Momentum Flux Balance at the Air‐Sea Interface

Abstract

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 ( τ oc ) to be identical to the air‐side stress ( τ a ); 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 τ oc can differ significantly from τ a in both direction and magnitude. The direction difference between τ oc and τ a ( DD ( τ oc , τ a )) and the normalized momentum flux ( ) decrease with increasing inverse wave age. In general, and DD ( τ oc , τ a ) are pronounced under wind‐following swell and wind‐crossing swell conditions, respectively. The occurrence frequencies of large and DD ( τ oc , τ a ) 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 τ oc and τ a ( DD ( τ oc , τ a ) > 5°). However, onshore winds predominate at . The τ a 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.