The seasonal cycle in a coupled experiment with an atmospheric GCM and a two-layer equatorial ocean model

Abstract

 The mean state and the seasonal cycle in the tropical Pacific are studied, using a new coupled tropical ocean-global atmosphere model. The atmospheric component is a general circulation model and the oceanic component is a two and a half layer model of the tropical Pacific. The coupling is based on delocalized physics: the spatial resolution of the physics of the atmospheric component is the same as the spatial resolution of the oceanic model. No flux corrections are applied. A 31 year experiment has been made with the climatological observed sea surface temperature outside the area of coupling. We observe a quick drift of the model which, after three years, reaches a warm mean state. The temperature bias varies geographically between 1 ^∘C and 2 ^∘C, but, in spite of this default, the eastern part of the basin remains colder than the west. This contrast is shown to be dependent on the shoaling of the thermocline east of 160^∘W. There is a significant seasonal cycle with an amplitude and phase of the seasonal variations which are well reproduced with respect to many other models. It is shown that interactions between the ocean and the atmosphere in the central and eastern Pacific are sufficient to explain the gross features of its evolution. In July, easterlies intensify in the Southern Hemisphere and lead to a strong upwelling and an enhanced evaporation in the eastern part of the basin. This induces a cooling throughout the area. The cooling reaches a first maximum in October in the easternmost part of the basin, then propagates westward along the equator with a decreasing amplitude. In January it is reinforced in the central part of the basin because of a divergence of the current, which is too strong. The mechanisms found here emphasize the role of the upwelling in maintaining the equatorial Pacific climate, and are in agreement with those found in other simplified coupled models.