Near-surface horizontal convergence and dispersion near the tidal-mixing front on Northeastern Georges Bank

Horizontal dispersion and convergence are evaluated from near-surface drifter data collected during the summer and autumn of 1988 and summer of 1989 in the vicinity of the tidal-mixing front on northeastern Georges Bank. The effective horizontal diffusivity coefficient, Ke, estimated from the relative motion of the drifters over the duration of the cluster tracking, increases as the cluster length scale, with a range from approximately −100 to 450 m2 s−1 for length scales of approximately 5 to 58 km. At length scales<20 km, the magnitude of Ke is near zero with many negative estimates, indicative of convergence. At length scales >20 km, Ke increases in magnitude and scatter. Above 30 km length scales, Ke is typically between 200–400 m2 s−1. The dispersion as a function of length scale is compared to published estimates using apparent diffusivities, which are adjusted for the finite size of the drifter cluster at release. Georges Bank shows lower apparent diffusivities at length scales<20 km and higher ones at scales >20 km. The latter are believed to be principally due to the large horizontal gradients in the seasonal flow field, although other processes perhaps associated with the large tidal currents may also contribute. At the shorter length scales, the low dispersion is due to convergent processes. Examples of convergence at different locations and under different wind and stratification conditions are presented. Convergence near the tidal-mixing front tends to occur under light wind conditions and was observed in both seasons and in both years. During two intense southwestward windstorms, drifters also converged in the Bankʹs vertically well-mixed zone, apparently due to a combination of spatially varying Ekman depth and topographic effects.