Previous climate change experiments with increased CO2 have shown greater mean south Asian monsoon precipitation and increased interannual variability. Confirming those previous results for an increase of CO2, the Parallel Climate Model (PCM), a global coupled climate model, shows greater mean south Asian monsoon precipitation and increased interannual variability. An ensemble of atmospheric model (NCAR CCM3) AMIP2-type sensitivity experiments with specified warmer tropical Indian and Pacific Ocean SSTs is analyzed to explore the mechanisms of these changes. Results show greater mean monsoon precipitation is due to increased moisture source from the warmer Indian Ocean. Additionally, increased south Asian monsoon interannual variability is mainly due to warmer mean tropical Pacific SSTs and the non-linear relationship between SST and evaporation. For a given interannual SST fluctuation with warmer mean SSTs, there is enhanced evaporation and precipitation variability that is communicated via the Walker Circulation to the south Asian monsoon to increase interannual monsoon precipitation variability there (even with no change in interannual SST variability in the Pacific).