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LT Kelly Taylor, USN

The shelf break frontal boundary separating cool, fresh water over the continental shelf and the warm, salty water of the continental slope is a constant feature of the Mid-Atlantic Bight.  The circulation in the region of the Mid-Atlantic Bight is southwest flow over the continental shelf from Georges Bank, past Montauk, New York, and finally turning offshore at Cape Hatteras, where it is drawn into the Gulf Stream.  The average offshore distance of the continental shelf break is 50-200 km offshore, coinciding with the location of the 100m isobath.  There are many methods for studying the frontal position: climatology, hydrographic surveys, moored arrays, and satellite imagery can be used to determine its location.  In all studies, there is a seasonal progression of the front, seen best in the temperature variability.  In the winter, temperatures over the shelf are homogenous due to increased vertical mixing, and range from 4-6oC.  This creates a very strong and narrow temperature gradient.  During this time the surface position of the front moves offshore.  In the summer, temperatures over the shelf and slope are strongly stratified.  This is due both to a decrease in wind stress and hence, vertical mixing, and increased solar insolation.  The salinity fields in the region of the front vary little seasonally, thus the density field tends to be similar to the temperature fields.  As the flow progresses to the southwest, the foot of the front shoals, most likely due to the shallower average shelf break depth to the south.  The frontal slope also changes seasonally, and two separate slopes are seen: a relatively constant slope in the lower half of the water column and a seasonally variable slope in the upper half of the water column.  Flow over the shelf itself is predominantly barotropic, due to weak horizontal density stratification.  There is a mean westward shelf flow of greater than 0.05 m/s to the west during both the summer and winter.  The dominant feature associated with the front is the baroclinic frontal jet, which can reach maximum velocities of 0.5 m/s.  The jet, which arises from differences between densities of the shelf and slope water, is continuous from south of Georges Bank to the region just north of Cape Hatteras.  Transport associated with the frontal jet is synoptically significant for this area. The differences in density between the shelf and slope water also drive geostrophically balanced offshore flows in the region of the front.  Onshore flow of magnitude 0.01 m/s has been detected within the bottom boundary layer at the foot of the front.  The foot of the front also undergoes fairly large horizontal migrations across the shelf break, indicating frontal motion at the foot can be as vigorous as the surface.  The area of the front acts as a barrier between the shelf water and the slope water, but can be influenced by warm core rings and the Gulf Stream.  There are instances, supported by satellite imagery, where warm core rings have pulled filaments of cold shelf water out over the slope.

Barth, J.A., D. Bogucki, S. D. Pierce, and P. M. Kosro, Secondary circulation associated with a shelfbreak front, Geophys. Res. Lett., 25, 2761-2764, 1998.

Gawarkiewicz, G., T. G. Ferdelman, T. M. Church, and G. W. Luther III, Shelf break frontal structure on the continental shelf north of Cape Hatteras, Cont. Shelf Res., 16, 1751-1773, 1996a.

Houghton, R. W., and M. Visbeck, Upwelling and convergence in the Middle Atlantic Bight shelfbreak front, Geophys. Res. Lett., 25, 2765-2768, 1998.

Houghton, R. W., F. Aikman III, and H. W. Ou, Shelf-slope frontal structure and cross-shelf exchange at the New England shelf break, Cont. Shelf. Res., 8, 687-710, 1988.

Linder, C. A., and G. Gawarkiewicz, A climatology of the shelfbreak front in the Middle Atlantic Bight, J. Geophys. Res., 103, 18405-18424, 1998.
 

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