Hadley and Walker Circulations in the Mid-Pliocene Warm Period Simulated by an Atmospheric General Circulation Model

Abstract

The mid-Pliocene warm period (∼3 million years ago; 3 Ma) is one of the plausible scenarios which provide insight into the climate system in a globally warmer world as projected by climate models for the future. The reconstructed sea surface temperature (SST) by the Pliocene Research, Interpretation and Synoptic Mapping phase 3 (PRISM3) reveals that salient warming occurs in the higher latitudes together with weakening of surface cooling in the equatorial and coastal upwelling regions. The sensitivity of an atmospheric general circulation model (AGCM) is studied by prescribing the surface condition based on the PRISM3 paleoenvironmental reconstructions. The simulated Walker circulation generally slows down, inducing convergent anomaly over the African continent and divergent anomaly over the Asian monsoon region at the lower troposphere; and vice versa at the upper troposphere in response to the reduced east-west gradient of the tropical SST in that period. The ascending branch of the Hadley cell expands poleward, implying a weakening of the meridional circulation in response to the warmer SST in the higher latitudes. To identify the physical reason for the modulation of the wet-dry climatological pattern in low latitudes, additional sensitivity experiments were conducted by changing the configurations of ice-sheet cover, vegetation and zonal patterns of the SST. The results indicate that the reduction of meridional and zonal gradient of tropical SST is the key factor for the expansion of proxy-suggested wetter climate over Africa.