Effect of Wave Directions on Orientation and Magnitude of Surface Wind Stress under Typhoon Megi (2010)

Abstract

Abstract The relationship between the wind-wave spectrum and surface wind stress τ in tropical cyclones, especially for the misalignment |ϕ| between the wind and τ, was investigated using data from three Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats deployed in Super Typhoon Megi (2010). The floats measured τ by integrating downward momentum flux in the ocean and, in a recent development, directional spectra of surface waves. The wind was captured by aircraft surveys. At wind speeds from 25 to 40 m s−1, the |ϕ| increased with the increasing angle between the wind and dominant waves. The |ϕ| was small near the eyewall, where wave energy concentrated in a narrow frequency band. At the location far away from the eyewall, where most spectra were bimodal in directions with similar frequencies, the stress direction might be similar to the high-frequency waves. The misalignment between the wind and propagating swell might affect the growth and directional spreading of wind waves under tropical cyclones. The resulting wave breaking might then release wave momentum into the ocean as most stress clockwise from the wind direction. C‖, the downwind drag coefficient, increased with increasing inverse wave age of dominant waves. |C⊥|, the magnitude of the crosswind drag coefficient, was significant when low-frequency waves deviated from the wind by more than 90°. The wave directions are used in the inverse wave age for scaling drag coefficients. The new parameterization based on wave dynamics can be useful for improving the prediction of wind stress curl under storms. Significance Statement Tropical cyclones can impact the maximum sea surface temperature cooling through the curl of surface wind stress τ, which causes divergence under the storm eye. To investigate the effect of surface waves on τ, measurements of downward momentum flux and surface waves from three EM-APEX floats deployed under Typhoon Megi in 2010 are used. The inverse wave age involving wind forcing on the wave directions of dominant waves and swells can significantly influence the momentum transfer efficiency in the downwind and crosswind directions, respectively. That is, the propagating swell under storms should play a crucial role in the downward momentum flux during its interaction with high-frequency waves and wind. Incorporating wave directions to parameterize the magnitude and orientation of τ will improve future models’ ability to predict wind stress curl and thereby the heat fluxes to storm intensification.