Modeling studies of the cross-frontal water exchange on Georges Bank: A 3-D lagrangian experiment

C. Chen, Q. Xu , R. C. Beardsley, P. J. S. Franks , and R. Schlitz

Characteristics of the cross-frontal water exchange over Georges Bank were examined by tracking the particle trajectories in the 3D flow field predicted prognostically by the primitive equation and turbulent closure model under the initial condition of bi-monthly climatological stratification. The effects of averaged and variable winds on particle trajectories in summer and winter also were studied based on the 1995 wind measurement taken on the southern flank buoy. The model revealed two distinct paths of the on-bank, cross-frontal water movement. One was at the northwestern flank of Georges Bank where the bottom topography changed sharply in both along and cross-bank directions, and the other was near the bottom around the bank where the tidal mixing front was located. On the northern flank, the cross-bank component of the Lagrangian residual current is generally opposite in direction to that of the Eulerian residual current, which led to cause a significant on-bank, cross-frontal water transport near the bottom. On the southern flank, the near-bottom water tended to converge toward tidal mixing and shelf-break fronts, respectively, which caused a divergence zone on the outer flank closed to the shelf break.

The response of the flow to the wind varied with water depth. In winter, the strong wind tended to cause a significant off-bank water transport, and hence wash out Georges Bank. In summer, the wind was too weak to alter the general pattern of tidal-driven particle trajectories within the mixed region and at the tidal mixing front. The wind tended to cause a remarkable off-bank water transport near the surface in the stratified region on the outer southern flank but has little influences on the water movement near the bottom. A relatively large off-bank water transport was found in the case with variable wind than in the case with averaged wind, although total inputs of the wind momentum for both the cases were the same.

A semi-analytical model was developed to examine the influences of the bottom slope on the particle trajectories. For given tidal and residual flow fields, the model revealed that the direction of tidal-cycle residual particle trajectories was sensitive to the bottom slope. It could be opposite to the direction of the Eulerian residual flow as the bottom slope becomes steep. This result supports our finding of particle trajectory pattern on the northern flank. The model results were consistent with previous modeling experiments on Georges Bank and also were in good agreement with the US GLOBEC Lagrangian drifter measurements.