Loop Current Eddy Paths in the Western Gulf of Mexico

Abstract

The paths of anticyclonic Loop Current eddies in the western Gulf of Mexico have been investigated using ARGOS-tracked drifters accompanied by hydrographic surveys. The analysis used orbit parameters derived from a least square fit of a translating ellipse kinematic model and showed that paths from four quite different eddies had a number of similar features. They are a general increase in rotational period over time, clockwise rotation of ellipse axes that slows with time and often becomes stationary in the far western Gulf, swirl velocities that decay quite slowly, and a tendency of the eddies to have low divergence. In three cases, 20- to 30-day oscillations of the orbit parameters were observed. Translation velocities of the orbits showed the characteristic stalls and sprints that have been previously observed. In two cases, stalls and deviations from solid body rotation could be attributed to the presence of vigorous lower continental slope cyclones situated to the northwest of the eddies in question. Comparison of relative vorticity, calculated from orbits and hydrography, showed reasonable agreement though deviations from solid body rotation were found in all cases. Vorticity also remained fairly constant for the 3?6-month periods investigated. Major perturbations were tentatively attributed to absorption of weaker, older Loop Current anticyclones in the western Gulf. Statistics on eddy characteristics, derived from drifters, were compiled for 10 eddies between 1985 and 1995. The paths were separated into two, east and west of 94°W, corresponding to the deep western basin and under the influence of the steep western slope, respectively. The paths occupied a broad band of 2°?3° latitude in width in the center of the basin with a mean west-southwest trend. There were no apparent preferred paths either in the main basin or near the western slope where eddies were equally likely to move northward or southward along the boundary. Eddy paths also showed frequent occurrences of 20- to 30-day anticyclonic perturbations similar to that found from the individual analyses.