The passage of hurricane Bonnie (1998) in the western Atlantic Ocean basin underscored the complexities of the ocean’s mixed layer response to both wind and wave forcing. Bonnie slowly drifted towards the north-northwest during an air-sea interaction experiment conducted from the NOAA WP-3D. During this period of time, there was marked upper ocean heat loss of about 64 KJ cm-2 (or four times the value required to sustain a tropical cyclone) where the storm had a maximum intensity of about 50 ms-1 and central pressures in the 950 mb range. Prior to this intense heat loss, the heat potential change was about 16 KJ cm-2 , but as the storm slowed down, Ekman divergence of the near-surface currents (i.e. upwelling) coupled with the enhanced air-sea fluxes caused a significant heat loss in the ocean mixed layer. As the storm accelerated, winds decreased to about 45 m s-1 , and the maximum upper ocean heat loss was about 32 KJ cm-2 based on thermal profiles from Airborne expendable Bathythermographs deployed on the 24 August research flight.

Directional wave spectra from the NASA Surface Radar Altimeter was concurrently observed form the NOAA WP-3D on this air-sea interaction mission. Wave spectra are used to evaluate mixing efficiency and dissipation associated with surface wave effects on the deepening and cooling surface mixed layer. The estimated significant slope, defines as the ratio of the RMS surface wave height and the wavelength of the dominant wave, ranged between 0.01 to 0.02. This mixing efficiency induced by surface waves is recast into coefficients that are commonly assumed in mixed layer models where mixing coefficients have a four-decade range in variability. Given the relative importance of current shear across the oceanic mixed layer base for deepening ocean mixed layers, the impact of the surface wave-induced orbital velocities, Stokes drift currents and Langmuir cells may be important to smaller scale physical processes in redistributing turbulence downward in the oceanic mixed layer particularly within + 2Rmax of the eye. Current and current shear measurements and oceanic mixed layer floats coupled with remotely sensed surface wave spectra will be valuable in sorting out these issues in a cooling and deepening ocean mixed layer during hurricane passage, which impacts the air-sea fluxes and hence storm intensity.