Topic Area
Shelf Break Fronts
Project Team Member(s)
LTjg L. Stacey Lewis,
USNR
Major Findings
The shelf break regions of many continental shelves divide shelf and slope water and generally contain large gradients in temperature, salinity, density, and nutrients. Often, shelf break fronts form along the edge of these continental margins and act as a boundary between relatively cool, fresh shelf water and warm, saline slope water. They coincide with the sharp change in bottom slope and tend to occur where the continental slope is fairly wide and where along shelf mean flow is in the direction of propagation of free coastal trapped waves. One of the more distinguishing characteristics of these fronts is that they persist even in the presence of strong external forces such as surface wind stress and deep-ocean eddies. Many times, the front will weaken, but quickly reappear after the forcing mechanism ceases. These fronts are more or less stationary with their mean position being controlled almost entirely by the shelf break location. Some specific examples of regions with a shelf break front include the Middle Atlantic Bight (MAB), eastern Bering Sea, and Celtic Sea.
Frontal maintenance occurs via three means: (1) Tidal mixing/stirring (2) Heating/cooling (3) Topography. It is common to have a well-defined frontal position where the tidal mixing just balances the heat input. On one side of the front, tidal mixing is stronger and the water remains well mixed. On the other side, the mixing is weaker (and often the water is deeper) to the point that surface heating cannot be mixed and causes the water to become thermally stratified. The thermocline development also acts to isolate the surface and bottom water and the heat input stays at the surface. Generally, this results in a horizontal pressure gradient in which flow is established along the isobars rather than across them.
Along the continental shelf of the
MAB, strong cross-isobath flows were believed to carry shelf water into
the deep ocean due to the proximity of the Gulf Stream. An experiment to
test this theory took place over a period from 1990 to 1991. During this
time, there were two hydrographic surveys that included cross-shelf transects
from the Chesapeake Bay to Cape Hatteras, transects along the 1000m isobath,
and cross-shelf transects off of New Jersey. They observed sharp salinity
and density gradients near the foot of the front between 10-40m isobaths,
a strong baroclinic jet, and a strong cross-frontal density gradient in
the seasonal pycnocline off the coast of New Jersey. The 1990 Cape observations
included Gulf Stream-derived water present over much of the shelf south
of 36°
N, which was less dense than further north, and low salinity shelf water
over the upper slope. In 1991, the Gulf Stream water was over the upper
slope at 10-30m (the level of the seasonal pycnocline), but not at the
shelf. The shelf water was also denser than the Gulf Stream water over
the upper slope and had southward velocities of 0.2 m/s at 50m. From these
observations, it was determined that two processes were responsible for
shelf water losses to the upper slope. The first was that Gulf Stream blocking
of the shelf flow was impeding the southward flow of shelf water and forcing
it offshore. Secondly, lateral detrainment was believed to be occurring
in which the Gulf Stream was acting to enhance the lateral velocity shears
across the front and displace or weaken the baroclinic jet structure in
the upper 30m of the water column (as seen in '91 off New Jersey).
References
Gawarkiewicz, G., "Linear stability models of shelfbreak fronts ." J. Phys. Oceanogr., Vol. 21, 471-488, 1996.
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.
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.
| This is a government-maintained internet site. Please read the U.S. Navy web page disclaimer and the dislaimer regarding external links. |