Permanent El Nino Conditions and Meridional Expansion of the Tropical Ocean Warm-Water Pool in the Early Pliocene: Modeling Global Impacts with an Atmospheric GCM

As new data emerges, our interpretation of the early Pliocene climate continually evolves. More and more data from the last few years indicate a persistence of a significantly reduced zonal temperature gradient in the equatorial Pacific, which is associated with climate conditions usually referred to as a "permanent El Niño". Here, data from a variety of regions in both Pacific and Atlantic oceans are collected to reconstruct the meridional temperature gradient in the ocean during the early Pliocene. Our results indicate a substantial reduction in the meridional temperature gradient from the equator to the subtropics (in comparison with today's climate) and a large poleward expansion of the warm-water pool in the tropical Pacific. The implications of a reduced meridional temperature gradient are studied with an atmospheric general circulation model forced with hypothetical surface temperature boundary conditions, representative of our new understanding of the early Pliocene. Changes in the global circulation and precipitation patterns are explored, along with inferences about the ocean heat transports. The Walker circulation is virtually removed by the imposed SST boundary conditions. The latitudinal extent of the Hadley cell increases only slightly, but the strength of the Hadley circulation is substantially reduced, which results in a weakening of the poleward heat transport by the atmosphere in the low- to mid- latitudes. The issue of the poleward heat transport in the early Pliocene is discussed in the context of the "Pliocene Paradox" (Fedorov et al, 2006). Further, the sensitivity of the modeling results to different choices of the surface boundary conditions is explored.