Simulation of monsoon variability in a warmer world
Akio Kitoh
Meteorological Research Institute, Tsukuba, Japan
Changes in the monsoons at the times of increased atmospheric CO2 and also at mid-Holocene are investigated with a global coupled GCM. 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. Precipitation variability 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.
Contrary to a year-round surface warming in the CO2-run, the 6ka-run reveals surface warming only in summer over Northern Hemisphere land. The SST at 6ka is colder than that at present due to different seasonal cycle in solar insolation. In the 6ka-run, there also is a northward shift and an intensification of the summer monsoon rainfall in the Afro-Asian region. Changes in the Australian summer monsoon rainfall is not large. The simulated SST variability in 6ka-run is weak and no ENSO-monsoon relationship nor Indian-Australian monsoon relationship was found.
