SEA ICE CONTROL OF PLIO-PLEISTOCENE TROPICAL PACIFIC CLIMATE EVOLUTION

Abstract

Marine proxies of sea surface temperature (SST) indicate that the tropical Pacific thermal gradient intensified through the Plio-Pleistocene and peaked during Pleistocene glaciations. The cause of this variability, which has been linked to the initiation of the Walker circulation, is uncertain. Here, we hypothesize that Plio-Pleistocene tropical climate variability was coupled to high-latitude Southern Hemisphere climate change, specifically sea-ice extent. We use a coupled ocean-atmosphere general circulation model to investigate the influences of sea-ice extent and atmospheric CO2 on the tropical Pacific thermal structure. In the model, CO2-radiative forcing in the absence of any sea-ice feedbacks has little influence on the tropical SST gradient. A 180 ppm reduction in CO2 causes the SST gradient to decrease by 0.4 °C. In comparison, an expansion of Southern Hemisphere, high-latitude sea ice reduces tropical SSTs and enhances the SST gradient in the tropical Pacific by as much as 3.5 °C. Tropical cooling is primarily due to the advection and upwelling of waters into the eastern equatorial Pacific that were cooled by sensible heat loss at the sea-ice margin in the Southern Pacific. An energy balance analysis indicates that the ocean heat flux into the eastern equatorial Pacific decreases by ~ 44%. This mechanism provides an intimate coupling between the tropical Pacific and the high-latitude Southern Hemisphere through the thermocline circulation. © 2006 Elsevier B.V. All rights reserved.