SEASONAL-VARIATION OF THE 3-DIMENSIONAL RESIDUAL CIRCULATION ON GEORGES BANK

The seasonal variation of the low-frequency circulation in the Georges Bank region is studied numerically by computing six bimonthly circula tion fields subject to forcing from the barotropic M2 tide, mean baroc linic pressure gradients, and mean wind stresses. The model is three d imensional, diagnostic, and nonlinear, with quadratic vertical eddy vi scosity that is stratification-dependent. Tidal forcing is imposed at the boundary of the domain, and an extensive set of density and wind d ata is used to determine climatological mean forcings. The magnitude o f the M2 tidally rectified around-bank velocities and transports is se nsitive to stratification influences on the eddy viscosity, with up to a 50% intensification relative to computations which assume no influe nces. The dependence occurs through both the local advective tidal str ess terms and the large-scale barotropic pressure field. The six bimon thly solutions (January-February, March-April, May-June, July-August, September-October, November-December) indicate important contributions from tidal rectification, baroclinic pressure gradients, and mean win d stress to the seasonal intensification of the Georges Bank gyre. Tid al rectification and baroclinicity are the dominant forcings, while wi nd stress generates opposing around-bank flow and cross-bank surface d rift in winter. Overall, the solutions are in approximate agreement wi th observed Eulerian around-bank currents and transports, although the re are both local and bank-wide discrepancies. The seasonal intensific ation of recirculation around Georges Bank is well produced by the mod el, with estimates of the recirculating transport increasing from unde r 0.02 Sv (January-February) to over 0.13 Sv (September-October).