Evolution and structure of a coastal squirt off the Mississippi River delta: Northern Gulf of Mexico

Abstract

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 flowing toward the southwest from the Mississippi River delta region. Satellite 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‐speed (>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 spread laterally, yielding a mushroom‐shaped feature, 75 km wide, which consisted of counter‐rotating vortices. Northeasterly wind forcing (averaging 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 currents. 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 event demonstrates that strong northeasterly winds over the northern Gulf of Mexico can initiate along‐shelf and off‐shelf flows of cooler coastal waters, contributing significantly to seasonal cooling and freshening of the continental shelf and to shelf/slope exchanges of water. During this event, approximately 100 km3 of inner shelf and river water was transported off the continental shelf, a volume equivalent to 17% of the average annual discharge of the Mississippi and Atchafalaya Rivers.