since it takes of order 1-2 months for the model particles to reach the bank (depending on starting location and vertical distribution), However, biological factors such as reproduction and predation are certainly important to the observed seasonal variation in abundance.

The flow fields have near-surface pathways from the northern and eastern Gulf to the southwestern Gulf where the SCOPEX study (along the 100-m isobath between Cape Cod and Georges Bank) investigated the feeding of right whales on high concentrations of Calanus.finmarchicus and other copepods (Kenney and Wishner, 1995). The trajectories suggest a seasonal transition whereby the near-surface Gulf flows provide pathways from the northern Gulf to Georges Bank in January/February and to the western and southern Gulf in May/June. The transit time from the release line in Fig. 4 to the 100-m isobath between Cape Cod and Georges Bank was 40-50 days in March/April and 25-50 days in May/June. These results support the assumption of Durbin et al. (1995b) that the northern and eastern Gulf populations were the "...source of Calanus observed in the southern GOM.. . "

3.3. Deeper releases

To examine the supply pathways below the surface Ekman layer, particles were released at a depth of 25 m on the release grid in Fig. 5. In the fixed depth cases only a small number (<20) of particles reached the bank, except in May/June (Table 2). The number reaching the bank was generally much lower than in the 5-m releases because the Gulf-wide circulation at 25 m (and deeper) did not deliver many particles to the northern edge of the bank and the mean velocities there are generally parallel to the isobaths. The particles which reached the bank were those which started very close to the bank (e.g. Fig. 8a~l). The greater number reaching the bank in May/June was due to the gyral circulation moving onto the Northeast Peak and southern flank.

In the 25-m passive cases the number of particles reaching the bank increased substantially in January/February (compared to the fixed-depth case), decreased in May/ June and stayed roughly the same in the other two seasons (Table 2). In all seasons, the particles reaching the bank upwelled substantially. The mean depths of the particles as they reached the bank were 8, 5, 15, and 8 m respectively, and the mean vertical velocities of these particles during their transit to the bank were of order 1 m day- 1. The increase in the number reaching the bank in January/February was due to the particles being upwelled


a) Jan/Feb 25 m fixed '~L~'.. L

: "".: " . . : ..... . . . ~

-~ " .. '.7.

.;.S b) Mar/Apr 25 rn fixed " ~ \

~ ..... :-.:. . ..... .--. .

":'~\t"~Jl# f / ? " i / :"

y ,/.. , . . / .... .K." . . j . . . / . . .

I I ~ - - ~. . / .~. .~. ...... I

c) May/Jun 25 rn f ixed" i ... =

\ \ t : i

d) Jul/Aug 25 rn fixed . .-'J ~ ~.,~, \

\ ".... • °

\

e) Mar/Apr 25 m passive ~ , i ~ " . . . . . . Y.. "~ ' ~ " "

. . . . . . :::i;;

f) May/Jun 25 m passive ".. i ... "... . . . . . . ".: \ ! '-. .; - : . . . . . . . . . . "-.\ ',

Fig. 8. Example particle trajectories for 25-m releases from the positions marked by O: (a) January/February, fixed depth; (b) March/April, fixed depth; (c) May/June, fixed depth; (d) July/ August, fixed depth; (e) March/April , passive; (t) May/June, passive• The trajectories have been

detided by sampling at tidal cycle intervals. The standard isobaths are included.

into the surface Ekman layer and carried onto the bank by the pathways seen at 5 m (Fig. 4a).

In all four seasons there was greater movement of particles onto the Northeast Peak, with greater subsequent supply of particles to the southern flank, in the passive case than the fixed case. This is illustrated by the particle trajectories for March/April and May/June (Fig. 8e,f) and arises from the upwelling together with greater on-bank flow higher in the water column (compare Figs 4 and 8). This may be an important supply mechanism to the "Stratified GB" region identified by Meise and O'Reilly (1996 their Fig. 5b) which has a similar spatial pattern to the drifter paths on the bank in Fig. 8e,f.

In the 25-m passive March/April case the cumulative number of particles on the bank grows slowly and after 60 days is roughly equal to the instantaneous number on the bank. This indicates that the particles do not leave the bank within 30-60 days, in contrast to the other seasons. In July/August the instantaneous number on the bank remains small but the


cumulative total is similar to that in March/April which generally has more particles on the bank. This is because the particles move much more quickly around the edges of the bank in July/August (when the recirculating gyre is nearing its maximum) and several particles come up onto the eastern end of the bank and then quickly move off(Fig. 8d). The numbers on the "central bank" are always less than 7 in the 25-m release cases, which together with the near- surface release results, suggests that the primary supply route to the "central bank" is the surface Ekman layer.

The results for the 100-m passive releases (Table 2) show that the deep flows do not deliver large numbers of particles to Georges Bank. Note that our definition of the bank (depths less than 90 m) means that 100-m fixed depth particles cannot reach the bank.

4. INTERPRETATION AND SENSITIVITY

4.1. T idal ly -rec t i f ied and baroclinic gyres

The cyclonic gyres in the Gulf of Maine and the anticyclonic Georges Bank gyre are important factors to regional transport. Variability in the strength and interaction of these gyres arises from a number of factors, including seasonal changes in the density field and inflows from the Scotian Shelf (Hannah et al., 1996), tidal rectification (Naimie et al., 1994), and the direct influence of the wind stress on the near-surface velocities (Section 4.2).

