A simple coupled atmosphere-ocean-sea ice-land surface model for climate and paleoclimate studies

The authors develop a coupled atmosphere-ocean-sea ice-land surface: model for long-term climate change studies that incorporates the seasonal cycle. Three ocean basins. the Antarctic Circumpolar Current region, and the major continents are resolved. The model variables are sectorially averaged acro ss the different ocean basins and continents. The atmosphere is represented by an energy-moisture balance model in which the meridional energy and moi sture transports are parameterized by a combination of advection and diffus ion processes. The zonal heat transport between land and ocean obeys a diff usion law, while the zonal moisture transport is parameterized so that the ocean always supplies moisture to the land. The ocean model is due to Wrigh t and Stocker, and the sea ice model is a zero-layer thermodynamic one in w hich the ice thickness and concentration are predicted by the methods of Se mtner and Hibler, respectively. In the land surface model, the temperature is predicted by an energy budget equation, similar to Ledley's, while the s oil moisture and river runoff are predicted by Manabe's bucket model. The above model components are coupled together using flux adjustments in o rder to first simulate the present day climate. The major features of this simulation are consistent with observations and the general results of GCMs . However, it is found that a diffusive law for heat and moisture transport s fives better results in the Northern Hemisphere than in the Southern Hemi sphere. Sensitivity experiment, show that in a global warming (cooling) exp eriment, the thermohaline circulation (THC) in the North Atlantic Ocean is weakened (intensified) due to the increased (reduced) moisture transport to the northern high latitudes and the warmer (cooler) SST at northern high l atitudes. Last, the coupled model is employed to investigate the initiation of glacia tion by slowly reducing the solar radiation and increasing the planetary em issivity, only in the northern high latitudes. When land ice is growing, th e THC in the North Atlantic Ocean is intensified, resulting in a warm subpo lar North Atlantic Ocean. which is in agreement with the observations of Ru ddiman and McIntyre. The intensified THC maintains a large land-ocean therm al contrast at high latitudes and hence enhances land ice accumulation, whi ch is consistent with the rapid ice sheet growth during the first 10 kyr of the last glacial period that was observed by Johnson and Andrews. The auth ors conclude that a cold climate is not responsible for a weak ol collapsed THC in the North Atlantic Ocean: rather it is suggested that increased fre shwater or massive iceberg discharge from land is responsible for such a st ate.