IMPACT AND FEEDBACK OF OCEAN WAVES ON THE ATMOSPHERE by P A

The air-sea momentum transfer is known to be affected by the state of the ocean waves. For example, young wind waves are usually steeper than old wind waves and therefore extract more momentum from the air flow. As a result airflow over young windsea is rougher than over old windsea. The sea state dependent momentum transfer is thought to be of importance in rapidly varying circumstances occurring near depressions and fronts and in fetch limited circumstances near coasts. In this paper we review previous results regarding the impact of ocean waves on the atmospheric climate and we discuss new results regarding the impact of waves on the atmosphere for medium-range weather forecasting. It is concluded that the impact of ocean waves on the atmosphere depends in a sensitive manner on the resolution of the atmospheric model. In addition, analysis differences induced by the coupled wind-wave model give a considerable increase of the impact of ocean waves on the atmosphere. Overall, a modest positive impact on the scores of the atmospheric model is found while the improvements in the forecast performance of the wave model are even larger. As a consequence the coupled atmosphere, ocean-wave model was introduced into the ECMWF operational forecast system on June 29 1998. After this date a number of improvements of the wave model's advection scheme and integration scheme have been introduced and the angular resolution of the wave spectrum was doubled. Combined with the recent increase of spatial resolution of the atmospheric model from Tl319 to Tl511, this resulted in a dramatic increase of forecast skill of geopotential height and significant wave height, in particular in the Northern Hemisphere Summer time.

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