Georges Bank residual circulation during weak and strong stratification periods: Prognostic numerical model results

The seasonal extremes of the three-dimensional residual circulation on Georges Bank are studied by numerically computing the circulation during weakly (March-April) and strongly (July-August) stratified periods. The computations are performed using a prognostic (i.e., including tidal-time advection of the density field), free surface, nonlinear, finite element model on realistic Georges Bank topography. The dynamics considered result from the dominant M(2) tide, baroclinic pressure gradients, wind stress, upstream boundary conditions, and advanced turbulence closure. Comparison of these solutions with earlier diagnostic model results illustrates the importance of processes which could not be included in these earlier simulations (such as density evolution and advanced turbulence closure in tidal time, inclusion of nonlinear baroclinic effects, improved treatment of other nonlinearities, etc.). During the strongly stratified July-August period, the newly modeled processes intensify the residual circulation and result in recirculating transport on Georges Bank of 0.25 Sv, which is twice the diagnostic result. The level of agreement between modeled and observed residual circulation improves very significantly for this period, with the prognostic model predicting around-bank currents as high as 46 cm s(-1) in the frontal zone on the Northeast Peak. Analysis of the tidal-time density and current structure in the frontal zone on the Northeast Peak indicates important tidal frequency dynamics and features which are supported by observational evidence. During the weakly stratified March-April period, the dynamical impact of the newly modeled processes is small. Hence the prognostic and diagnostic models predict similar global structure (with a recirculating transport on Georges Bank of 0.05 Sv), although there are local differences which result in the prognostic simulation agreeing more closely with the observed circulation.