The cyclonic circulations over the Gulf and its major basins contribute to the supply pathways to Georges Bank. In particular, the western limb of the gyre arising from tidal rectification and baroclinic flow over Georges Basin (Naimie, 1996) supplies the northern- flank jet in the Northeast Peak region, although there is only a limited supply onto the bank in the absence of other flow components such as the surface Ekman drift (compare the 5- and 25-m trajectories for January/February; Figs 4a and 8a). Similarly, the western and southern limbs of the baroclinic gyre in the southwestern Gulf (e.g. Chen et al., 1995) partially supply the Georges Bank gyre, although particle supply onto the bank can be minimal. On a larger scale, the movement of particles from the western limb of the Jordan Basin gyre to the Georges Basin gyre (Fig. 4) illustrates how adjacent gyres can combine with the surface Ekman drift to provide cross-gulf supply routes, similar to recent drifter observations of near-surface flow from Jordan Basin to Georges Bank (N. Pettigrew, Univ. Maine Orono, cited by Bisagni, 1995). In these simulations, the additional flow components are the surface Ekman layer and the inflows to the Gulf; however high-frequency wind events or other current fluctuations could give the same result.

The most intense circulation feature is the anticyclonic gyre over Georges Bank, which arises from a combination of tidal rectification and baroclinic circulation (Naimie, 1996). This gyre is the primary contributor to particle drift around the bank's perimeter and controls the exposure time of the particles to cross-bank exchange by other flow components (such as the wind-driven ftow or horizontal mixing; see also Perry et al., 1993). An example of increased along-bank velocity reducing the cross-bank exchange is the reduction in the number of particles reaching the bank in July/August compared to May/June. In both seasons the Georges Basin gyre moves particles south towards the bank but in July/August the northern flank jet is wider and faster which prevents the particles from reaching the bank. The jet can also provide on-bank flow on the Northeast Peak and a rapid supply route from the southwestern Gulf to the bank's northern flank.

In March/April, May/June and July/August, there is upper-ocean on-bank flow on the


Northeast Peak associated with the tidally-rectified and/or baroclinic flow components. Particle-tracking in a model flow field for barotropic tidal rectification (Loder et al., 1997) indicates only weak on-bank drift along the northern edge of the Northeast Peak, with greatest magnitudes near the bottom. Moored current measurements across the northern edge in summer (Brickman and Loder, 1993; Loder et al., 1993) show on-bank flow increasing towards the surface, consistent with a seasonal migration of the tidal front onto the bank and associated baroclinic circulation across the Northeast Peak. It thus appears that the summertime development of tighter baroclinic recirculation pathways across the Northeast Peak is the primary origin of the on-bank supply pathway in the present trajectories, although the recirculation pathway is not as far on-bank in the present model solutions as in observations (Loder et al., 1993; Limeburner and Beardsley, 1996). Possible contributors to the latter difference include inadequacies of the initial density field in this area (e.g. Tremblay et al., 1994 also found current discrepancies in this area; see their Fig. 12), and mixed area expansion during the present prognostic computations.

4.2. Ekman-layer transport

As noted in Section 3, wind-driven flow in the surface Ekman layer plays an important role in the seasonal variation in the number of particles reaching the bank. A theoretical estimate of the seasonal-mean velocity profile due solely to the local wind stress can be calculated using the local mean vertical eddy viscosity profile, the wind response structure functions (Lynch and Werner, 1987; Hannah and Wright, 1995), and the wind stress. Here, we compare the estimated wind-driven velocity profile with the seasonal-mean velocity profile extracted from the 3-d model fields for a location on the northern flank of Georges Bank.

In January/February (Fig. 9a,b), the relatively strong wind stress and weak stratification lead to a deep mixed layer, and the cross-bank velocities in the upper 40 m are clearly wind- driven. To a lesser extent, the near-surface vertical structure in the along-bank flow is also influenced by wind stress. In contrast, in May/June (Fig. 9c,d) the wind stress is weaker, the stratification stronger and the wind-driven currents are largely confined to the upper few metres. The substantial shear in the cross-bank currents below the shallow wind-driven layer is not wind-driven.

An indication of the spatial and seasonal changes in the depth over which local wind- driven currents are distributed is provided by the surface Ekman layer depth in the model solutions (Fig. 10), computed as the vertical distance in which the estimated wind-driven velocity falls offto e-1 of its surface magnitude. In the Gulf of Maine, the estimated Ekman depths are greatest in January/February (--~20 m) and least in July/August (<5 m). In contrast, the strong tidal currents on Georges Bank result in Ekman depths generally exceeding the water depth, and hence the abrupt change in the depth of the wind influence across the bank's northern flank, with weaker wind-driven currents on the bank. This transition zone migrates on-bank in the summer, associated with the seasonal evolution of the stratification and on-bank migration of the tidal front (e.g. Loder et al., 1993).

The particle tracking and the velocity profiles support the idea that the seasonal changes in wind stress and mixed-layer depth can play a major role in the near-surface transport of Calanusfinmarchicus and other biota onto the bank. In seasons (or months or years) with relatively-strong mean wind stress, such as in the present January-February case, the surface Ekman layer can provide a relatively-fast near-surface conveyor belt for transport across the Gulf. For typical winter winds from the northwest, this drift is generally


E v ¢ -

" e l

g e--

"1o

0

-20

-40

-60

-80

-100

0

-20

-40

-60

-80

-100

f *

/ a)

I I I

I I I

-0.1

d) I I I I

0 0.1 0.2 0.3 -0.1 0.05 Along-bank flow (m/s) Cross-bank flow (m/s)

I I

I I

I I

i\ on bank I I I ]~ l

i i

-0.05 0

off bank

I I

I I

off bank

I

b)

I

0.1

Fig. 9. Mean current profiles on the northern flank in (a,b) January/February and (c,d) May/June. The seasonal-mean model profile (solid) and the local wind-driven profile (dashed) have been rotated into along- and cross-bank components where, for each season, the along-bank direction is defined as

the direction of the seasonal-mean velocity at 25 m. The site (h = 103 m) is indicated in Fig. 1.

