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Regional Modeling Activities in the Gulf of Maine/Georges Bank

The Gulf of Maine (GoM), located on the North American continental shelf between Cape Cod and Nova Scotia, is a semi-enclosed basin opening to the North Atlantic Ocean (Fig. 1). The geometry of the GoM is characterized by several deep basins and shallow submarine banks. On the seaward flank of the GoM is Georges Bank (GB), which is separated from the Nantucket Shoals to the west by the Great South Channel (GSC) and from the Scotian Shelf to the east by the Northeast Channel. The bank is roughly elliptical in plane view with a length of about 200 km along the major axis and a width of about 150 km along the minor axis. The cross-bank bottom topography rises steeply with a slope of about 0.01 from 1000 m on the slope to 100 m at the shelf break, increases slowly with a slope of 0.0004 to 40 m over a distance of about 150 km on the top of the Bank, and then decreases rapidly with a slope of about 0.03 to a depth of 300 m in the deep basin just north of GB. GSC runs approximately north-south, with a sill depth of about 50 m located near 40 degree and 45' to separate the mid and outer continental shelf to the south from the deeper GoM to the north with depths in exceed of 150 m. The deep connection between the GoM and the open ocean is mainly through Northeast Channel where the sill depth is about 230 m.

The first systematic study of the general circulation in the GoM can be traced back to Bigelow (1927). Based on a large number of surface drifters and hydrographic observations, he suggested that the summertime (stratified season) surface circulation in the GoM is dominated by two relatively large-scale gyres: a cyclonic circulation around Jordan Basin and an anticyclonic circulation around Georges Bank (Fig. 1). This circulation pattern has been demonstrated by long-term direct Eulerian and Lagrangian current measurements since 1970.

Our current understanding of the general circulation in the GoM is mainly based on a synthesis of field observations summarized in Fig. 1. The near surface circulation is controlled by wind stress, tidal rectification, seasonal heating/cooling, river discharge, Scotian Shelf inflow, and warm-core ring intrusions. The deep current is controlled by intrusions of warm and saline water through the Northeast Channel, and is influenced by wind mixing and surface cooling.

Flow over GB is dominated by strong M2 tidal currents. These currents exhibit a rotary character over the bank and increase as the water becomes shallower. The maximum velocity is about 30 cm/s near the shelf break on the southern flank and about 100 cm/s at the edge of the northern flank (Moody et al., 1984). When a long barotropic tidal wave propagates from the deep ocean onto variable bottom topography, a clockwise-rectified flow is generated over the bank due to the nonlinear transfer of vorticity and momentum from tidal currents (Loder, 1980; Zimmerman, 1978 and 1980; Greenberg, 1983). This topographically tidal rectified current moves eastward as a current jet of about 20 cm/s on the northern flank and re-circulates westward as a relatively broad and weaker flow of 1-3 cm/s on the southern flank. Whenever stratification is involved, strong tidal-induced vertical mixing creates a well-defined tidally mixed front (TMF) around the shallow cap of the Bank. During summer, the TMF is located near the 40-m isobath on the northern flank and about the 50 to 60-m isobath on the southern flank (Flagg, 1987; Chen et al., 1995a). During winter, the TMF disappears over the southern flank as strong wind mixing and surface cooling homogenize the local water column on the top and flanks of the Bank (Fig. 2). Because of the TMF, the clockwise re-circulation gyre over GB varies significantly with seasons, strongest during the summer in which the maximum along-bank current can reach 30 to 40 cms-1 on the northern flank, and about 8 to 10 cms-1 on the southern flank, and weakest during the winter when the very weak tidally driven rectified residual currents are in competition with wind- and buoyancy-driven flow (Butman et al.,1982; Loder and Wright, 1985; Chen et al. 1995a; Limeburner and Beardsley, 1996).

The unique physical processes of the clockwise residual circulation gyre, tidal mixing, and tidal mixing front on GB make it one of the most productive shelf ecosystems in the world (Riley, 1941, O'Reilly et al., 1987, Horne et al., 1989; Wiebe and Beardsley, 1996; Franks and Chen, 1996, 2001). The total annual production on the bank is about two-to-three times higher than the mean value of the annual production over continental shelves in the world. The patterns of high plankton patchiness are often recorded on GB, with an unusually high concentration of chlorophyll in the vertically well-mixed region and frontal zone (O'Reilly et al., 1987, Horne et al., 1989; Mountain and Taylor, 1996). Cod and haddock spawn on the northeast flank of GB in late winter and early spring (Smith and Morse, 1985). Their eggs and young larvae move following the clockwise residual circulation gyre and arrive on the southern flank in late April and May, with a high abundance of cod and haddock larvae found in the stratified region between tidal and shelf break fronts on the southern flank (Lough 1984; Townsend and Pettigrew, 1996; Lough and Mountain, 1996). These larval fishes continue to move westward and the northeastward following the recirculation and grow to pelagic juveniles on the western flank in late spring. Demersal juveniles occupy the mixed region on GB in early summer, suggesting a significant cross-frontal onbank transport on GB (Lough and Manning, 2001).

The GoM/GB is a challenging region for modeling due to complexity of geometry, nature of near-resonance tidal motion, shelf-estuarine interaction, and significant influence from the upstream inflow and frequent intrusion with the Gulf Stream warm-core rings. A successful model should be capable to 1) resolve the resonance nature of tidal motion in the Bay of Fundy; 2) the asymmetric clockwise rectified gyre over steep bottom topography, and 3) subtidal current jet associated at shelf break. This model should also be capable to resolve the short-term variability due to tidal motion and a long-term variability due to the seasonal variation of wind stress and surface heating, river discharge, Scotian Shelf inflow, and eddy intrusion.

Funded by the US GLOBEC/Georges Bank Program and the Massachusetts Marine Fisheries Institute (MFI) at SMAST/UMASSD, an integrated GoM/Georges Bank model system was built by the joint effort between the MEDM laboratory at SMAST and Woods Hole Oceanographic Institution. This model system has been carefully validated for long-term simulation and assimilation application of the 1995 and 1999 circulation and stratification in the GoM/GB. This system is ready for the forecasting application for fishery management on GB and hindcasting studies of the physical and biological interaction process controlling the ecosystem dynamics in this unique environment.

Many scientists have expended effort on the GoM/GB model system development. The coupled meso-scale meteorological (MM5) and FVCOM GoM/GB model was developed in collaboration with Dr. Robert C. Beardsley, Senior Scientist at Woods Hole Oceanographic Institution. Biological models were developed in collaboration with Dr. Peter Franks, Professor at Scripps Institution of Oceanography, Dr. David Townsend at University of Maine. Dr. Jamie Pringle, Assistant Professor at the University of New Hamisphere, helped setting up an upstream open boundary condition to include the impact of the Scotian Shelf water, Dr. Ken Brink, Senior Scientist at Woods Hole Oceanographic Institution, provided invaluable insight into the dynamics and interpretation of model results. Dr. Brian Rothschild, Co-Director of the Massachusetts Marine Fisheries Institute provided both scientific insight on modeling fish recruitment dynamics as well as funding for personnel and computer facilities.

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