The seasonal response of a general circulation model to changes in CO2 and sea temperatures

Abstract

The seasonal response of an atmospheric general circulation model to changes in atmospheric carbon dioxide concentrations and sea surface temperatures is discussed. The model has five layers and a quasi‐uniform 330km horizontal grid. Sea surface temperatures, sea ice extents, and zonally mean cloud amounts are prescribed from climatology, so that feedbacks between these variables and the rest of the model are ignored. Soil moisture, snow depth and boundary layer height are modelled explicitly, and both diurnal and seasonal variations of solar zenith angle are included. Two experiments are carried out, and compared with a three‐year control integration. In each case, the model's response varies with season and location. In the first experiment the effect of increasing atmospheric carbon dioxide concentrations with prescribed present day sea surface temperatures is examined. The model's troposphere becomes warmer, thereby increasing the low level static stability over the ocean and reducing evaporation and precipitation. The warming is larger over land than over the oceans. In summer, this results in an increase in precipitation along the eastern coasts of continents. In the second experiment, the sea surface temperatures are increased by 2 K and the carbon dioxide concentration is doubled. The land surface temperature rises by 3 K. Evaporation increases markedly over the oceans. Precipitation increases in the main regions of atmospheric convergence and decreases in some regions of the subtropics. The magnitude of the model's response is shown to be reasonably consistent with that found in other three‐dimensional climate models.