Deep Sub-Surface Eddies in the Gulf of Mexico

The study is focused on the analysis of the kinematic structure and dynamics of eddy currents that develop in sub-surface layers of the Gulf of Mexico (GOM); that is, eddy currents that do not have a clear surface signature. Such eddies have a weak surface expression and therefore are difficult to detect and monitor based on remote sensing data. Although in the past studies of such eddies received certain attention, our understanding of their characteristics, generation mechanisms, and dynamics remains poor, due to the lack of direct measurements and process-oriented modeling studies. In our study, we use publicly available current profile data, collected in the GOM to meet the requirements of data collection associated with the regulations imposed by the US government, in combination with the surface drifter data used to describe the oceanographic context behind the measurements and results from the HYCOM, Mercator, and TOPS hydrodynamic models. The results of our analyses show that, for the most part, these sub-surface events appear to correspond with the presence of cyclonic eddies. The surface signature of the eddies is usually weak, but the near-surface drifting buoys do pick up the circulation. Also, in some cases, the maps of surface currents reveal the presence of a Loop Current Eddy in the vicinity of a deep sub-surface flow, suggesting frontal instabilities that lead to the development of meso-scale (mostly cold-core) features along frontal boundaries as a plausible eddy generation mechanism. Our comparison of model data with observations, for the periods of the occurrence of deep subsurface flows, did not reveal any signatures of deep sub-surface flows in the outputs of the hydrodynamic models. Moreover, screening of the model data for the presence of deep sub-surface flows with a weak surface expression did not turn up even a single such case, suggesting that the models do not have the right physics to resolve deep subsurface flows. The latter result emphasizes the timeliness of the study for a better understanding of flow dynamics in the GOM.