The Onshore Transport of Cold Water During an Upwelling Regime on the New Jersey Shelf

A comprehensive dataset obtained in summer of 1996 on the New Jersey shelf off Atlantic City is analyzed to determine the pathway for cold water during a period of sustained upwelling. The data include shipboard CTD/ADCP surveys, time series from three across-shelf lines of moorings and remote sensing. An upwelling event that occurred from June 30 through July 11, 1996, is studied. The event comprised three stages: first, the cold water was upwelled through the bottom Ekman layer; second, the onshore flow concentrated in the pycnocline; third, mesoscale eddy-like features developed. The first gust of upwelling-favorable wind that lasted approximately 2 days generated a northward alongshelf current through the whole water column with corresponding onshore Ekman transport in the bottom mixed layer. After that, an alongshelf pressure gradient was set, which forced alongshelf flow in the opposite direction compared to the wind stress. As a result, southward flow was observed near the bottom until the end of upwelling cycle prohibiting onshore Ekman transport. The onshore flow was maintained through the pycnocline, with the maximum velocities at 10-12 m depth. At the same time, the temperature anomaly transport (pathway for colder water) was centered at 14-17 m depth that corresponded to the lower part of the pycnocline. This onshore flow was primarily balanced by alongshelf pressure gradient while the acceleration of alongshelf current was less important. Approximately six days after the onset of upwelling, mesoscale currents began to dominate the study area, thus establishing three-dimensional flow field with spatially localized (in alongshelf direction) onshore currents. In particular, the cyclone of approximately 25 km in diameter occupied the northern part of the study area.