Momentum Flux Balance at the Air-Sea Interface

Abstract

Ocean waves can spatiotemporally redistribute the momentum flux at the air-sea interface, which varies with the sea state. Traditional atmosphere-ocean coupled systems assume the ocean-side stress (τoc) to be identical to the air-side stress (τa); consequently, the role of ocean waves is neglected. In this study, the wave impacts on the air-sea momentum flux are investigated based on 1-year high-resolution model simulations in the Baltic Sea using an atmosphere-wave coupled model (Uppsala University-Coupled Model, UU-CM). The simulation results show that τoc can differ significantly from τa in both direction and magnitude. The direction difference between τoc and τa (DD(τoc, τa)) and the normalized momentum flux ((Formula presented.)) decrease with increasing inverse wave age. In general, (Formula presented.) and DD(τoc, τa) are pronounced under wind-following swell and wind-crossing swell conditions, respectively. The occurrence frequencies of large (Formula presented.) and DD(τoc, τa) are higher nearer the coast; statistically, both decrease significantly with increasing water depth because of the joint effect of dissipation processes. Based on four selected areas, we find that alongshore winds (winds blowing parallel to the coastline) are favorable for large angular differences between τoc and τa (DD(τoc, τa) > 5°). However, onshore winds predominate at (Formula presented.). The τa in the wave model is generally less than that obtained from the atmospheric model under low-moderate wind conditions if the wave model feeds only the Charnock coefficient (roughness length) back to the atmospheric model in coupled systems.