Simulation of ocean skin temperature modulation by swell waves

Abstract

Three different mechanisms proposed to explain the observed modulation of the ocean skin temperature by swell waves were simulated and tested in an effort to understand and model the modulation better. The mechanisms include compression of the thermal boundary layer, localized enhancement of the wind stress along the waveform, and preferential breaking along the phase of the swell due to hydrodynamic effects. To implement the preferential-breaking mechanism, a new model for the time evolution of the skin temperature that directly simulates disruption of the skin layer by wave breaking and surface renewal events was developed. The predicted modulation due to the different mechanisms was compared with direct observations made in open-ocean conditions. The model incorporating preferential breaking best predicts the overall observed temperature modulation, simultaneously approximating the mean skin temperature and the amplitude of the observed temperature modulation throughout the range of observed wind speeds when the wind and swell are aligned. The model incorporating enhancement of the wind stress is unable to predict the temperature modulation at lower wind speeds, but enhanced wind stress appears to have a significant contribution at high winds. Thinning of the thermal boundary layer due to streamline compression is not sufficient to explain the observed temperature modulation. These results demonstrate that preferential wave breaking can have an important effect on both the modulation of the skin temperature and the mean bulk-skin temperature difference. Copyright 1998 by the American Geophysical Union.