Observation of Shelfwater Overrunning the Southern Slope Sea
Analyses of two years (1992 and 1993) of high resolution (1.47 km2) sea surface temperature satellite images of the southern Mid-Atlantic Bight (MAB), showed that unusually extensive overhang of shelf water occurs episodically, and coherently over along shelf distances of several hundred kilometers. These episodes are dubbed overrunning of the Slope Sea by shelf water. The overrunning volume has a "face" and a "back" (southern and northern limit). It transports substantial quantities of shelf water southward, and does not retreat onto the shelf, but eventually joins the western edge of the Gulf Stream in the vicinity of Chesapeake Bay. The correlation between satellite and in situ surface temperature, the location of shelf-slope front, and the relation of surface temperature to the deeper water mass were explored in order to quantify the overrunning episodes. The satellite extracted temperature profile agreed well with in situ surface temperature and coincident Expendable Bathythermographs (XBT) transects. The combined analyses of satellite imagery and various in situ data further demonstrated that the shelf water that over-run the Slope Sea are not mere surface features but can reach depths between 40 ∼ 60 m. Results confirm previous concepts on shelf circulation, shelf-slope exchange, fate of shelf water. They also sheds new light on shelfwater budget: overrunning of Slope Sea and southwest transport by upper slope current constitutes an important conduit for shelf water transport. Further analyses of potential causes for the overrunning events showed that wind stress due to winter storms moved the shelf-slope front and with it shelf water offshore to distances ∼10 to 40 km. The offshore displacement of shelf water also appeared to be related to the onshore veering of the Gulf Stream near Cape Hatteras. This veering of the Stream caused sea level to rise near Cape Hatteras producing a blocking effect on the shelf circulation. Such a blocking effect of the southwestward flow of shelf water in the MAB appeared to be the reason for the overruning of shelf water off New Jersey. In addition, the excess fresh water discharge from the St. Lawerence is also observed to be related to the flooding of shelf water off New Jersey.
Remote sensing, Slope Sea, Shelfwater, Geophysics and Seismology, Oceanography
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