Ocean Waves and the Atmospheric Climate

Abstract

Ocean waves are generated by wind and, as a consequence, there is a considerable transfer of momentum from atmosphere to ocean. This momentum transfer depends, however, on the stage of development of the ocean waves. Ocean waves that are just generated by wind ("young? windsea) are usually steeper and are capable of extracting more momentum from the air flow than older waves that are more gently sloped. In this paper the consequences are discussed of the sea-state-dependent momentum transfer on the seasonal climate of the European Centre for Medium-Range Weather Forecasts (ECMWF) model. Here 15 simulations of the 1990/91 winter season were performed. Because of the large variability in the extratropics, results of the ensemble mean are presented. They show that the sea-state-dependent momentum transfer has a definite impact on atmospheric and wave climate in both hemispheres. In certain aspects, there is an improvement in the coupled climate when compared to the analyzed climate. We mention the favorable changes over the Eurasian continent in the mean 500-hPa height field and high-frequency variability. The blocking frequency over the North Pacific has shown an unfavorable reduction, however. The reduction of mean surface wind speed over the Southern Ocean seems favorable in view of the positive error in wind speed in the 10-day ECMWF forecast when verified against the analysis.