Turbulence in the Ocean Boundary Layer Below Hurricane Dennis

Direct measurements of the ocean boundary layer beneath Hurricane Dennis were made using air-deployed neutrally buoyant floats and an AXBT survey. The floats closely follow the three-dimensional motion of water parcels while measuring their temperature and can thus measure both the turbulence in the boundary layer and the resulting vertical heat transport. Three floats were deployed ahead of Dennis using a chartered skydive King Air aircraft; the eastern side of Dennis' eyewall passed close to, or over, the easternmost float. The AXBT's were deployed from a NOAA-P3 aircraft after Dennis had passed and thus measured the net cooling of the ocean.

On the left side of the eye, the oceanic mixed layer cooled nearly 3 degrees C and deepened to at least 65m in response to Dennis. Both turbulent entrainment and downwelling contributed to the increased mixed layer depth. Vertical speeds within this layer were 6.5 cm/s (rms) at the peak of the storm with peak speeds of 20 cm/s. Kinetic energy dissipation rates within the mixed layer were approximately 1e-5 W/kg and both these and the vertical velocity appear to scale with the wind stress. The float trajectories were dominated by large eddies which repeatedly crossed the entire mixed layer in a regular pattern, cooling at the surface and then carrying this heat into the bulk of the layer. The surface cooling, roughly 1000 W/m^2, accounted for only a small part of the net cooling of the mixed layer, roughly 5000 W/m^2. Most of the cooling was due to cold water entrained from below.