-25 ¢-

~. -50 q ) a

-75

-1000 5'0 100 150 Distance (km)

Fig. 10. Variation in Ekman depth across the Gulf of Maine and northern flank of Georges Bank for January/February (dash) and July/August (dashdot). The section runs from Jordan Basin

through site x (Fig. I). Georges Bank bottom topography profile is shown in the lower right.

southward and provides a b road supply route from much of the G u l f to Georges Bank. The surface E k m a n drift can also have an impor tan t effect by moving particles across saddle regions and separat ion lines between adjacent gyres under weaker and /or more variable wind stress.


An additional important feature of the wintertime conveyor belt from the Gulf of Maine to Georges Bank is an effective horizontal convergence mechanism, which may contribute to increased retention. The increase in the Ekman layer depth results in a decrease in the magnitude of the southward Ekman drift over the bank, so that drifting particles slow down. Notice the generally small velocities at 5 m over most of the central bank in the January/February and March/April solutions (Fig. 2). The slowing down results in a horizontal convergence of the particles on the bank as illustrated in Fig. 5, which shows a decrease in the north-south extent of the particle distributions for the March/April 5-m fixed-depth release. Recent drifter observations from the US GLOBEC program (Limeburner et al., 1996), as well as earlier observations (Drinkwater et al., 1992), provide support for the occurrence of this mechanism.

In the May/June and July/August velocity fields, particle trajectories in the Gulf of Maine are sensitive to their vertical position within the upper few metres. This arises because of the use of only the bimonthly mean component of the wind stress, which is weak in these periods. Consequently, the surface Ekman layer is generally an underestimate of the oceanic average and the mean surface velocity is too high (Fig. 9). However, this does not affect most of the results presented here since the 5-m release positions are below the spring/summer high-velocity layer, and there is minimal upwelling of particles to the surface (with the exception of the passive 5-m releases in May/June and July/August).

4.3. Upwelling

In all seasons the mean vertical velocity fields show a coherent band of upwelling between the 100- and 200-m isobaths along the northern side of the bank. This upwelling is primarily due to compression of the water column as the large scale Gulf flows ride up onto the side of the bank, with vertical velocities of a few m day- 1. This is consistent with our result that the 25-m passive release particles which reached the bank had upwelled, with mean vertical velocities of order 1 m day- i.

In the January/February 25-m releases, this upwelling led to a large increase in the number of particles reaching the bank and was primarily due to the small mean vertical velocities rather than the larger vertical velocities at the M2 frequency (Table 2). Along the bank's northern edge the shear in the cross-bank mean flow is generally favourable to increased on-bank flow for upwelled particles (Fig. 9). While the upwelling velocities are small they are sufficient to displace particles up into the wind-driven layer within a few days, where they are advected onto the bank along pathways similar to those shown in the 5-m releases.

In the other seasons the primary pathway onto the bank crosses the northern edge of the Northeast Peak region, where tidal rectification and baroclinic circulation dominate. Understanding the influence of upwelling on the number of particles reaching the bank in these seasons is more difficult because the vertical velocity fields have a much more complex structure than in January/February.

4.4. Horizontal velocity fluctuations

Here we briefly discuss the sensitivity of the advective pathways to the effects of unmodelled horizontal current fluctuations using a horizontal random walk (Section 2.2). For these experiments, the vertical position is fixed, the release grid is as shown in Fig. 5, the


Table 3. Cumulative number of particles reaching Georges Bank after 60 days for the horizontal mixing sensitivity study

Cumulative number

Release depth (m) Diffusivity (m 2 s- I ) Jan/Feb Mar/Apr May/Jun Jul/Aug

5 0 195 94 99 46 5 10 204 110 109 50 5 50 200 114 106 70

25 0 16 19 77 20 25 10 21 26 87 30 25 50 39 40 81 29

time step At=360 s, and the horizontal diffusivity is homogeneous and isotropic (K=K,.=Ky). For K = 1 0 m Z s -1, the rms step size is 85m and, in the absence of advection, the expected displacement from a particle's initial position after 24 h is 1.3 km. For K = 50 m 2 s - ~ the step size and 24-h displacement are 190 m and 3 km, respectively.

The horizontal random walk generally increases the supply of particles to the bank and reduces the seasonal changes, but does not change the qualitative nature of the results (Table 3). The results are most sensitive to horizontal mixing when the circulation is around the bank rather than across it. For example, the January/February 5-m releases show weak sensitivity to horizontal mixing in contrast to the January/February 25-m releases and the July/August releases at both depths. The increased supply to the bank is consistent with the notion that the Gulf gyres provide a mechanism for particle delivery to the bank edge but additional processes are needed to move particles across the northern flank jet.

4.5. Vertical turbulent mixing

Here we investigate the effects of allowing the particles to experience simulated vertical turbulent motions. The turbulence is represented by the vertical random walk process described in Section 2.2, with parameters derived from the mean turbulence fields for March/April as predicted by the circulation model (Werner et al., 1995).

Figure 11 shows particle locations 20 and 40 days after release in the March/April flow field for a release grid in Georges Basin and different cases of vertical movement. As before, particles with depth fixed at 5 m move directly onto the bank (Fig. 1 la), while those with depth fixed at 25 m remain in the Gulf of Maine (Fig. 1 lb). When the particles released at 5 m are allowed to move with the model's vertical velocity together with vertical turbulent kicks, the pattern changes substantially with the particles advected eastward in the northern-flank jet and then onto the Northeast Peak (Fig. l lc). The pattern changes because the vertical mixing causes the particles to spend time being advected by the flows below the wind-driven on-bank velocities at 5 m. The vertical velocities associated with the vertical mixing in the upper 10m (for this field) are typically in the range +0 .5× 1 0 - 2 m s - I , which is much larger than the mean vertical velocities of order 10- 5 m s - J from the model flow field. The simulations suggest that, in late winter and early spring with turbulence levels appropriate to tidal forcing and seasonal-mean wind stress, vertical velocities due to "behaviour" (of zooplankton or other biota) must be the order of 10-3m s -1 (100 m per day) or more to have a substantial impact on the distribution of


mf' I • .:4. ." / I a) 5 ixed i .'.: . . . . . t .......... . ... . . _ [ . % . . . " * ' " ..,, - -

". . . . . . . . . ~ l l l . ".. " - _ : " " . . ." : : ) i o ~ e e e . . . . . . , "~. ; ' , . ; ' . e o o o e o e o o " ' . . ,

" . .. " ' " : : l l l l l l l " : : I ; . . . " 'A l l O o e ~ A . . . . . . . ' ", ' : - .." ~ " " ~ - ' - - " ' £ - Z ' " " .

