INFLUENCE OF NORTHERN HEMISPHERE ICE SHEETS ON GLOBAL CLIMATE

Proxy records of Pleistocene glacial-interglacial climate have demonstrated linkages between high latitude and tropical regions. The climate processes that link these distant regions are not well understood but are usually thought to involve changes in atmospheric CO₂ and/or North Atlantic Deep Water production. Northern hemisphere ice-sheet orography is known to influence stationary wave patterns, cyclogenesis, and snowfall in the mid-latitudes, 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) ice sheets, flat LGM ice sheets, and the absence of ice sheets on the global climate. Our model results demonstrate that ice-sheet orography can lead to substantial warming of tropical surface temperatures, similar in magnitude to the warming induced by raising atmospheric CO₂ from glacial to interglacial levels. The tropical warming in the full ice-sheet case is initiated by changes in the stationary wave pattern over eastern Asia, which cause surface warming in the northwestern Pacific. Over several decades, the anomalously warm North Pacific water is transported to the tropics through the thermocline circulation leading to tropical and eventually global warming. In contrast, the flat ice sheet experiment causes tropical cooling. The model results demonstrate an ocean-atmosphere-ice sheet dynamic that directly links tropical and extratropical regions.