The Influence of Ocean Surface Temperature Gradient and Continentality on the Walker Circulation. Part II: Prescribed Global Changes

Abstract

The mechanisms responsible for forcing the global Walker circulation are investigated with a series of experiments using an atmospheric model developed at the Goddard Institute for Space Studies. These experiments feature worldwide changes in ocean surface temperatures (OSTs), topography and/or continents. The results show that the primary factor affecting the circulation in the model is the global distribution of continents and oceans. The OST gradients are also important, but the topography is relatively unimportant. Both continentality and OST gradients by themselves force the model atmosphere by introducing zonal variations of surface heating, which give rise to vertical motions. These in turn give rise to variations in moisture convergence and condensation, which reinforce the vertical motions. The forcing by OST gradients is partly nonlocal in character, and the and the atmospheric response is affected by the continentality. In particular, in the south tropical Pacific the model response is greatly enhanced by the continentality, and is controlled by distant OST gradients more than by local OST gradients. In all cases the zonal variations of the vertical motions are highly correlated with condensation (i.e., precipitation) and this in turn is correlated more with moisture convergence than with local evaporation.