Eddies and Jets Over the Slope of the Northeast Gulf of Mexico

Abstract

The eddy circulation over the northeastern Gulf of Mexico slope and the DeSoto canyon has been characterized using data from a 13 mooring array and 7 hydrographic surveys over a period of two years. The presence of cyclones and anticyclones over the lower Mississippi-Alabama slope is strongly influenced by the positions of cyclonic frontal eddies on the Loop Current or a Loop Current warm eddy to the south of the study region. Both anticyclones and cyclones feed an eastward surface jet on the upper slope that can follow the rim of the canyon or meander across the slope. Since this eastward jet often transports part of the discharge of the Mississippi delta, it provides a mechanism to transfer nutrient-rich, river-derived water to offshore or, on occasion, to the outer west Florida shelf. A westward countercurrent, between 200 and 500 m depth against the Alabama slope, often accompanies this surface jet. This usually occurs in conjunction with a cyclone over the slope of the west Florida terrace. The eastward jet and its countercurrent are replaced by westward cyclonic flow if a Loop Current frontal eddy extends northwards onto the middle and upper slope. A time scale for the dominant eddies is of order 100 days, with eddy diameters and swirl speeds of order 100-150 km and 50 cm/s, respectively. Over the head of the canyon smaller scale cyclones and anticyclones occur with shorter time scales (∼ 20 days), and their interaction with the Alabama slope eddies can cause strong cross-slope currents, which are an effective mechanism for the transport of material on and off the shelf. The predominance of the eastward jet and anticyclonic currents promotes upwelling at the head of the canyon and along the shelf break of the west Florida shelf, where the water is ∼ 2° C colder on average than further west.