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.