A climatology of the shelfbreak front in the Middle Atlantic Bight

Abstract

Description of the shelfbreak front in the Middle Atlantic Bight is hampered by the extreme variability of the front. In order to gain insight into both the seasonal variability and regional variations in the mean frontal structure and associated baroclinic jet, historical data are used to produce two‐dimensional climatological fields of temperature and salinity for the region south of Nantucket shoals, along the south flank of Georges Bank, and off the coast of New Jersey. Associated cross‐shelf fields of density and geostrophic velocity are also computed. The climatological temperature and salinity are consistent with previous descriptions of the frontal hydrography. The temperature contrast across the front varies seasonally between 2° and 6°C. The salinity contrast is 1.5–2, with little seasonal variation. The near‐surface density gradients are strongest during the winter and weakest during the summer, when the seasonal thermocline is established. The cross‐frontal density gradients are strongest near the foot of the front. Despite the inherent smearing of frontal gradients incurred by averaging over large temporal and spatial scales, the geostrophic velocity field south of Nantucket shows a strong (0.2–0.3 m s−1) baroclinic jet associated with the frontal density gradients. The core of the jet, having a width of 15–20 km, is located near the 150‐m isobath. Transport calculations for the flow over the outer shelf and slope are in the range of 0.2–0.3 Sverdrups (Sv) to the west. This is comparable to the estimated transport (0.4 Sv) shoreward of the 100‐m isobath.