"%\ ". '.~ ." ..-'-~_~.f--iB~,_ \ "..

\_ / ; .) o .b" l ~/ , . i " i .W r

• .:{ : I I c) t u r b u l e n t k i c k s ..."~ ........ t

" ' . . . , . . " ., ~ , , . _ .

/ :. ". " , ^ " 0 " ~ " • o . ... . 4 i . ~ o o e o v ' . . .

• . . . . : : : o l o e o e o . . . . . . . . "% " . . : • e e o o e o e e "*. I : ,- . . . . . . . . . : a , ~o ...~

: , ,' . ~ o e o e e l ~ _ e n z ,-

\ "" : 0 " . . . . . %

y o ,":: i . ~ ¢ _ ¢ ., / i } ~ . , " " - " / ' t A '~ . ,0 '

i I( ~ J ' I 6" i

• • : : : / I . b) 25 m fixed , ~ , . ." t _

-, < . . . . . . . . : . i . . . ~- .... . ,.; . - ~ l l l l ~ \ ~ " i . . . . ~ - , : l l , " "..."~

o ~ - . . . . . ~ , : ~ • - .. . . . . . ." . O " ~ i . . - I , ~ e e e e ~ ' . - \a,. :-" . . . . . . ' . . . sn j ,_ ,~ ........ -.

\ .... "... - . . . .~/ - , . , . ...... ) -

" - . i ? " i / " _ ' \ i ' ~ , I / . . " ,,,j .; %

I l/ ,.-i" I ./-'/ i

d) t u ~ u l e n t kicks + 0 . 0 5 c m / s i _ ..% , . - " ' " " * . - - - -

- I [ '". f • ~ ~ - ~ • . ' ~ o e o '* \ %

I , , # , ~ . . 4 r o o o o o " • . . . . ~ o i e o o o o . . . . • t " . . . : ' . o e o o o o o e o " . . \

I ~ ' . . - ' . e o o o o e o ".

• . .° . " e e o ~ a . . . . . . . . . . . . *. \ ,. ,° .. ~ ~ : ......

. . ~ ~ .... ~ . . ~ ~ ~ 1 g ~ - \ / ~ ~ . "

\ / i ) ~ < ....... - ~ / ] . i ' o ~"

I I~ , . J ' l .6" I • ; . . : / I

I~ e ) t u r b u l e n t k i c k s + 0 . 2 c m i s ( r - I l~ f) d ,e l m , g r a t , o n 5•25 m .... t -~

. , . . . e ' " ' " ' . . . - - - ' " " % - - ~:. . . . ~: . ;. , - - , , ~ ("...... , . " . . , <_

: .. .~. , )oeo "-.. \ : ".. " .~,)oeo ".. \ t . . . . . . . . -", "",'~::::: . . . . . . "-'1 | . . . . . . . . . . . . . . . "' 4 ) ' .. : " ' . . . .' ~ : O l O O e o 0 . . . . . * " "~ / .... ": :" . . . . . . . . . . " . . . . . ' / ~ ", '...> " : : : = : : : l m ..... \

~ : . ; -" eee l e l , ". I'-- ; • • ¢ i") J m : I o e o I R l l K I " ";-"

. . . . " ' ° " " ii" i " ~ " t : f " ~ .. : . : .~Jr,,t .v,,. , . . . . . . . . ". , . . . . . . . _ , , . _

\ "-... \ "..,..- .-....'~, - \ , ~.. " ~ . ~ ' ~ l ~ r ' ~ l l ' ~ 1 . 4 y '~ ~.~.2"~ ! " o ~ ~-

• / t i 6 . . . , \ / : ~ue ' ,~

",- / ~ . . . . . / . :

I i v , i .: i l i i ) . / , - " ' I /.-

Fig. l 1. Locations after 20 days (IS]) and 40 days (O) of particles released in Georges Basin (release positions O) in the March/April flow field: (a) for fixed depth at 5 m; (b) for fixed depth at 25 m; (c) for release at 5 m and vertical velocity from the 3-d circulation model with turbulent vertical mixing; (d) as in (c) with an additional surface-seeking velocity of 0.05 cm s- i; (e) as in (c) with an additional surface-seeking velocity of 0.2 cm s- i: and (f) for a diel migration at speed of I cm s- i between the 5-

and 25-m levels. The total number of particles is 49.

vert ical pos i t ions (and resul t ing hor izon ta l advect ion) . As examples we show the results for par t ic les with surface-seeking velocit ies o f 0 .05× 10 - 2 m s - 1 (Fig. l l d ) and

0.2 x 1 0 - 2 m s -~ (Fig. 1 le), a d d e d to the model velocities and tu rbu len t kicks. Hence, add i t i ona l veloci t ies o f the o rde r o f 10 - 3 m s - ~, which are within the capabi l i t ies of la ter stages o f Calanusfinmarchicus (e.g. D u r b i n et al., 1995a,b), can largely overcome the effects o f the ver t ical tu rbu lence assoc ia ted with the seasona l -mean and t idal forcings.

