Abstract
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 characterized by two aspects ; one is a relatively high resolution of the oceanic part in the low latitudes to simulate El Nino phenomena, and the other is an elaborate sea ice model to simulate seasonal variation of sea ice coverage and thickness. Time integration has been performed up to 70 years over which the CO2 concentration doubles. The globally averaged surface air temperature increases 1. 6°C during this period. Atmospheric response to the CO2 increase is slow in the Southern Hemisphere and over oceanic areas. However, the surface air temperature increase in the high latitudes in the Northern Hemisphere is not dominant up to the year 50. This speed of CO2-induced warming is affected by 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 is observed. This mode shows interannual variations in the Pacific with a dominant period of about 6 years, which is close to the typical time scale of El Nino. It also shows variations of interdecadal time scales, with implication of predictability for a few decades.
