Understanding the climate of the last glacial maximum using a climate system model

Sang-Ik Shin
University of Wisconsin

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A coupled atmosphere-ocean-sea-ice model is used to understand the climate change at the Last Glacial Maximum (LGM, around 21,000 years ago).

In the tropics, the simulation shows a moderate cooling of 3C over land and 2C in the ocean, in zonal average. This change is about 1C cooler than the CLIMAP estimates but consistent with recent estimates of both land and sea surface temperature changes. A sensitivity experiment shows that about half of the simulated tropical cooling of the LGM originates from reduced atmospheric concentrations of greenhouse gases. In addition to the reduced greenhouse effect, the positive water-vapor feedback and the enhanced ocean ventilation of the thermocline and the intermediate waters also contribute to the simulated LGM tropical cooling.

The model simulates a shallower and weaker thermohaline circulation than the modern with an enhanced northward flow of Antarctic Bottom Water as suggested by paleoceanographic evidences. These deep circulation changes are attributable to the increased surface density in the Southern Ocean caused by sea-ice expansion and the accompanying increase of oceanic vertical stability in the Atlantic Ocean. Both the Gulf Stream and the Kuroshio are intensified due to the overall increase of the wind stress curl over the subtropical oceans. The intensified zonal wind stress and southward shift of its maximum in the Southern Ocean effectively enhances the transport of the Antarctic Circumpolar Current by more than 50%.

Subtropical waters are cooled by 2-2.5C, in agreement with recent estimates. Simulated conditions in the North Atlantic are warmer and with less sea-ice than indicated by CLIMAP estimates again, in agreement with more recent estimates. The increased meridional SST gradient at the LGM results in an enhanced Hadley Circulation and increased midlatitude storm track precipitation. The increased baroclinic storm activity also intensifies the meridional atmospheric heat transport.

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13 Feb, 2002
2 PM/ DSRC 1D 403
(Coffee at 1:50 PM)
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