Seasonal variation of the three‐dimensional mean circulation over the Scotian Shelf

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Abstract
The seasonal‐mean circulation over the Scotian Shelf is studied numerically by computing mean and tidal current fields for winter, spring, and summer using a three‐dimensional nonlinear diagnostic model. The mean current fields are forced by seasonal‐mean baroclinic pressure gradients, tidal rectification, uniform wind stresses, and associated barotropic pressure gradients. A historical hydrographic database is used to determine the climatological mean baroclinic forcing. Upstream open boundary conditions are estimated from the density fields to give no normal geostrophic bottom flow and are specified as either along‐boundary elevation gradients or depth‐integrated normal velocities. The numerical solutions for nominal bimonthly periods (January–February, April–May, and July–August) reveal the dominant southwestward nearshore and shelf‐break flows of relatively cool and fresh shelf water from the Gulf of St. Lawrence and Newfoundland Shelf, with speeds up to about 20 cm/s. The seasonal intensification of the southwestward flows is reproduced by the model, with the transport increasing from 0.3 Sv in summer to 0.9 Sv in winter on the inner Halifax section. There are also pronounced topographic‐scale influences of submarine banks, basins, and cross‐shelf channels on the circulation, such as anticyclonic gyres over banks and cyclonic gyres over basins. Baroclinicity is the dominant forcing throughout the domain, but tidal rectification is comparable on the southwestern Scotian Shelf (e.g., about 0.2 Sv recirculating transport around Browns Bank for all the periods). The mean wind stress generates offshore surface drift in winter. The solutions are in approximate agreement with observed currents and transports over the Scotian Shelf, although there are local discrepancies.