Analysis of a resonant-like regime in the oceanic mixed layer induced by a hurricane

The impact of an idealized hurricane on the ocean upper layer dynamics and thermodynamics is studied by using an 1.5-layer ocean model adapted from Gaspar (1988). This is an integral model derived from the vertically integrated full-primitive equations system over the mixed-layer depth. The turbulent entrainment parameterization at the mixed-layer base has been updated for hurricane wind conditions in order to take into account the mixing source associated with the vertical shear of the horizontal current.

The experimental conditions simulate a wide (20° x 10°) oceanic domain forced by an idealized hurricane. A Holland (1980) wind profile with a maximum wind speed of 50 m/s and a maximum wind radius of 40 km is used for the vortex. Its trajectory is along a constant latitude (30°N) and is covered at constant speed. The ocean is initially uniform, at rest and forced by the surface fluxes induced by the hurricane.

The model simulates realistic mixed layer deepening and cooling rates, and particularly the asymmetric cooling and deepening on each side of the trajectory. Those characteristics are qualitatively consistent with previous studies and observations. A set of simulations with hurricane translation speeds ranging from 1 to 12 m/s allows us to investigate the simulated mixed layer dynamics and thermodynamics.

Special attention is paid to the coupling mechanisms between the wind stress vector and the currents. When the wind-stress energy flux is maximum, the shear between the mixed layer and the thermocline is dramatically increased. This leads to enhanced turbulent entrainment and a strong mixed-layer heat loss. For a hurricane translation speed of about 4m/s, the inertial rotating mixed-layer currents period is roughly equal to the surface wind inversion time span. A resonant-like regime is then reached. In this regime, a high correlation is obtained between the heat and kinetic energy budgets.