The f ixed-depth s imula t ions in the M a r c h / A p r i l flow fields do not p rovide pa thways f rom the S C O P E X region (a long the 100-m i soba th between Cape Cod and Georges Bank) on to Georges Bank. The 5-m releases stall a t the western edge o f the bank (Fig. 4b) and the 25-m releases move ea s tward a long the 100-m i soba th para l le l to the nor thern side o f the bank (Fig. 8b). However , the a d d i t i o n o f vert ical mixing al lows roughly one-ha l f o f the part icles to reach the bank (not shown). The par t ic les are advected eas tward when mixed down and then on to the bank when mixed up into the near-surface flows (a diel migra t ion would have


a similar effect). In addition, particle releases in this region are expected to be sensitive to unmodelled horizontal velocity fluctuations.

These numerical experiments indicate that vertical turbulent mixing, in conjunction with vertical shear in the horizontal currents, can play an important role in the transport of biota and physical properties from the Gulf of Maine to Georges Bank. The mean turbulent mixing associated with the M2 tide and seasonal-mean forcings in late winter and early spring is predicted to be sufficient to mix passive particles out of the near-surface layer, pointing to even greater effects upon the inclusion of the more energetic storm-band wind forcing. This suggests that biota that are passive and neutrally buoyant will not be able to remain on the near-surface Ekman-layer conveyor belt from the Gulf of Maine to Georges Bank, although model simulations of the time evolution of the Ekman layer for realistic time-varying wind stress are required to address this. Nevertheless, sensitivity simulations with additional surface-seeking behaviour indicate that realistic vertical swimming speeds for later stages of Calanus finmarchicus can substantially modify and often override the influences of vertical turbulent mixing.

4.6. Biological behavior

The simulations described above show that the horizontal movement of particles or biota in the upper waters of the Gulf of Maine can strongly depend on their vertical position in relation to vertical shear in the horizontal currents. Thus, the different vertical migration and distribution patterns that have been reported for Calanus finmarchicus (e.g. Clarke, 1933, 1934; Durbin et al., 1995a,b) can be expected to have important implications for the extent and timing of their supply to Georges Bank.

The Georges Basin examples presented above (Fig. 1 I) illustrate how different patterns of vertical movement in the southern Gulf can, in conjunction with the early spring surface Ekman layer and the underlying gyres, lead to markedly different drift patterns. Biota that stay near the surface ride the conveyor belt to central Georges Bank, those that stay below the belt remain over the deep Gulf, while those that spend some time near the surface move to the bank's northern edge and are advected via the jet onto the Northeast Peak. When the vertical distribution is specified to be a diel migration between the 5- and 25-m levels (Fig. 11 f), the drift pattern in the March/April flow field is qualitatively similar to the cases in which particles move in and out of the surface layer due to turbulent kicks (Fig. 1 lc and to a lesser extent, Fig. 1 ld). The supply of particles to the central bank is reduced compared to the 5-m fixed-depth case (Fig. 1 la) with most particles moving onto the Northeast Peak via the (subsurface) northern flank jet. These examples suggest that different behavioural migration patterns in the upper ocean in typical winter and early spring seasons will lead to a range of intermediate states - - different combinations of drift onto the central bank, around the Northeast Peak and in the Gulf's circulation - - with the detailed drift pathways depending on factors affecting the fraction of time spent on the surface conveyor belt and on the belt's temporal and vertical structure (i.e. wind stress variability and Ekman layer structure).

Additional particle-tracking experiments for the wider release grid (Fig. 5), and with vertical positions specified as a diel migration between 5 m and 25 m confirm that the extent of supply to Georges Bank is intermediate between the cases with particles released at the 5- or 25-m levels (Table 2). Experiments with a deep diel migration (down to 150 m or the bottom) generally show a further reduction in supply to the bank (Table 2), consistent with


• I-Mar/Apr5/150m ....... - , H I I-Jan/Feb 5/150 m ...... - , e l ................... ' l r - " , - - - - ..' , ~ ~111 | - " , - - - - -" \ ~ ~11 r . - - " / ...... • / ~ 1 I V - ' " ." ..... : / 7 ~'.. ' " " \ : . . . . . . . " " ' • ' " \

,., " ' • ' - . ~ - -

k ~ ....... ' 2"-,,,] k i ......... ."'". :" ' ........ ."---~ , , ..... q L,: :

1. , .... - . . . . . . - - x - - - -, "~', ....... :- , ¢ : , . g - ~ , ~ , - - - , - I

- - 7 - . . , , / ( i ,.../ .-'7. , I,,G ! .,..'."-1 "~1 \ , t / ' j / ~ ' ~.-:i / " ~ ~ - ~ I j.:'

Fig. 12. Isochrons of transit time to Georges Bank for particles undergoing a diel migration between 5 m and 150 m (or the bottom). The isochrons are 50 days (solid) and 25 days (dashed).

Otherwise as for Fig. 6.

the particles spending even less time near the surface. For a fixed horizontal release position, the transit time to the bank (Fig. 12) is roughly doubled in the January/February experiment compared with the 5-m fixed depth case (e.g. release positions which are on the 25-day isochron in Fig. 6 are on the 50-day isochron in Fig. 12). The transit times in March/April are also increased. Nevertheless, even with this large-amplitude migration, with particles spending about half the time in the (relatively) quiescent lower water column, the near- surface conveyor belt can provide an effective transport mechanism to the bank.

The fraction of time that a particle is required to spend in the surface layer in order to be advected to Georges Bank depends on how far and how fast the particle needs to travel. For the 5- to 150-m diel migration the particles spend one-third of the time at 5 m. In the January/February flow field, the transit time for particles starting in the central Gulf was about 50 days. Faster transit times require more time in the surface layer and shorter distances require less. These simulations do not preclude the occurrence of additional supply pathways from other regions (e.g. from the Scotian Shelf) or in flow fields with more complex temporal variability• For example, the near-surface vertical structure and horizontal excursions of storm-band currents may contribute to additional supply mechanisms, especially in conjunction with vertical migrations of similar period• The present results indicate that such mechanisms are not necessary to explain the Calanus finmarchicus supply suggested by Meise and O'Reilly (1996), provided that there are biological mechanisms for the Calanus to offset downward turbulent mixing and spend a significant fraction of time (roughly one-third to one-half) in the surface Ekman layer.

