Wind-Wave-Current Interaction in Tropical Cyclones

The passage of a tropical cyclone (TC) over a warm ocean represents one of the most extreme cases of air-sea interaction. The most apparent effects of TC passage are marked sea surface temperature (SST) cooling of 1 ^oC to 5 ^oC, strong current velocities of more than 2 m s^-1, and large surface gravity waves. It is well established that the intensity of a TC over an open ocean may be significantly affected by the cooling of SST caused by air-sea interaction (Khain and Ginis 1991; Schade and Emanuel 1999, Cione and Uhlhorn 2003, Ren and Perrie 2006).

Previous numerical modeling studies of the ocean response to TCs (Price 1981, Ginis and Dikinov 1989, Jacob et al., 2000, Morey et al. 2006) pointed out that the major factor that governs the SST response to hurricanes is the momentum flux at the sea surface. Although many experimental and theoretical studies have shown that momentum flux is strongly dependent on the wave-induced processes near the ocean surface (Drennan et al., 2003; Hara and Belcher 2004; Moon et al. 2004a,b; Fan et al. 2008a,b), the role of wind-wave-current coupled processes are not well understood and therefore are often ignored.

Proper evaluation of the sea state dependence of air-sea fluxes requires modeling the wave boundary layer (lower part of the atmospheric boundary layer that is affected by surface waves) and the equilibrium range of wave spectra. Traditionally, the momentum and turbulent kinetic energy (TKE) fluxes from wind to waves is assumed to be identical to the flux into subsurface currents due to wave breaking based on the assumption that no net momentum (or TKE) is gained (or lost) by surface waves. This assumption, however, is invalid when the surface wave field is not fully developed. Especially under TC conditions, the surface wave field is complex and fast varying in space and time and may significantly affect the air-sea flux budget.

Theoretical (Kenyon and Sheres 2006) and numerical (Tolman et al. 1996) studies have also pointed out that if the ocean currents have a large horizontal gradient, they may significantly affect the surface gravity wave field. Since strong surface currents with large horizontal gradients are typically observed under TC forcing, wave-current interaction can be important in the air-sea interaction processes during TCs. The strong TC-induced ocean currents may also affect the momentum flux into the currents due to the difference between the wind and the current.