A climatology of the Middle Atlantic Bight shelfbreak front

Abstract

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.