Variability of currents in late spring in the northern Great South Channel

The residual flows computed from detided shipboard ADCP data collected in late spring 1988 and 1989 clearly show different circulation patterns in the near-surface and deeper regions in the Great South Channel (GSC). In the upper 50 m, the residual flow in the northern GSC consists of three principal currents: (1) a southward coastal current located along the western flank of the GSC; (2) a broad cyclonic circulation crudely following the local topography in the interior region of the northern GSC; and (3) a northeastward current along the western flank of Georges Bank. Below 50 m, the residual flow tends to be cyclonic along the local 100-m isobath in the northern GSC. These circulation patterns are consistent with the vertical distributions of water properties and the trajectories of satellite-tracked drifters drogued at 5 and 50 m. Comparisons with geostrophic current shears and numerical model results suggest that the residual flow in spring is driven primarily by tidal rectification over the shallower sides of the northern GSC and by buoyancy forcing over the deeper flanks of the GSC. The southward transports of low-salinity plume surface water and Maine Intermediate Water (MIW) were about 0.07 ± 0.03 Sv and 0.31 ± 0.38 Sv in April 1988 and about 0.12 ± 0.06 Sv and 0.66 ± 0.14 Sv in June 1989. The larger transports of low-salinity plume water and MIW found in June 1989 are believed to be due to the increased freshwater river discharge in 1989 and occurrence of a subsurface coastal jet current along the western flank of the GSC.