Seasonal variation of the three‐dimensional residual circulation on Georges Bank

Abstract

The seasonal variation of the low‐frequency circulation in the Georges Bank region is studied numerically by computing six bimonthly circulation fields subject to forcing from the barotropic M2 tide, mean baroclinic pressure gradients, and mean wind stresses. The model is three dimensional, diagnostic, and nonlinear, with quadratic vertical eddy viscosity that is stratification‐dependent. Tidal forcing is imposed at the boundary of the domain, and an extensive set of density and wind data is used to determine climatological mean forcings. The magnitude of the M2 tidally rectified around‐bank velocities and transports is sensitive to stratification influences on the eddy viscosity, with up to a 50% intensification relative to computations which assume no influences. The dependence occurs through both the local advective tidal stress terms and the large‐scale barotropic pressure field. The six bimonthly solutions (January–February, March–April, May–June, July–August, September–October, November–December) indicate important contributions from tidal rectification, baroclinic pressure gradients, and mean wind stress to the seasonal intensification of the Georges Bank gyre. Tidal rectification and baroclinicity are the dominant forcings, while wind stress generates opposing around‐bank flow and cross‐bank surface drift in winter. Overall, the solutions are in approximate agreement with observed Eulerian around‐bank currents and transports, although there are both local and bank‐wide discrepancies. The seasonal intensification of recirculation around Georges Bank is well produced by the model, with estimates of the recirculating transport increasing from under 0.02 Sv (January–February) to over 0.13 Sv (September–October).