A TRANSIENT CO2 EXPERIMENT WITH THE MRI CGCM QUICK REPORT

A transient response experiment to the gradual increase in atmospheric CO2 concentration at a compound rate of 1%/yr has been performed with a coupled atmosphere-ocean general circulation model (CGCM) developed at the Meteorological Research Institute (MRI). The model is characte rized by two aspects; one is a relatively high resolution of the ocean ic part in the low latitudes to simulate El Nino phenomena, and the ot her is an elaborate sea ice model to simulate seasonal variation of se a ice coverage and thickness. Time integration has been performed up t o 70 years over which the CO2 concentration doubles. The globally aver aged surface air temperature increases 1.6 degrees C during this perio d. Atmospheric response to the CO2 increase is slow in the Southern He misphere and over oceanic areas. However, the surface air temperature increase in the high latitudes in the Northern Hemisphere is not domin ant up to the year 50. This speed of CO2-induced warming is affected b y interdecadal variation of sea ice found both in the transient and in the control runs. It is also suggested that leads in sea ice act as a strong negative feedback on changes in sea ice volume, affecting the timing of the warming. Analysis of sea surface temperature shows that the dominant air-sea coupled mode in the model is very close to what i s observed. This mode shows interannual variations in the Pacific with a dominant period of about 6 years, which is close to the typical tim e scale of El Nino. It also shows variations of interdecadal time scal es, with implication of predictability for a few decades.