The Response of Quasi-Geostrophic Oceanic Vortices to Tropical Cyclone Forcing

The response of quasi-geostrophic (QG) oceanic vortices to tropical cyclone (TC) forcing is investigated using an isopycnic ocean model. Idealized oceanic currents and wind fields derived from observational data acquired during hurricane Katrina are used to initialize this model. It is found that the upwelling response is a function of the curl of wind-driven acceleration of oceanic mixed layer (OML) currents rather than a function of the wind stress curl. Upwelling (downwelling) regimes prevail under the TC's eye as it translates over cyclonic (anticyclonic) QG vortices. OML cooling of ~1^oC occurs over anticyclones due to the combined effects of downwelling, instantaneous turbulent entrainment over the deep, warm water column (weak stratification) and vertical dispersion of near-inertial energy. By contrast, OML cooling of ~4^oC occurs over cyclones due to the combined effects of upwelling, instantaneous turbulent entrainment over regions of tight vertical thermal gradients (strong stratification) and trapping of near-inertial energy that enhances vertical shear and mixing at the OML base.

The rotational rate of the QG vortex affects the kinematics and dynamics of TC-forced near-inertial waves. As rotation is increased in both QG cyclones and anticyclones, the near-inertial response is shifted toward more energetic frequencies that enhance vertical shear and mixing. Horizontal Doppler shift gradients in QG vortices build up near-inertial vertical shear by rotating the wavenumber vector at differing rates at various depth levels. This process is accentuated in strong QG vortices, leading to enhanced mixing over the upper ocean, which reduces the amount of near-inertial energy available for mixing at depth. TC-induced temperature anomalies in QG vortices propagate westward with time, deforming the cold wake. Therefore, to accurately simulate the impact of TC-induced OML cooling and feedback mechanisms on storm intensity, coupled ocean-atmosphere TC models must resolve geostrophic ocean eddy location as well as thermal, density, and velocity structures.