Abstract
Air-sea energy and momentum fluxes constrain the available energy for tropical cyclone intensification. The surface wave field modifies these fluxes in numerous ways. In this study we focus on two sea-state dependent processes. The first is the sea-state dependence of the wind stress over the ocean under tropical cyclone force winds. The second is the sea-state dependence of the upper-ocean turbulence due to Langmuir turbulence, which modifies the mixed-layer deepening and cooling of the ocean surface during the passage of a tropical cyclone. In this study we utilize the WAVEWATCH III (WW3) surface wave model, the one-dimensional General Ocean Turbulence Model (GOTM), and the Princeton Ocean Model (POM) to investigate these processes. The impact of the surface wave field (sea-state) on the wind stress over the ocean is investigated with fetch-dependent seas under uniform wind and with complex seas under idealized tropical cyclone winds. Two different approaches are employed to calculate the wind stress and the mean wind profile. The near-peak frequency range of the surface wave field is simulated using the WAVEWATCH III model. The high frequency part of the surface wave field is empirically determined using a range of different tail levels. The results suggest that the drag coefficient magnitude is very sensitive to the spectral tail level but is not very sensitive to the drag coefficient calculation methods. The drag coefficients at 40 m/s vary from 1 x 10-3 to 4 x 10-3 depending on the saturation level. The misalignment angle between the wind stress vector and the wind vector is sensitive to the stress calculation method used. In particular, if the cross-wind swell is allowed to contribute to the wind stress, it tends to increase the misalignment angle. Our results predict similar amounts of sea state dependence of the drag coefficient regardless of the approaches taken. More sea-state dependence of the drag coefficient is predicted for tropical cyclones than for aligned growing wind conditions, this enhancement of the drag coefficient may be attributed to swell that is significantly misaligned with local wind. The amount of sea-state dependence of the drag coefficient is sensitive to the tail level and the translation speed of the tropical cyclone. The upper-ocean turbulence is significantly modified by the Stokes drift of the surface waves because of the Craik-Leibovich vortex force (Langmuir turbulence). Under tropical cyclones the contribution of the surface waves varies significantly depending on complex wind and wave conditions. Therefore,
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