Observed and Modeled Changes in the South Asian Summer Monsoon over the Historical Period

The behavior in the South Asian summer monsoon (SASM) was analyzed in Coupled Model Intercomparison Project (CMIP3) multimodel historical (20c3m) simulations and in modern observational and reanalysis data. The CMIP3 simulations capture the observed trend of weakening of the SASM circulation over the past half century, but are unable to reproduce the magnitude of the observed weakening trend. While the observations indicate a slight decrease in SASM-related precipitation, the CMIP3 simulations indicate on average a very slight increase, albeit with very large intermodel and intramodel variabilities. The CMIP3 simulations reproduce the observed negative relationship between the SASM and ENSO. The observed weakening trend in this relationship in recent decades, which has been attributed in some studies to anthropogenic forcing, appears to be well within the variability of the CMIP3 multimodel ensemble. For some models, distinct realizations indicate both strengthening and weakening trends that are larger in magnitude than the observed weakening trend.