Comparison of monsoon changes simulated in a greenhouse warming and for 6000 years before present

Changes in the monsoon at the times of increased atmospheric CO2 and also at mid-Holocene (6,000 years before present: 6ka) are investigated with a global coupled atmosphere-ocean GCM (AOGCM). The one (CO2-run) is a transient CO2 experiment where atmospheric CO2 is increased at the 1% compound rate for 150 years, and the results are compared with the control run. The other (6ka-run) is a sensitivity experiment to the orbital parameters at 6,000 years before present, when Northern Hemisphere land masses get increased heating in summer and cooling in winter, respectively.

In the CO2-run, there is a northward shift and an intensification of the monsoon rainfall in the Afro-Asian region. The Australian summer monsoon is also intensified. Interannual variability of precipitation generally increases in a warmer world, except over the tropical and subtropical Pacific where simulated SST variability decreased. The ENSO-monsoon relationship (less Indian summer monsoon rainfall during El Nino years) holds both in the control and in the CO2-run, but its interdecadal variability is large. There is a significant correlation between the Indian summer (JJA) monsoon rainfall and the following Australian summer (DJF) monsoon rainfall in the control run, but this relationship becomes weaker and weaker when warming proceeds.

For the 6ka-run, results are compared with those performed with the atmospheric GCM (AGCM). Both the AGCM and the AOGCM run show enhanced seasonal variations in terms of surface temperature changes from February to August, but its magnitude and spatial pattern is different between the AGCM and the AOGCM. The African and Asian summer monsoons are more vigorous in the 6ka simulation with a northward shift of the rain band. This is more clearly seen in the AOGCM. A change in the Australian monsoon is small. The AOGCM 6ka-run showed a 0.35C cooling of the global mean SST and more extensive sea-ice extent.