Eddy statistics from Lagrangian drifters and hydrography for the northern Gulf of Mexico slope

Cyclones and anticyclones on the northern Gulf of Mexico slope have been statistically characterized using upper layer drifter orbits and repeated hydrographic surveys. A total of 243 slope eddy drifter orbits (151 cyclones and 92 anticyclones) were analyzed in terms of size (mean radius and eccentricity) and rotation period. Orbits in large Loop Current anticyclones were excluded. Eddies of both signs scale approximately as the baroclinic radius of deformation with relatively smaller radii on the upper slope, where water depths are less than 1000 m. The median radius for all eddies was ∼30 km, and median periods were 12 and 13.5 days for cyclones and anticyclones, respectively. Anticyclones also tend to be more circular than cyclones. Relative vorticity maps were constructed for seven hydrographic surveys, which mapped the slope at 4–5 month intervals. A total of 26 cyclones and 15 anticyclones had statistics that were consistent with drifter orbit analyses. The surveys indicate that at any one time, between 2 and 5 cyclones and 1 and 3 anticyclones may be present. Both drifters and surveys indicate that cyclones have a slight numerical dominance.