The influences of the SST and land surface temperature on the onset of the Asian summer monsoon are studied by using an atmospheric GCM, Global Spectral Model (GSM) of Japan Meteorological Agency (JMA). After confirming from five-year control run with the climatological SST that the model performs the Asian summer monsoon well, we make the impacts of two kinds of main experiments, i.e., SST fixed run and solar fixed run, focusing on the Asian summer monsoon onset. In the SST fixed run, the model is run from April 1st until June 30th under the SST, which is fixed at the value of April 1st. To see impacts systematically, an ensemble average over five cases is compared with that of the control run. The fixed SST considerably reduces the Somali jet, the cross equatorial water vapor transport, the northern-hemispheric evaporation, and then suppresses the ITCZ jump from the southern hemisphere to the northern hemisphere. It also reduces the low-level westerlies from Southeast Asia, which brings the monsoon onset. In the solar fixed run, the model is run from April 1st until June 30th under the solar condition, which is fixed at that of April 1st. The fixed solar condition reduces the land-sea thermal contrast, suppresses the heat low over the continent, and reduces low level westerlies in a geostrophic sense. The above experiments indicate that both land-sea thermal contrast and SST considerably contribute to the onset of the Asian summer monsoon. The land-sea thermal contrast induces low-level wind surrounding the Eurasian continent, and makes a primary contribution to the formation of the Asian monsoon westerlies. The seasonal march of the SST destabilizes the stratification, induces the ITCZ jump from the southern to the northern hemisphere, strengthens the Hadley circulation, and then enhances the monsoon westerlies through effective transport of the absolute angular momentum.
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