The Seasonal Variation of the Tropical Circulation as Simulated by a Global Model of the Atmosphere

Abstract

A mathematical model of the atmosphere with a seasonal variation of insulation and sea surface temperatures is integrated numerically with respect to time over three model years. The model has a global computational domain and a realistic distribution of mountains. It contains a highly idealized parameterization of convection, i.e., dry and moist convective adjustment. It is found that the model accurately simulates the seasonal variation of the location of the tropical rainbelt as well as that of the flow field associated with it. Over the continental regions of the model, the tropical rainbelt tends to form very close to the equator, whereas, in certain oceanic regions, it has a tendency to form away from the equator. Based upon a comparison of these results with those of another numerical experiment, it is concluded that this tendency is not due to an inherent characteristic of the rainbelt of the model to avoid the equator in oceanic regions, but rather it is due to the equatorial belt of low sea surface temperatures which is not favorable for the formation of a rainbelt. Over the sea, the surface temperature distribution seems to be the primary factor in determining the location of the rainbelt and accompanying tropical disturbances. The primary source of kinetic energy of the disturbances in the model tropics is the conversion of eddy available potential energy which is generated by the effects of moist convection. A secondary source is the energy supplied from middle latitudes through pressure interaction. This effect has a significant magnitude in the subtropics of the model. The belt of maximum eddy conversion moves from one summer hemisphere to the other with respect to season in a manner similar to the tropical rainbelt. On the other hand, the contribution of pressure interaction to the production of eddy kinetic energy is significant in the winter hemisphere and thus supplements the contribution of eddy conversion. In general, the rate of eddy conversion due to transient eddies is particularly large in areas of relatively warm sea surface temperatures, where the tropical rainbelt and its accompanying disturbances predominate.