Sea ice control of Plio–Pleistocene tropical Pacific climate evolution

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.