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

Abstract

A coarse-mesh, global climate model developed at the Goddard Institute for Space Studies (GISS) has been used to assess the influence of ocean surface temperature (OST) gradient and continentality on the Walker circulation. The basic model climate was established by a five-year integration in which the prescribed seasonal cycle in OST distribution was identical for each year. In the model climate, the Walker circulation is characterized in the zonal plane by three pairs of clockwise and counterclockwise cells to the troposphere. Three separate winter experiments were performed in which the normal west-to-east OST gradients in the tropical Pacific were replaced by a uniform distribution in the band from 8°N to 16°S. Each experiment was characterized by OSTs set at the warmest, coldest, or mean temperatures in the band. The model response features statistically significant changes in the intensity of the various cells and branches with small shifts in the east-west extent. The overall structure in the zonal plane for the experiments with the coldest or mean temperatures, however, remained unchanged. A major disruption of the six-cell structure did result for the experiment with the warmest temperature and resultant net heat source. The fourth prescribed changed experiment involved the replacement of the South American continent by an ocean with the OSTs linearly interpolated from the eastern Pacific to the western Atlantic. In this case, a dramatic change in the structure of the Walker circulation also took place as the upward branch over South America was reduced sufficiently to eliminate the corresponding counterclockwise cell and thereby allow two clockwise cells to merge into one large cell. The Hadley cell was less intense and shifted northward with the South American continent removed. In summary, these experiments with the GISS model seem to indicate that both continentality and OST gradient are important as forcing mechanism of the overall structure of the Walker circulation and the intensity of the individual cells. The details of the forcing, however, are likely to be different for the two mechanisms.