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Statistics of the Air-Sea Fluxes of Momentum and Mechanical Energy in a Coupled Wave-Atmosphere Model

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  • 1 Royal Netherlands Meteorological Institute, De Bilt, the Netherlands
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Abstract

An atmospheric general circulation model (GCM) and a wind wave model are coupled through the wind stress. The wind stress which forces the wave model, depends in the coupled model on the stage of development of the wave field. As the waves depend on the local and instantaneous wind as well as the earlier wind elsewhere, nonlocal and memory effects are thus introduced in the momentum flux parameterization. It is examined how strong these effects are compared to the local and instantaneous wind contribution. A second variable considered is the energy flux from the atmosphere to the wave field, which can be determined diagnostically from the wave model.

Almost all of the momentum and energy passed to the wave field is lost quasi-instantaneously and locally to the ocean, whereas only a small fraction remains in the local wave field or propagates away. The momentum and energy contained in the wave field are determined by the small difference between input and dissipation. This small difference depends strongly on the memory of the wave field. Wave propagation also influences the local balance. The case for the fluxes is different. A memory effect is only visible in the momentum flux for storms that last relatively long. In general, the typical time scale of wind changes is smaller than that of wave evolution. Nonlocal effects can be neglected in both the momentum flux and the energy flux. These results indicate that, at the present state of the art in wave and climate modeling, the wave effect on the air-sea fluxes can be described adequately in (coupled) atmosphere-ocean GCMs by the local and instantaneous wind.

Abstract

An atmospheric general circulation model (GCM) and a wind wave model are coupled through the wind stress. The wind stress which forces the wave model, depends in the coupled model on the stage of development of the wave field. As the waves depend on the local and instantaneous wind as well as the earlier wind elsewhere, nonlocal and memory effects are thus introduced in the momentum flux parameterization. It is examined how strong these effects are compared to the local and instantaneous wind contribution. A second variable considered is the energy flux from the atmosphere to the wave field, which can be determined diagnostically from the wave model.

Almost all of the momentum and energy passed to the wave field is lost quasi-instantaneously and locally to the ocean, whereas only a small fraction remains in the local wave field or propagates away. The momentum and energy contained in the wave field are determined by the small difference between input and dissipation. This small difference depends strongly on the memory of the wave field. Wave propagation also influences the local balance. The case for the fluxes is different. A memory effect is only visible in the momentum flux for storms that last relatively long. In general, the typical time scale of wind changes is smaller than that of wave evolution. Nonlocal effects can be neglected in both the momentum flux and the energy flux. These results indicate that, at the present state of the art in wave and climate modeling, the wave effect on the air-sea fluxes can be described adequately in (coupled) atmosphere-ocean GCMs by the local and instantaneous wind.

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