Response of Coupled Atmosphere-Ocean GCM to Ice Age Conditions and CO2 Changes

Reconstructions of the Last Glacial Maximum (LGM, 21ka) climate from paleoclimatic data is expected to provide an opportunity for an extreme test of the possibility of Coupled Atmosphere-Ocean GCM's to simulate climate changes and help us understand the mechanism of climate change. To simulate the LGM and compare it with global warming (cooling) experiments by CO2 increase (decrease), here we use a coupled GCM of CCSR/NIES/FRCGC renewed and frozen for the submission of IPCC/AR4 optimized to be run on a super-computer "Earth Simulator". The model has a resolution of T42 (2.8 deg. for longitude and latitude) layers 20 for the atmospheric part and 1 to 0.5 deg. with 48 levels for the ocean part, which are synchronously coupled without flux adjustment and without any acceleration techniques. To separate the role of greenhouse gases and other components (mainly the ice sheet part), numerical experiments with LGM conditions as well as Glacial CO2 level are performed for 1000 years. The preliminary results show that the total LGM forcings bring about 3.2 deg of global cooling in 250 years, while the glacial CO2 has an impact to cool the climate by 2.2 deg C, and the low latitude climate sensitivity is within the range paleo-data indicate. The sensitivity of east Pacific is larger than the West Pacific whose pattern is quite similar for global warming experiment. The impact of ice sheet upon the ocean temperature is pronounced in the North Atlantic but even in the northwest Pacific region. The NADW is first strengthened from 20 Sv level to 30 Sv level in case of full LGM condition in 250 years and to 25 Sv level in case of Glacial CO2 case, although it is gradually weakening and cross the control level after 600 years. Further investigation for atmospheric circulation and ocean response compared with other GCMs are discussed in the presentation.