Simulating the baroclinic ocean current response to a tropical cyclone

The response of the ocean mixed layer embedded in a pre-existing baroclinic current is examined using an idealized numerical model. The ocean is initialized based on observations of mass density structure within, and in the periphery of, the Gulf of Mexico Loop Current. On the warm (light) side of the current, a deep mixed layer with weak thermocline stratification is observed, while to the cold (heavy) side, the mixed layer is shallower and stratification greater. Separating these two water masses is a vertically-sheared current jet with maximum balanced geostrophic velocity at the surface. The ocean is modeled in a multi-layered reduced gravity framework, whereby the top layer absorbs wind energy from a hurricane-like vortex, and transmits this energy downward via entrainment mixing across the base of the mixed layer, as well as internal wave energy emission through inertial pumping.

Results indicate mixed-layer kinetic energy excited by the storm is rapidly attenuated within the current relative to the initially homogeneous case, which is consistent with observations in Hurricane Lili (2002). Specifically, mixed-layer energy is reduced to background levels less than 2 inertial periods after storm passage. Both kinetic energy advection and pressure work by the near-inertial (ageostrophic) current are responsible for the rapid decay at the point of peak surface wind forcing. Additionally, it is shown that shear stresses at the mixed layer base are relatively reduced in these current systems, which may modulate ocean cooling, and ultimately, thermal feedback to a cyclone. These results have implications for ascertaining surface energy exchange in tropical cyclones within current systems using budget methods, as the currents will quickly remove this storm-generated energy.