A GCM Simulation of Global Climate Trends: 1950–1988

Abstract

An atmospheric general circulation model has been forced with observed mean monthly sea surface temperatures for the period 1950?1988 and the trends in annual mean fields from an ensemble of three such experiments analyzed. During this period, Northern Hemisphere oceans cooled while the southern oceans warmed. The simulated changes in land-surface temperature over this period are less than observations would indicate, suggesting that factors other than just SST changes, including greenhouse warming, may have been operating. The results show decreases in rainfall for North Africa consistent with the observed desiccation of the Sahel region over the same period. These decreases are statistically significant but not as severe as observed. Simulated rainfall trends for other regions are less definite but are suggestive that SST changes may have contributed to some of the observed trends. The use of global-scale sensitivities based on precipitation changes as a function of temperature changes is demonstrated to be unsatisfactory in this particular study. Simulated trends in the global circulation reveal a tendency towards a boreal winter-type circulation in which atmospheric moisture is transported from the Northern Hemisphere to the Southern Hemisphere, while a net mass exchange occurs in the reverse direction. Trends in upper- and lower-level winds indicate a strengthening of the Walker circulation in the Pacific. While some of these trends are consistent with available observations, there are certain observed features that are not reproduced and therefore not explained by the SST changes.