EFFECTS OF SURFACE WAVES ON WIND STRESS AND UPPER OCEAN RESPONSE UNDER TROPICAL CYCLONES

Xiaohui Zhou, University of Rhode Island

Abstract

Evolution of a tropical cyclone (TC) is strongly affected by the supply of heat from a warm ocean surface. When strong TC wind enhances upper ocean turbulence, deepens the mixed layer, and cools the sea surface temperature (SST), the reduced SST decreases the heat flux to the atmosphere and weakens the storm. The cool-water entrainment can be further enhanced by three-dimensional processes, notably by upwelling due to Ekman pumping. Therefore, accurate prediction of these upper ocean responses to a TC is critically important for improving TC intensity forecasts.

Since the upper ocean responses are driven by the applied wind stress, accurate model predictions require a reliable wind stress (drag coefficient) parameterization. Previous observations suggest that the drag coefficient varies widely in tropical cyclone conditions, and its dependence on sea states (surface wave fields) is poorly understood. In Chapter 1 of this thesis, the drag coefficient under tropical cyclones and its dependence on sea states are investigated by combining upper ocean current observations (using EM-APEX floats deployed under five tropical cyclones) and a coupled ocean-wave (Modular Ocean Model 6 - WAVEWATCH III) model. The estimated drag coefficient averaged over all storms is around 2-3 x 10-3 for wind speeds 25-55 m/s. While the drag coefficient weakly depends on wind speed in this wind speed range, it shows stronger dependence on sea states. In particular, it is significantly reduced when the misalignment angle between the dominant wave direction and the wind direction exceeds about 45o, a feature which is underestimated by current models of sea state dependent drag coefficient. Since the misaligned swell is more common in the far front and in the left front quadrant of the storm (in the Northern Hemisphere), the drag coefficient also tends to be lower in these areas and shows a distinct spatial distribution. Our results therefore support ongoing efforts to develop and implement sea state dependent parameterizations of the drag coefficient in tropical cyclone conditions.

Although the upper ocean responses to a TC are mainly driven by the applied wind stress, they are further modified by ocean surface waves. The most significant surface wave effect is the enhancement of upper ocean turbulence and mixing by the interaction between the wave-induced Stokes drift and the Eulerian current vorticity (Langmuir turbulence). In addition, the momentum input to the upper ocean can be different from the wind stress when surface waves are growing or decaying (air-sea momentum flux budget), and the upper ocean currents interact with surface waves through the Coriolis-Stokes force, Stokes-shear force, and Stokes advection (wave-current interactions). These three wave effects have not been fully investigated in previous studies. In Chapter 2 of this thesis, impacts of surface waves (the Langmuir turbulence, the air-sea momentum flux budget, and the wave-current interactions) on upper ocean responses under TCs are investigated by combining upper ocean observations (using EM-APEX floats and Lagrangian floats deployed under five tropical cyclones) and a coupled ocean-wave (Modular Ocean Model 6 - WAVEWATCH III) model. The results present two clear evidences of enhanced upper ocean mixing due to the Langmuir turbulence. First, the observed mixed layer vertical velocity variance is significantly enhanced relative to the shear only turbulence value, being consistent with previous LES studies. Second, the observed weak near surface current shear is consistent with the model prediction with the enhanced Langmuir turbulence mixing. The results also show weaker upper ocean responses on the left of the storm, which are likely evidences of the reduced drag coefficient due to dominant waves misaligned from wind. The models underestimate upper ocean cooling and mixed layer deepening even if the mixing scheme includes the Langmuir turbulence enhancement, suggesting possible deficiency of the mixing scheme developed based on idealized LES results. The effects of the air-sea momentum flux budget and the wave-current interactions are secondary compared to the dominant effects of the Langmuir turbulence but are not trivial.