Simulations of warm tropical conditions with application to middle Pliocene atmospheres

During the early and mid-Pliocene, the period from 5 to 3 million years ago, approximately, the Earth is believed to have been significantly warmer than it is today, but the reasons for the higher temperatures are unclear. This paper explores the impact of recent findings that suggest that, at that time, cold surface waters were absent from the tropical and subtropical oceanic upwelling zones. El Niño was in effect a perennial rather than intermittent phenomenon, and sea surface temperatures in low latitudes were essentially independent of longitude. When these conditions are specified as the lower boundary condition for an atmospheric GCM, we find that the trade winds along the equator, and hence the Walker Circulation, collapse. The low-level stratus clouds in low latitudes diminish greatly, thus reducing the albedo of the Earth. The atmospheric concentration of water vapor increases, and enhanced latent heat release due to stronger evaporation warms up the tropical atmosphere, particularly between 40 S and 20 N. Moreover, teleconnection patterns from the Pacific induce a warming over North America that is enhanced by surface albedo feedback, a process that may have helped to maintain this region ice-free before 3 Ma. The results presented here indicate that the suggested absence of cold surface waters from the tropical and subtropical oceanic upwelling zones could have contributed significantly to the Pliocene warmth.