Influence of Sea Ice on Plio-Pleistocene Tropical Climate

Proxy records of Plio-Pleistocene climate record enhanced east-west sea surface temperature (SST) gradients in the tropical Pacific region on both secular and glacial-interglacial time scale. The climate processes that cause the variations in thermal gradient are not well understood but are generally thought to involve changes in intensification of the southeast Trades and/or long-term cooling of deep water through the Cenozoic. High latitude sea-ice extent is sensitive to climate forcing and is known to influence the heat budget and mid- latitude SST on the sea-ice margin, but its effect on tropical regions has not been examined. We have completed a series of coupled ocean-atmosphere general circulation model simulations that test the influence of Last Glacial Maximum (LGM) sea ice extent, modern sea ice extent, and the absence of sea ice on the global climate. Our model results demonstrate that Southern Ocean sea-ice expansion can lead to a substantial increase of the tropical Pacific thermal gradient, similar in magnitude to that indicated by proxy records. The enhanced tropical Pacific SST gradient is caused by cooling of mid-latitude Southern Ocean surface water, which is subducted and advected through the eastern boundary current to form the water at the base of thermocline in the eastern equatorial Pacific (EEP). The cold upwelling water in the EEP reduces the ocean heat flux to a greater degree in the EEP than the western equatorial Pacific, increasing the equatorial Pacific SST gradient and intensifying the Walker Circulation. These model results demonstrate an ocean-sea ice-atmosphere dynamic that directly links tropical and extratropical regions. Due to an indirect ocean pathway between the North Pacific and the EEP, it is unlikely that Northern Pacific sea-ice has had such a large influence on the cooling of the EEP. We will report on new climate simulations that test the climate implications of this connection.