Impact of the sea state on the atmosphere and ocean

Ocean waves represent the interface between the ocean and the atmosphere, and, therefore, wave models are needed to compute not only the wave spectrum, but also the processes at the air-sea interface that govern the fluxes across the interface. This was one of the reasons for developing a wave prediction model, called the WAM model, that determines the sea state dependence of the air-sea fluxes. As a first step, the study of the two-way interaction between ocean waves and atmospheric circulation was undertaken. Modest improvements in forecasting waves and winds were obtained, and, as a consequence, since the 29th of June 1998 ECMWF produces weather and ocean wave analyses and forecasts using a coupled IFS-WAM forecasting system. In this paper, we briefly discuss our recent experience with two-way interaction. Nowadays, there is a substantial impact on weather and wave forecasting, and reasons for this increase in impact are given. A future task is to couple the ocean waves and the ocean circulation. It is argued that prediction of the ocean circulation would benefit from the inclusion of currents in the calculation of fluxes. In addition, it is shown that the thickness of the ocean mixed layer is to a large extent determined by the energy flux associated with breaking ocean waves, and furthermore, the momentum flux is controlled to a lesser extent by breaking waves. Finally, in the presence of ocean waves, many authors have shown that there is an additional force on the mean flow, given by the cross product of the Stokes drift and the Coriolis parameter (Coriolis-Stokes forcing). This would have a large impact on the Ekman transport. It is pointed out, however, that in a Eulerian frame there is also a surface drift caused by the waves which exactly cancels the total momentum of the Coriolis-Stokes forcing. There may still be an impact of the Stokes drift on the circulation in the deeper layers of the ocean, but this depends on the interaction of the surface drift and oceanic turbulence.