WHOI PO

Wave slopes and wind-wave alignment in hurricanes

Ocean surface waves mediate the air-sea fluxes critical to tropical cyclone intensification. Wave slope, the ratio of a wave’s height to its length, is often a key metric used in the parameterization of wave-induced stress. Observations from drifting buoy arrays deployed into several recent hurricanes show that wave slopes saturate at high wind speeds (> 25 m/s), a trend that is qualitatively similar to the wind speed dependence of the air-sea drag coefficient used to model momentum transfer in hurricane and coastal impact forecasts. This saturation is coincident with a transition of the spectral tail from wind equilibrium to a dissipation-dominated form, suggesting that wave breaking is ubiquitous and thereby limits slope.

Wave slopes also exhibit a secondary dependence on the alignment of the wind and longer waves, or wind-wave alignment. At a given wind speed, waves are steeper where wind and waves are closely aligned and less steep where they cross. Ongoing work combines buoy and profiling float observations to relate spatial variations in the surface wave field to upper ocean response. Future analyses include the characterization of wave-driven mixing contributions, the influence of near-surface currents on wave generation, and the incorporation of measured spectra into high-resolution LES models to explore wave-induced drag and turbulent structures within the atmospheric boundary layer.