Abstract
Model study of the cross-frontal water exchange on Georges Bank: A three-dimensional Lagrangian experiment
Model study of the cross-frontal water exchange on Georges Bank: A three-dimensional Lagrangian experiment
Changsheng Chen
School for Marine Science and Technology, University of Massachusetts-Dartmouth, New Bedford, Massachusetts, USA
Qichun Xu
School for Marine Science and Technology, University of Massachusetts-Dartmouth, New Bedford, Massachusetts, USA
Robert C. Beardsley
Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA
Peter J. S. Franks
Marine Life Research Group, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California,
USA
Characteristics of the cross-frontal water exchange over Georges Bank are investigated through a sequence of numerical model
experiments featuring realistic three-dimensional (3-D) bathmetry, bi-monthly averaged climatological stratification, tidal
forcing, and mean and observed wind-forcing. The model used in this study is the ECOM-si version of the
Blumberg and Mellor [1987]
primitive equation model with
Mellor-Yamada [1982]
level 2.5 turbulence closure. The model domain includes Georges Bank and the Gulf of Maine, and is forced at the open ocean
boundary by the semidurnal M2 tide. Fluid particles were tracked in the 3-D, time-dependent Eulerian flow field to examine the Lagrangian flow field, and
passive tracer experiments were conducted to investigate the relative roles of advection and turbulent diffusion on cross-frontal
exchange. Two distinct paths for the on-bank water movement were detected on Georges Bank: one is over the northwestern flank
of the bank where the bottom topography changes sharply in both along- and cross-bank directions, and the other is near the
bottom around the bank where the tidal mixing front is located. Over the northern flank, the cross-bank component of the Lagrangian
residual current is generally opposite in direction to that of the Eulerian residual current, resulting in an on-bank, cross-frontal
water transport near the bottom. Over the southern flank, the near-bottom water tends to converge toward the tidal mixing
and shelf-break fronts, so that the near-bottom flow over the shelf between these two fronts is divergent. The response to
wind-forcing varied with ambient stratification and water depth. In winter, strong winds can drive a significant off-bank
water transport, tending to “wash out” the bank. In summer, winds are generally too weak to alter the general pattern of tidal-driven
particle motion within the mixed region on the crest of the bank and the surrounding tidal mixing front. Some wind-driven
off-bank transport occurs near the surface in the stratified region on the outer southern flank, but this has little influence
on water movement near the bottom. Passive tracer experiments reveal that the net cross-frontal water flux near the bottom
is caused primarily by advection and horizontal diffusion. Tidal-induced vertical diffusion tends to make the tracer mix rapidly
upward, thus reducing the percent of the cross-frontal flux due to advection. Our new model results are consistent with previous
model studies of Georges Bank, and agree in general with drifter and dye patches observed in the U.S. GLOBEC NW Atlantic/Georges
Bank program.
Received 21
April
2000;
accepted 6
November
2002;
published 13
May
2003.
Citation: Chen, C., Q. Xu, R. C. Beardsley, and P. J. S. Franks
(2003),
Model study of the cross-frontal water exchange on Georges Bank: A three-dimensional Lagrangian experiment,
J. Geophys. Res.,
108(C5),
3142,
doi:10.1029/2000JC000390.