Physical Oceanography of the Northwest Atlantic Continental Margin

by Peter C. Smith and John W. Loder

The dominant driving forces for circulation and mixing on the continental shelves of the northwest Atlantic are the tides, surface wind stress, offshore currents and eddies, and buoyancy input associated with freshwater runoff and melting sea ice. At large scales, a continuous, buoyancy-driven coastal current has been traced over 5000 km from the west coast of Greenland to the Mid-Atlantic Bight (Fig. 8-3; Chapman and Beardsley, 1989). The basis for this inference is the observation that the freshwater component of the southern shelf waters is highly depleted in the oxygen isotope, 18O, suggesting sources in the Labrador Sea such as glacial or sea-ice melt or Hudson Bay runoff. Strong seasonal variability has also been identified in both the salinity and transport of this current, consistent with that of the buoyancy sources (Smith and Schwing, 1990). Off Cape Sable, N.S., the maximum alongshore transport in Jan.-Feb. carries a pulse of low- salinity water from the Scotian Shelf into the Gulf of Maine, and the associated buoyancy flux then contributes to the development of the vernal circulation in the Gulf (Brooks, 1985).

Imbedded within the large-scale coastal flow are mesoscale [10-100 km)] circulations associated with the numerous submarine banks which populate the shelf. Vorticity constraints and certain forcing mechanisms (e.g., tidal rectification; Loder, 1980) tend to produce anticyclonic (clockwise) residual gyres over these banks, but variable characteristics of physical size and circulation produce important differences in the significance of the gyres to the banks' marine ecosystems. These differences may be particularly relevant to the comparative analyses of GLOBEC.

Loder et al. (1988a) have examined and compared the physical regimes of four northwest Atlantic Banks using data from various multidisciplinary field experiments conducted over the last decade (Table 1). On the shallow plateau of Georges Bank, enhanced currents and mixing maintain vertically-uniform temperature year-around (Fig. 8-4b), whereas strong thermal stratification develops over the Newfoundland banks (Fig. 8-4a). [A smaller well-mixed zone also occurs over Browns Bank, but is not resolved by the smoothed hydrographic data in Fig. 8-4b.] Since the annual temperature cycle on Georges Bank is determined primarily by seasonal warming, the difference between the observed mixed-water temperature and that which would result from the net heat input at the surface may be used to estimate the rate of horizontal heat (and by inference, salt and nutrient) exchange (Loder et al., 1982).

The depth-averaged mean circulation consists of a clockwise gyre on each of the four banks except Southeast Shoal, where a weak westward flow prevails (Fig. 8-5). The currents with periods of a day or less (e.g., tidal) are typically 2 to 5 times stronger than the mean and low-frequency currents, but the excursion (║udt) variance, affecting horizontal exchange, is dominated by the low-frequencies. To characterize the physical exchange rates for the different banks, the time scales for mixing in three perpendicular directions have been estimated from moored current and density data, Lagrangian drifters and heat or salt budgets (Loder et al., 1988a):

TR = residence time (cross-isobath),
TG = recirculation time (along-isobath), and
Tv = vertical exchange time.

A schematic summary of these scales (expressed as rates proportional to T-1; arrow size and normalized by the residence time; Fig. 8-6) reveals that the recirculation time is typically of the same order as the residence time, suggesting that the gyres are of limited significance in "retaining" water or drifting particles on the banks. There is a suggestion, however, that Georges Bank maintains the highest ratio of residence to recirculation time. The vertical exchange rates are maximum on Georges and Browns Banks and minimum on Flemish Cap, where nutrient supply rate may limit production in the surface layer. The physical time scales are also useful in characterizing the bank ecosystems by comparison to the biological time scales, such as phytoplankton doubling or the duration of a critical phase in the early life history of fish eggs or larvae.

Some potentially important biophysical interactions in the Gulf of Maine have been explored as part of the recent field programs (Table 1). For instance, 7-year records from a mooring off Cape Sable, N.S., collected before and during the Fisheries Ecology Program (FEP), have revealed that deviations from the seasonal cycles of current and salinity are related to similar anomalies in the alongshore component of wind stress and, to some extent, the presence of warm-core Gulf Stream rings at the mouth of Northeast Channel (Smith, 1989a). The mechanism appears to be related to the wind- driven intrusion of Slope Water into the Gulf via Northeast Channel, and subsequent upwelling off southwest Nova Scotia. Such an event appears to have enhanced stratification and nutrient supply on Browns Bank in the spring of 1985, leading to the highest phytoplankton and zooplankton biomasses of the three-year FEP observation program (Perry et al., 1989).

FEP variations of surface drift and particle dispersion, using clusters of satellite-tracked drifters, revealed a "leaky" gyre on the western cap of Browns Bank, characterized by consistent exit of drogues from the northern flank to the inshore area (Smith, 1989b). The residence time for the drogues was estimated at 10-14 days, equivalent to the average hatching time for haddock eggs, so the advection and dispersion of eggs and larvae may have important implications for survival and subsequent recruitment. Unfortunately, attempts to model the drogue trajectories using a 2-D barotropic model with wind and tidal forcing (Page and Smith, 1989) have been largely unsuccessful due to: 1) poor resolution and 2) the absence of baroclinic effects. In addition, model- derived interannual differences in particle displacements from the Bank during the spawning season could not be consistently related to gadoid egg and larval mortality (Campana et al., 1989), but better baroclinic models are required before any biophysical conclusions may be reached.

