On the structure and motions of cyclones in the northern Gulf of Mexico

Abstract

The eddy field on the northern slope of the Gulf of Mexico has been investigated using high‐resolution AXBT and AXCP aircraft surveys and ARGOS tracked drifters. A number of vigorous cyclones were mapped. Their vertical structures showed solid body rotation with radii of 50–75 km with a subsurface maximum (∼40 cm s−1) at the outer edge at about 150 m. Flows had little shear in the upper 400 m and only weak currents below depths of 800 to 1000 m. Cyclones were observed to move on and off the slope under the influence of swirl velocities of larger warm‐core eddies, including Loop Current (LC) anticyclones. Rotational periods range from about 7 to 18 days. The mapped eddy fields contained both cyclones and anticyclones with a range of diameters from about 40 to 150 km. The presence of many relatively small‐scale features along the upper slope strongly affects features of the shelf break velocity field that were measured by a moored current meter array. The directions and vertical shears of the moored currents were consistent with flows caused by the northern edges of cyclones and anticyclones. These eddies can cause persistent events and strong cross‐isobath fluctuations of the currents. Counterrotating pairs of eddies on the upper slope seem to be a mechanism for the transport of material on and off the Louisiana and east Texas shelves. Coherences between moorings, 74 km apart, were generally not significant, as might be expected when the currents are dominated by small‐scale eddies. However, an EOF analysis of the shelf break array indicates that about 40% of the along‐slope current variances could be attributed to westward propagating continental shelf waves.