2006 Ocean Sciences Meeting SEACOOS Poster - Cold event in the South Atlantic Bight during summer of 2003: Model simulations and implications

For the program on "Coastal Upwelling: Coevolution of Physical, Chemical, and Biological Theory and Practices II Posters," OS36J-07. A series of interdisciplinary studies, primarily from the mid-1970's to mid-1980's, characterized the major features of upwelling along the shelf break of the South Atlantic Bight (SAB) off the SE U.S. (including physical mechanisms and biological responses). During the summer of 2003, unusually cold subsurface seawater temperatures were observed over much of the mid-to-outer SAB continental shelf. Cold coastal seawater anomalies were also observed from the East Florida shelf to the Mid-Atlantic Bight. Unusually persistent upwelling favorable wind along the entire U.S. Atlantic seaboard was a major factor contributing to upwelling and widespread cold water anomalies. The present study investigates additional factors that may have contributed to an unusually intense upwelling response in the central SAB off Georgia. Comparison of T-S characteristics of the subsurface SAB shelf water in mid-summer 2003 with shelf and Gulf Stream climatology indicated that the upwelled water originated from depths greater than 200 meters in the Gulf Stream. The contribution of saline and thermal stratification in late spring and summer to the strength of the upwelling and shoreward extent of the cold subsurface intrusion is proposed as one additional mechanism resulting in enhanced upwelling. The enhanced response was produced by feedback mechanisms associated with a reduction of Ekman depth and an increase of the off-shore velocity in the upper layer. In this study, a set of model simulations is used to determine the relative importance of the principal forcing mechanisms during 2003. Updated mass field boundary conditions from basin-scale HYCOM simulations are tested to provide off-shore and upstream influences in a one-way nesting sense. Model skill is estimated by comparing model results with observations of velocity, water level and surface and bottom temperature. The model simulations show the intrusion of cold water onto the shelf occurred in the near-bottom portion of the water column along the shelf-break, especially south of 32 N. Nitrate flux onto the shelf is estimated using temperature-nitrate relationships to evaluate the potential response in primary production. *A L Aretxabaleta, J R Nelson, H E Seim, B O Blanton, F E Werner, E P Chassignet