The modern and glacial overturning circulation in the Atlantic ocean in PMIP coupled model simulationsReport as inadecuate

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1 KNMI - Royal Netherlands Meteorological Institute 2 Center for Climate System Research Tokyo 3 Hadley Center for Climate Prediction and Research 4 School of Earth and Ocean Sciences 5 Potsdam Institute for Climate Impact Research 6 MRI - Meteorological Research Institute Tsukuba 7 NCAR - National Center for Atmospheric Research Boulder 8 Department of Physics

Abstract : The simulation of the Atlantic thermohaline circulation THC during the Last Glacial Maximum LGM provides an important benchmark for models used to predict future climatic changes. This study analyses the THC response to LGM forcings and boundary conditions in nine PMIP simulations, including both GCMs and Earth system Models of Intermediate Complexity. It is examined whether the mechanism put forward in the literature for a glacial THC reduction in one model also plays a dominant role in other models. In five models the THC reduces during the LGM by 5?40%, whereas four models show an increase by 10?40%. In all models but one a reduced enhanced THC goes with a stronger weaker reversed deep overturning cell associated with the formation of Antarctic Bottom Water AABW. It is found that a major controlling factor for the THC response is the density contrast between AABW and North Atlantic Deep Water NADW during the LGM as compared to the modern climate. More saline AABW is consistently found in all simulations, while all models but one show less cooling of AABW as compared to NADW. In five out of nine models a reduced enhanced THC during the LGM is associated with more less dense AABW at its source region, which in turn is determined by the balance between the opposing effects of salinity and temperature on the density of AABW versus that of NADW. The response in net evaporation over the Atlantic basin is relatively small in most models, so that changes in the freshwater budget are dominated by ocean transports. In only two models is the THC response during the LGM directly related to the response in net evaporation.

Author: S. L. Weber - S. S. Drijfhout - A. Abe-Ouchi - M. Crucifix - M. Eby - A. Ganopolski - S. Murakami - B. Otto-Bliesner - W. R. Pelt



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