Finally, the recent Georges Bank Frontal Study (Loder et al, 1988b) is providing an unprecedented, detailed picture of the physical processes occurring along the Bank's northern edge, while accompanying biological measurements promise to enhance our understanding of the ecosystem. In particular, repeated Batfish sections across the Bank edge indicate the formation of an internal hydraulic jump during off-bank tidal flow (Figs. 8-7, 8-8), and subsequent propagation onto the Bank as a packet of large-amplitude internal waves after the tide turns. This phenomenon considerably complicates the classical pictures of residual current (e.g., Loder and Wright, 1985) and turbulence (e.g., Garrett et al., 1978) generation by the barotropic tidal current. In essence, the dynamics of the northern front on Georges Bank appears to be a combination of those of 1) a tidal-mixing front and 2) a tidally-forced stratified fluid at the shelf break. In addition, drifter measurements from the recent study suggest that there is a surface convergence at the front which, in conjunction with the along-bank jet, would distribute passive surface particles widely around the Bank, while retaining them in the frontal zone. The biological implications of these processes are presently under study. Clearly, however, understanding the biophysical interactions in this complex system requires consideration of both region-wide and local physics, including 1) an adequate thermohaline circulation model of the Gulf as a whole and 2) a careful representation of the physical processes in the vicinity of fronts and abrupt topography.


TABLE 1. Recent Multidisciplinary Experiments on the Northwest Atlantic Continental Shelf
EXPERIMENT YEARBANKTARGET
SPECIES
Flemish Cap Int'l Exper.1979-81Flemish Capredfish,
cod
S.E. Shoal Exchange Study1986-89S.E. Shoacapelin
SWNS Fish. Ecol. Program1983-85Browns Bankhaddock,
cod
Georges Bank Larval
Herring Patch Study
1978Georges Bankherring
Georges Bank Frontal Study1988Georges Banklobster,
scallop

8.5.1 References

Brooks, D. A. 1985. Vernal circulation in the Gulf of Maine. J. Geophys. Res. 90, 4687-4705.

Campana, S. E., K. T. Frank, P. C. F. Hurley, P. A. Koeller, F. H. Page and P. C. Smith. 1989. Survival and abundance of young cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) as indicators of yearclass strength. Can. J. Fish. Aquat. Sci. 46 (suppl. 1), 171-182.

Chapman, D. C. and R. C. Beardsley. 1989. On the origin of shelf water in the Middle Atlantic Bight. J. Phys. Oceanogr. 19, 384-391.

Garrett, C. G. R., J. R. Keeley and D. A. Greenberg. 1978. Tidal mixing versus thermal stratification in the Bay of Fundy and Gulf of Maine. Atmos.-Ocean 16, 403-413.

Loder, J. W. 1980. Topographic rectification of tidal currents on the sides of Georges Bank. J. Phys. Oceanogr. 10, 1399-1416.

Loder, J. W., D. G. Wright, C. Garrett and B.-A. Juszko. 1982. Horizontal exchange on central Georges Bank. Can. J. Fish. Aquat. Sci. 39, 1130-1137.

Loder, J. W. and D. G. Wright. 1985. Tidal rectification and frontal circulation on the sides of Georges Bank. J. Mar. Res. 43, 581-604.

Loder, J. W., C. K. Ross and P. C. Smith. 1988a. A space- and timescale characterization of circulation and mixing over submarine banks, with application to the northwestern Atlantic continental shelf. Can. J. Fish. Aquat. Sci. 45, 1860-1885.

Loder, J. W., K. F. Drinkwater, E. P. W. Horne and N. S. Oakey. 1988b. The Georges Bank Frontal Study: An overview with preliminary results. EOS 69, 1283.

Page, F. H. and P. C. Smith. 1989. Particle drift in the surface layer off Southwest Nova Scotia: Description and evaluation of a model. Can. J. Fish. Aquat. Sci. 46 (Suppl. 1), 21-43.

Perry, R. I., P. C. F. Hurley, P. C. Smith, J. A. Koslow and R. O. Fournier. 1989. Modelling the initiation of spring phytoplankton blooms: a synthesis of physical and biological variability off Southwest Nova Scotia. Can. J. Fish. Aquat. Sci. 46 (Suppl. 1), 183-199.

Smith, P. C. 1989a. Seasonal and interannual variability off Southwest Nova Scotia. Can. J. Fish. Aquat. Sci. 46 (Suppl. 1), 4-20.

Smith, P. C. 1989b. Circulation and dispersion on Browns Bank. Can. J. Fish. Aquat. Sci. 46, 539-559.

Smith, P. C. and F. B. Schwing. 1990. Mean circulation and variability on the continental shelves of eastern Canada. Cont. Shelf Res., in press.


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