SEA-ICE DYNAMICS AND CO2 SENSITIVITY IN A GLOBAL CLIMATE MODEL

Present-day results and CO2 sensitivity are described for two versions of a global climate model (GENESIS) with and without sea-ice dynamics. Sea-ice dynamics is modelled using the cavitating-fluid method of Flato and Hibler (1990, 1992). The atmospheric general circulation model originated from the NCAR Community Climate Model version 1, but is heavily modified to include new treatments of clouds, penetrative convection, planetary boundary-layer mixing, solar radiation, the diurnal cycle and the semi-Lagrangian transport of water vapour. The surface models include an explicit model of vegetation (similar to BATS and SiB), multilayer models of soil, snow and sea ice, and a slab ocean mixed layer. When sea-ice dynamics is turned off, the CO2-induced warming increases drastically around approximately 60-80-degrees-S in winter and spring. This is due to the much greater (and unrealistic) compactness of the Antarctic ice cover without dynamics, which is reduced considerably when CO2 is doubled and exposes more open ocean to the atmosphere. With dynamics, the winter ice is already quite dispersed for 1 X CO2 so that its compactness does not decrease as much when CO2 is doubled.