The Spring 1998 Northeastern Gulf of Mexico (Negom) Cold Water Event: Remote Sensing Evidence for Upwelling and for Eastward Advection of Mississippi Water (Or: How an Errant Loop Current Anticyclone Took the Negom for a Spin)

Temperatures of coastal waters along the Florida Panhandle in the northeastern Gulf of Mexico were 2-6 C lower in May-July 1998 than is normal for these waters at this time of the year. Surface temperatures were up to 7 C colder than air temperatures measured nearby off Alabama at National Data Buoy Center (NDBC) buoy 42007. Satellite data from the Advanced Very High Resolution Radiometer, Sea-viewing Wide-Field-of-View Sensor, the Ocean Topography Experiment, and European Remote-sensing Satellites altimeter sensors were combined with wind, air temperature, and water temperature data gathered at NDBC buoys to document the anomalous wind, temperature, and river plume dispersal patterns. The difference between the water and air temperatures suggests that upwelling caused the cooling of coastal waters. Eastward, upwelling-favorable winds stronger than 3 m·s-1 occurred along the Alabama coast up to 32% of the time between May and August 1998 compared with only about 20-26% during the same months in 1996, 1997, and 1999. The altimeter data suggested that a large anti-cyclonic eddy migrated far into the northeastern Gulf of Mexico, interacting with shelf waters through August 1998. Upwelling was probably enhanced by the presence of baroclinic structure associated with the eddy. The eddy contributed also to convergence near the Mississippi River Delta, eastward flow along the coast, and offshore flow in the region between Fort Walton Beach and Cape San Blas, FL. The region experienced anomalously high river discharge during this period as a result of increased precipitation over northern Florida (a teleconnection to El Nino of 1997-1998), and further, turbid Mississippi River water dispersed eastward along the coast. Decay of local phytoplankton blooms off northern Florida in late June and early July 1998 probably followed after eastward winds relaxed, westward winds increased, and low-salinity waters led to vertical stability in the water column, possibly leading to hypoxia along these shores