5. DISCUSSION AND SUMMARY

The particle-tracking experiments described above illustrate the upper-ocean transport pathways from the Gulf of Maine to Georges Bank and the potential importance of different physical and biological processes in supplying Calanus finmarchicus and other biota to the bank. The basic circulation structure is a two-layer flow regime: a surface Ekman layer overlying topographic-scale gyres. Both the Ekman layer and the gyres evolve on seasonal time-scales. The resulting horizontal and vertical structure of the currents provide the potential for different and variable transport pathways depending on the vertical and horizontal distributions of the biota of interest. Variations in these distributions due to vertical migrations, turbulent vertical mixing or local upwelling can


determine whether the biota remain in the Gulf of Maine, move onto Georges Bank, or exit from the Gulf.

The primary upper-ocean supply mechanism in the flow fields examined here (for winter through summer) is surface Ekman-layer drift associated with the mean northwesterly wind stress in winter and early spring. This drift provides a near-surface conveyor belt from the Gulf of Maine basins, where Calanusfinmarchicus arise from diapause in winter and spring, to Georges Bank where increased tidal mixing provides a physical retention mechanism through reduced Ekman drift speeds. The routes, speeds and destinations of these pathways can be expected to vary seasonally and interannually due to variability in the wind stress.

The near-surface conveyor belt is augmented by the increased large-scale flow through the Gulf of Maine in winter, and by branches of the gyres over the basins and banks. In particular, the western limbs of the cyclonic gyres over Georges and Wilkinson Basins result in persistent and faster pathways to the northern edge of Georges Bank and the northern Great South Channel region, respectively. Below the surface Ekman layer, the basin gyres also provide local supply routes to the southern gulf but the present realizations of these gyres generally do not provide complete pathways to Georges Bank.

The strong jet along the northern flank tends to isolate the bank by reducing residence times over the northern edge and thus reducing cross-bank exchange by other flow components. However, the jet can contribute to transport from the northern Great South Channel region to the Northeast Peak (along the bank's northern edge) in spring and summer. The exploratory experiments with horizontal random walks indicate that unmodelled horizontal velocity fluctuations should contribute to increased cross-bank exchange, but are not likely to alter the qualitative features of a seasonally-varying two- layer flow regime with strong coupling of the Gulf of Maine and Georges Bank via the near- surface layer.

The model solutions indicate that there are upwelling zones along the bank's northern edge which tend to displace passive biota up onto the near-surface conveyor belt. However, the sensitivity studies with turbulent vertical mixing indicate that passive biota generally will be mixed downwards from the near-surface region and will not be able to remain on the conveyor belt. Further investigations of the variability and influences of key features are required, such as the surface Ekman layer, cross-bank exchange along the northern flank and local upwelling.

The present results point to some specific biological implications. If Calanusfinmarchicus swimming ability can substantially offset the turbulent mixing, then the present simulations provide strong support for the suggestion of Meise and O'Reilly (1996) of a seasonally- varying supply of Calanus to Georges Bank via the near-surface wind-driven flow from the Gulf of Maine. This supply pathway is predicted to be most effective in winter and early spring, which is qualitatively consistent with Meise and O'Reilly's description of peak abundance over the central bank in March-April. The sensitivity experiments indicate that diel vertical migrations in and out of the near-surface layer provide a significant supply of Calanus to Georges Bank in winter and early spring, and that surface-seeking swimming speeds of order 1 0 - 3 m s - j on average can substantially offset the turbulent mixing associated with the seasonal-mean forcings. The key questions are: what are the vertical distributions of Calanus finmarchicus in relation to the surface Ekman layer? and what biological and/or environmental factors affect these distributions and their variability?

The simulations are qualitatively consistent with the summer distributions of Calanus finrnarchicus reported by Meise and O'Reilly (1996) and Perry et al. (1993) which indicate a


sharp reduction in abundance across the northern edge of Georges Bank consistent with increased summertime isolation of the bank by the seasonal frontal system. The suggestion here of a supply pathway to the Northeast Peak in May/June and July/August is consistent with the Meise and O'Reilly distributions of higher abundance in the Tidal Front GB and Stratified GB subareas than in the Mixed GB subarea in late spring and summer (their Fig. 6). This does not preclude an inflow of Calanusfinmarchicus from the Scotian Shelf to outer Georges Bank in the spring and summer; indeed, the present flow fields generally show transport across the mouth of the Northeast Channel (Fig. 2). Further, we have not considered food limitation and predation which are likely contributors to the decline of Calanusfinmarchicus abundance on the bank in spring and summer (e.g. Davis, 1987).

Perhaps the most significant result of the present simulations is the demonstration of the alternative drift pathways that can arise for particles with different vertical velocities in a highly-structured flow field; specifically, a surface layer with a broad drift overlying topographic-scale gyres. Physical and biological processes affecting the vertical positions of biota in such a regime can have first-order effects on the horizontal drift pathways, pointing to the importance of realistic descriptions of both the ocean velocity field and the behaviour of the biota of interest in order to quantify exchange and supply rates.

Acknowledgements--We are grateful to our many colleagues who have provided observational information and contributed to the circulation model and related algorithms, especially Dan Lynch for his lead role in the model development, lan Perry for his enduring support for this study, and Brian Blanton for development of the particle- tracking algorithms. We also thank Alex Herman, Greg Lough, Carol Meise, lan Perry and Doug Sameoto for input on Calanusfinmarchicus distributions and life cycles; Dave Greenberg and Justin Ip for contributions to the model application and development; and Ken Drinkwater and Jeff Runge for internally reviewing the manuscript. This is contribution 79 of the U.S. GLOBEC program, jointly funded by NOAA and NSF. The model application has also been supported by the (Canadian) Interdepartmental Panel for Energy, Research and Development.

