Upwelling and mixed layer deepening in mesoscale oceanic eddies during the passage of tropical cyclones

Tropical cyclones in the Gulf of Mexico often propagate over the Loop Current (LC), and warm and cold core eddies (WCE and CCE, respectively). These robust mesoscale oceanic features are present at any time, and they often impact the oceanic mixed layer (OML) thermal response to tropical cyclones (TC) and feedback mechanisms to storm intensity. For example, observational data acquired during the passage of TCs Katrina and Rita over the eastern Gulf indicate that both storms rapidly intensified over warm, anticyclonically rotating oceanic features (LC and WCEs) where the OML cooling response was reduced (ΔT < 1°C). By contrast, they rapidly weakened over cyclonically rotating CCEs where OML cooling was enhanced (ΔT ~ 4.5°C).

In this context, the goal of this paper is to delineate the contrasting thermal response to TCs in mesoscale oceanic eddies during the forced stage (relatively short stage where the TC is overhead). To this end, the TC-induced upwelling is delineated in presence of very rapidly rotating mesoscale oceanic eddies (cyclones and anticyclones), based on data acquired in the LC system during the passage of Katrina and Rita, theoretical arguments, and numerical experiments. Moreover, the isopycnic ocean model used here (MICOM) includes a turbulence closure for the OML that is used to investigate the OML deepening due to wind erosion in both WCEs and CCEs.

Observational data, theoretical predictions, and numerical experiments indicate that the wind-driven horizontal current divergence under the TC's eye is a function of the underlying geostrophic relative vorticity. Upwelling (downwelling) regimes develop when the wind stress vector is with (against) the geostrophic OML velocity vector. The reduced cooling of less than 1°C in WCEs is a consequence that the interaction of the wind stress and the OML geostrophic flow produces horizontal convergence of warm water under the storm's eye, and wind erosion occurs over a warm, deep, and nearly homogeneous water column. By contrast, in CCEs, the increased OML cooling of more than 3°C results because the interaction of the wind stress and the OML geostrophic currents drives upwelling of cold water under the storm's eye, and wind erosion takes place over a shallow OML, and over a near-surface water column with sharp vertical thermal gradients. From a broader perspective, numerical models must include mesoscale oceanic features to reproduce the contrasting thermal response to atmospheric forcing, and feedback mechanisms to atmospheric processes.