EVOLUTION AND STRUCTURE OF A COASTAL SQUIRT OFF THE MISSISSIPPI RIVERDELTA - NORTHERN GULF-OF-MEXICO

In early October 1992, satellite-derived sea surface temperature data revealed a 200 km long and 10 to 30-km-wide stream of cool water flowi ng toward the southwest from the Mississippi River delta region. Satel lite imagery and in situ measurements have enabled a detailed study of the squirt's kinematics and subsurface characteristics over a 2-week period. In its early stages, the squirt appeared as a narrow, high-spe ed (> 75 cm/s)jet of water which flowed westward over the Mississippi Canyon, forcing a semi-submersible drilling rig to suspend operations from October 2 to 4. After crossing back onto the shelf, the squirt sp read laterally, yielding a mushroom-shaped feature, 75 inn wide, which consisted of counter-rotating vortices. Northeasterly wind forcing (a veraging 10-15 m/s) and water level setup east of the delta appear to have been the primary mechanisms for evolution of the high-velocity cu rrents. Satellite and in situ measurements demonstrate that the dipole eddy was comprised of a cool low-salinity, low-density water mass at least 26 m deep in the center and 16 m deep along its margins. This ev ent demonstrates that strong northeasterly winds over the northern Gul f of Mexico can initiate along-shelf and off-shelf flows of cooler coa stal waters, contributing significantly to seasonal cooling and freshe ning of the continental shelf and to shelf/slope exchanges of water. D uring this event, approximately 100 km(3) of inner shelf and river wat er was transported off the continental shelf, a volume equivalent to 1 7% of the average annual discharge of the Mississippi and Atchafalaya Rivers.