REFERENCES

Lynch and Werner, 1987 cited in manuscript but not in references. Please supply reference details. Backus, R. H. and Bourne, D. W. (1987) Georges Bank. MIT Press, Cambridge, 593 pp. Berg, H. C. (1993) Random Walks in Biology. Princeton University Press, 152 pp. Bigelow, H. B. (1926) Plankton of the offshore waters of the Gulf of Maine. Bulletin of the U.S. Bureau of

Fisheries 40, 1-509. Bisagni, J. (1995) Satellite and drifter evidence for Maine coastal current flow onto Georges Bank. In Report of

the U.S. GLOBEC Georges Bank Program Scientific Investigators" Workshop, 16-18 October 1995. Woods Hole, Ma, p. 11.

Blanton, B. O. (1995) DROG3D: User's Manual.for 3-Dimensional Drogue Tracking on a Finite Element Grid with Linear Finite Elements. Program in Marine Sciences, University of North Carolina, Chapel Hill, NC, 13 pp.

Blumberg, A. F., Galperin, B. and O'Connor, D. J. (1992) Modelling vertical structure of open channel flows. Journal of Hydraulic Engineering 118, 1119-1134.

Bollens, S. M. and Frost, B. W. (1989) Predator induced diel vertical migration in a planktonic copepod. Journal of Plankton Research 11, 1047-1065.

Brickman, D. and Loder, J. W. (1993) The energetics of the internal tide on Georges Bank. Journal of Phys&al Oceanography 23, 409-424.

Brooks, D. A. (1985) Vernal circulation in the Gulf of Maine. Journal of Geophysical Research 90, 4687-4705. Brown, W. S. and Irish, J. D. (1992) The annual evolution of geostrophic flow in the Gulf of Maine: 1986-1988.

Journal of Physical Oceanography 22, 445-473. Bumpus, D. F. and Lauzier, L. M. (1965) Surface circulation on the continental shelf of eastern North America

between Newfoundland and Florida. In Serial Atlas of the Marine Environment. American Geographical Society, New York, folio 7, 8 pp. + 8 pl.


Chen, C., Beardsley, R. C. and Limeburner, R. (1995) Variability of currents in late spring in the northern Great South Channel. Continental Shelf Research 15, 451-473.

Clarke, G. L. (1933) Diurnal migration of plankton in the Gulf of Maine and its correlation with changes in submarine illumination. The Biological Bulletin 65, 402-436.

Clarke, G. L. (1934) Further observations on the diurnal migration of copepods in the Gulf of Maine. The Biological Bulletin 67, 432-460.

Csanady, G. T. (1984) The free surface turbulent shear layer. Journal of Physical Oceanography 14, 402-411. Davis, C. S. (1987) Zooplankton life cycles. In Georges Bank, eds. R. H. Backus and D. W. Bourne, pp. 256-267.

MIT Press, Cambridge. Drinkwater, K. F., Gra~a, M. J. and Loder, J. W. (1992) Lagrangian current measurements from the Georges

Bank Frontal Study, 1988-89. Part I: Drift buoy trajectories. Can. Data Rep. Hydrogr. Ocean Sci. No. 116: iv + 105 pp.

Durbin, E. G., Campbell, R. G., Gilman, S. L. and Durbin, A. G. (1995a) Diel feeding behavior and ingestion rate in the copepod Calanusfinmarchicus in the southern Gulf of Maine during late spring. Continental Shelf Research 15, 539-570.

Durbin, E. G., Campbell, R. G., Gilman, S. L. and Durbin, A. G. (1995b) Abundance, biomass, vertical migration and estimated development rate of the copepod Calanus finmarchicus in the southern Gulf of Maine during late spring. Continental Shelf Research 15, 571-591.

Flagg, C. N., Magnell, B. A., Frye, D., Cura, J. J., McDowell, S. E. and Scarlet, R. I. (1982) Interpretation of the physical oceanography of Georges Bank. EG&G Environmental Consultants, Waltham, Mass. Final report prepared for U.S. Dept. of Interior, Bureau of Land Management, 901 pp.

Galperin, B., Kantha, L. H., Hassid, S. and Rosati, A. (1988) A quasi-equilibrium turbulent energy model for geophysical flows. Journal of the Atmospheric Sciences 45, 55~2.

Hannah, C. G. and Wright, D. G. (1995) Depth-dependent analytical and numerical solutions for wind-driven flow in the coastal ocean. In Quantitative Skill Assessment for Coastal Ocean Models, eds. D. R. Lynch and A. M. Davies, pp. 125-152. American Geophysical Union.

Hannah, C. G., Loder, J. W. and Wright, D. G. (1996) Seasonal variation in the baroclinic circulation in the Scotia Maine region. In Buoyancy Effects on Coastal and Estuarine Dynamics, eds. D. G. Aubrey and C. T. Friedrichs, pp. 7-29. American Geophysical Union.

Holloway, G. (1994) Comment: on modeling vertical trajectories of phytoplankton in a mixed layer. Deep-Sea Research 41, 957-959.

Kenney, R. D. and Wishner, K. F. (1995) The South Channel Ocean Productivity Experiment. Continental Shelf Research 15, 373-384.

Legg, B. J. and Raupach, M. R. (1982) Markov chain simulation of particle dispersion in inhomogeneous flows: The mean drift velocity induced by a gradient in Eulerian velocity variance. Boundary-Layer Meteorology 24, 3-13.

Limeburner, R. and Beardsley, R. C. (1996) Summertime Lagrangian circulation on Georges Bank. Deep-Sea Research H 43, 1547-1574.

Limeburner, R., Beardsley, R. C. and Brink, K. H. (1996) Lagrangian measurements of near-surface flow over Georges Bank. EOS, Transactions of the American Geophysical Union 76, OS197.

