Publication Date:
2022-05-25
Description:
Submitted in partial fulfillment of the requirements for the degree of Master of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1996
Description:
Description of the shelfbreak front in the Middle Atlantic Bight is hampered by the
extreme variability of the front. In order to gain more insight into the mean frontal structure
and associated baroclinic jet, historical data is used to produce two dimensional climatological
fields of temperature and salinity for the region south of Nantucket shoals.
Associated cross-shelf fields of density, geostrophic velocity, relative vorticity, and shallow
water potential vorticity have also been computed. Historical data from a quality-controlled
database (HydroBase) in the region 69-72°W, 39.5-41°N is included. Cross-shelf
sections are obtained by averaging the data in nine depth bins with an average cross-shelf
spacing of 10 km but an increased resolution of 4 km near the shelfbreak. The vertical
averaging interval was 10 m over the shelf and upper slope waters, increasing to 50 m in
the deep slope waters. The data were averaged in bimonthly periods to study seasonal
trends. For inter-regional comparison, similar analyses were performed for the south flank
of Georges Bank and the shelf off New Jersey.
The climatological temperature and salinity are consistent with previous descriptions of
the frontal hydrography (e.g. Wright [1976], Beardsley and Flagg [1976], and Flagg
[1987]). Most importantly, features such as the "cold pool", the upper slope pycnostad,
and the frontal boundary are well resolved when compared with synoptic sections. The
temperature contrast across the front varies seasonally between 2-6°C near the surface
and at depths of 45-65 m. The salinity contrast is 1.5-2 PSS, with little seasonal variation.
The resulting cross-frontal near surface density gradients are strongest during the winter
and weakest during the summer, when the seasonal thermocline is established. The crossfrontal
density gradients are stronger near the bottom outcrop of the front, consistent with
previous modeling studies [Gawarkiewicz and Chapman, 1992].
Despite the inherent smearing of frontal gradients incurred by averaging over large temporal
and spatial scales, the geostrophic velocity field shows a strong (20-30 cm s-1) baroclinic
jet associated with the cross-frontal density gradients. The core of the jet, having a
width of 15-20 km, is located between the 100-120 m isobaths. The core of the jet is well
shoreward of the surface expression of the front, resulting from strong density gradients at
the foot of the front. The horizontal velocity shear on the cyclonic, offshore edge of the front is roughly 0.2-0.4 * 10-4 s-1, with shears on the anticyclonic, offshore edge of the jet
being half as large. The potential vorticity structure is drastically affected by the seasonal
pycnocline during the summer but remains relatively uniform during the winter months.
Comparisons of the mean fields from Georges Bank, Nantucket Shoals, and New Jersey
show that the foot of the front shoals as the flow progresses to the southwest. The seasonal
migration of the frontal boundary experiences a phase shift consistent with an alongshelf
propagation of minimum salinities to the southwest.
Finally, transport calculations for the flow over the outer shelf and slope give values in the
range of 0.1-0.6 Sv to the west. This is comparable to the estimated transport shoreward of
the 100 m isobath of 0.38 Sv [Beardsley et al., 1985], which suggests that the shelfbreak
frontal jet may be an extremely important element in the alongshelf transport of fresh
water in this region.
Keywords:
Climatology
;
Continental shelf
Repository Name:
Woods Hole Open Access Server
Type:
Thesis
Format:
application/pdf
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