Springtime hydrography of the southern Middle Atlantic Bight and the onset of seasonal stratification

The Ocean Margins Program was a major multi-disciplinary observational effort in the southern Middle Atlantic Bight (MAB), focusing on the transformation, retention, and export of biogenic materials from the shelf. The observational effort peaked in the spring and summer of 1996 with four ship-based surveys, augmented by an array of 26 moorings supporting 126 temperature and 118 salinity sensors. The data from the cruises and moored array are used to describe how the springtime hydrographic evolution takes place in an area in which five water masses locally vie for dominance and that is subject to strong wind stress, heat flux, and offshore forcing. The results show that the region is subject to large-scale intrusions from both the north and south, which materially affect the timing and development of stratified conditions. The intrusions from the north are wind driven and provide cold, moderately saline, unstratifed water, delaying the development of stratified conditions. Intrusions also occur from the south, where warmer and generally more saline waters from the South Atlantic Bight are driven into the area by alongshore winds and/or intrusions of Gulf Stream waters pushed shoreward by Gulf Stream frontal eddies. In 1996, an intrusion of saline water from the south, combined with a reversal of the alongshore winds from the north, slowed the southward flow of cold MAB water, and subsequently caused low-salinity Virginia Coastal Waters to spread out from the coastal plume over the denser water from the north. With the reduced alongshelf flow and an initial stratification provided by the low-salinity coastal water, solar insolation and sensible heat fluxes were then able to warm the surface layer, permanently establishing the seasonal thermocline/halocline for the area not directly impacted by intrusions from the Gulf Stream and South Atlantic Bight.