Loder, J. W., Drinkwater, K. F., Oakey, N. S. and Horne, E. P. W. (1993) Circulation, hydrographic structure and mixing at tidal fronts: the view from Georges Bank. Philosophical Transactions of the Royal Society of London A343, 447-460.

Loder, J. W., Shen, Y. and Ridderinkof, H. (1997) Characterization of three-dimensional Lagrangian circulation associated with tidal rectification over a submarine bank. Journal of Physical Oceanography 27, 1729-1742.

Lynch, D. R. and Naimie, C. E. (1993) The M2 tide and its residual on the outer banks of the Gulf of Maine. Journal of Physical Oceanography 23, 2222-2253.

Lynch, D. R. and Werner, F. E. (1987) Three-dimensional hydrodynamics on finite elements. Part I: Linear Harmonic Model. International Journal for Numerical Methods in Fluids 7, 871-909.

Lynch, D. R., Ip, J. T. C., Naimie, C. E. and Werner, F. E. (1996) Comprehensive coastal circulation model with application to the Gulf of Maine. Continental Shelf Research 16, 875-906.

Lynch, D. R., Werner, F. E., Greenberg, D. A. and Loder, J. W. (1992) Diagnostic model for the baroclinic, wind-driven and tidal circulation in shallow seas. Continental Shelf Research 12, 37-64.

MeUor, G. L. and Yamada, T. (1982) Development of a turbulence closure model for geophysical fluid problems. Reviews of Geophysics and Space Physics 20, 851-875.


Meise, C. and O'Reilly, J. E. (1996) Spatial and seasonal patterns in abundance and age-composition of Calanus finmarchieus in the Gulf of Maine and on Georges Bank: 1977-1987. Deep Sea Research H 43, 1473-1501.

Miller, C. B. (1995) The life history paradigm for Calanusfinmarchicus. In Report of the U.S. GLOBEC Georges Bank Program Scient(fie Investigators' Workshop, 16-18 October 1995. Woods Hole, Ma, pp. 53-54.

Naimie, C. E. (1995a) On the modeling of the seasonal variation in the three-dimensional circulation near Georges Bank. Ph.D. thesis, Dartmouth College, Hanover, NH, 266 pp.

Naimie, C. E. (1995b) Georges Bank Bimonthly Residual Circulation Prognostic Numerical Model Results. Report # NML 95-3, Numerical Methods Laboratory, Dartmouth College, Hanover, NH.

Naimie, C. E. (1996) Georges Bank residual circulation during weak and strong stratification periods -- Prognostic numerical model results. Journal of Geophysical Research 101, 6469q5486.

Naimie, C. E., Loder, J. W. and Lynch, D. R. (1994) Seasonal variation of the 3-D residual circulation on Georges Bank. Journal of Geophysical Research 99, 15967-15989.

Perry, R. I., Harding, G. C., Loder. J. W., Tremblay, M. J., Sinclair, M. M. and Drinkwater, K. F. (1993) Zooplankton distributions at the Georges Bank frontal system: retention or dispersion? Continental Shell" Research 13, 357 383.

Sameoto, D. D. and Herman, A. W. (1990) Life cycle and distribution of Calanusfinmarchicus in deep basins on the Nova Scotia shelf and seasonal changes in Calanus spp. Marine Ecology Progress Series 66, 225 237.

Sherman, K. (1980) MARMAP, a fisheries ecosystem study in the NW Atlantic: fluctuations in ichthyoplankton zooplankton components and their potential for impact on the system. In Advanced Concepts .for Ocean Measurements in Marine Biology, eds. F. P. Diemer, F. J. Vernberg and D. Z. Mirkes, pp. 9-37. University of South Carolina Press.

Sherman, K., Smith, W. G., Green, J. R., Cohen, E. B., Berman, M. S., Marti, K. A. and Goulet, J. R. (1987) Zooplankton production and the fisheries of the northeastern shelf. In Georges Bank, eds. R. H. Backus and D. W. Bourne, pp. 268-282. MIT Press, Cambridge.

Thomson, D. J. (1987) Criteria for the selection of stochastic models of particle trajectories in turbulent flows. Journal of Fluid Mechanics 180, 529-556.

Tremblay, J. M., Loder, J. W., Werner, F. E., Naimie, C. E., Page, F. H. and Sinclair, M. M. (1994) Drift of sea scallop larvae Plaeopecten magellanicus on Georges Bank: a model study of the roles of mean advection, larval behavior and larval origin. Deep Sea Research H 41, 7~,9.

Werner, F. E., Page, F. H., Lynch, D. R., Loder, J. W., Lough, R. G., Perry, R. I., Greenberg, D. A. and Sinclair, M. M. (1993) Influence of mean 3-D advection and simple behavior on the distribution of cod and haddock early life stages on Georges Bank. Fisheries Oceanography 2, 43-64.

Werner, F. E., Perry, R. I., MacKenzie, B. R., Lough, R. G. and Naimie, C. E. (1995) Larval trophodynamics, turbulence, and drift on Georges Bank: a sensitivity analysis of cod and haddock. ICES Annual Science Conference, 21-26 September 1995, ICES C.M. 1995/Q:26, ,~alborg, Denmark.

Wiebe, P. and Beardsley, R. C. (1996) Physical-biological interactions on Georges Bank and its environs. Deep- Sea Research H 43

Wishner, K. F., Schoenherr, J. R., Beardsley, R. and Chen, C. (1995) Abundance, distribution and population structure of the copepod Calamtsfinmarehieus in a springtime right whale feeding area in the southwestern Gulf of Maine. Continental Shelf Research 15, 475-507.

Woodruff, S. D., Slutz, R. J., Jenner, R. L. and Steurer, P. M. (1987) A comprehensive ocean-atmosphere data set. Bulletin of the American Meteorological Society 68, 1239-1250.

Documents

Upper-ocean transport mechanisms from the Gulf of Maine to Georges Bank, with implications for Calanus supply