Simulated Changes in the Asian Summer Monsoon at Times of Increased Atmospheric CO₂

Abstract

Possible changes in the Asian summer monsoon due to increased atmospheric CO₂ are investigated by an MRI global coupled atmosphere-ocean general circulation model. The summer (June-August) monsoon rainfall in India increases significantly with global warming. On the other hand, the monsoon wind shear index, defined as the difference between 850hPa and 200hPa zonal winds over the northern Indian Ocean, decreases. At 850hPa, the westerly wind shifts northward and intensifies from the Sahel to northwest of India, but the monsoon westerly over the Arabian Sea weakens. It is found that increased moisture content in the warmer air leads to larger moisture flux convergence, contributing to the increased rainfall. Therefore, the monsoon wind shear index is not a good indicator for identifying any change of monsoon accompanying global warming. In contrast to the increased rainfall in India, change in rainfall is little over China where soil moisture becomes drier at times of increased CO₂. It is also noted that the northern Eurasian continent becomes wetter in the increased-CO₂ climate.
The magnitude of the interannual variability of the Asian summer monsoon rainfall becomes larger in the CO₂ experiment than in the control experiment, particularly in the later stage of the experiment after CO₂ doubling. However it should be noted that the interdecadal variation of this interannual variability is also large both in the control and the CO₂ experiments.