The importance of land surface processes for predicting climate change in the Asian monsoon area

The possible impact of anthropogenic climate change on the Asian summer monsoon is investigated in several time-slice experiments using prescribed sea-surface temperature (SST) and sea-ice anomalies. The study is carried out with different atmospheric general circulation models (GCMs), each being involved in two time-slice experiments differing only by the treatment of the land surface processes. The objective is to assess the robustness of the predicted climate change, and its possible sensitivity to the land surface hydrology. Despite the use of identical SST anomalies, the various GCMs do not give similar responses in the Asian summer monsoon. While the weakening of the Walker and Hadley circulations is consistent in all the experiments, there is a significant spread in the summer precipitation anomalies, showing that the sensitivity of the monsoon rainfall is not solely related to the changes in the circulation but also to changes in the atmospheric water content. Though it does not represent the main source of uncertainty, the treatment of the surface hydrology is liable to affect significantly the monsoon response to CO2 doubling. A slight change in evapotranspiration is enough to induce a significant change in precipitation. A simple analysis of the regional water budget indicates that this sensitivity is not only related to changes in the horizontal transport of water vapor, but also to changes in the precipitation efficiency, which depends on soil moisture and, therefore, on the treatement of the land surface hydrology.