The Global Monsoon Variability Simulated by CMIP3 Coupled Climate Models

The global monsoon climate variability during the second half of the twentieth century simulated by 21 coupled global climate models (CGCMs) that participated in the World Climate Research Programme's Coupled Model Intercomparison Project phase 3 (CMIP3) is evaluated. Emphasis was placed on climatology, multidecadal trend, and the response of the global monsoon precipitation to volcanic aerosols. The impact of the atmospheric model's horizontal resolution on the group ensemble mean (GEM; obtained from the three resolution groups) simulations of global monsoon climate is also examined. The CMIP3 CGCMs' multimodel ensemble simulates a reasonably realistic climatology of the global monsoon precipitation and circulation. The GEMs are also able to capture the gross features of the global monsoon precipitation and westerly domains. However, the spreading among the rainfall GEMs is large, particularly at the windward side of narrow mountains (e. g., the western coast of India, the Philippines, Mexico, and the steep slope of the Tibetan Plateau). Main common biases in modeling rainfall climatology include a northeastward shift of the intertropical convergence zone (ITCZ) in the tropical North Pacific and a southward migration of the North Atlantic ITCZ during boreal winter. The trend in the Northern Hemisphere land monsoon index (NHMI) detected in the CMIP3 models is generally consistent with the observations, albeit with much weaker magnitude. The significant decreasing NHMI trend during 1951-85 and 1951-99 occurs mainly in the models with volcanic aerosols (VOL models). This volcanic signal is detectable by comparison of the forced and free runs. It is estimated that from about one-quarter to one-third of the drying trend in the Northern Hemisphere land monsoon precipitation over the latter half of the twentieth century was likely due to the effects of the external volcanic forcings. On the other hand, the significant increasing trend in the global ocean monsoon index (GOMI) during 1980-99 appears chiefly in those models that are free of volcanic aerosols (No-VOL models). The exclusion of the volcanic aerosols is significant in simulating the positive GOMI trend against the internal variability of each model. These results suggest the climatic importance of the volcanic forcings in the global monsoon precipitation variability.