Response of the Indian monsoon and ENSO-monsoon teleconnection to enhanced greenhouse effect in the CNRM coupled model

The present study is an assessment of a two-member ensemble of transient climate change simulations, with a focus on the Indian summer monsoon and ENSO-monsoon teleconnection. The CNRM ocean-atmosphere coupled model is integrated from 1950 to 2099 and driven by changes in concentrations of greenhouse gases and sulfate aerosols. The simulated monsoon climate is first validated against available observations and NCEP/NCAR reanalyses over the second half of the 20(th) century. The model captures the main features of the Indian monsoon climate and the main mode of variability found in the tropical regions, namely the El Nino Southern Oscillation, reasonably well. During the second half of the 21(st) century, both scenarios indicate a significant increase in the annual mean surface air temperature (about 2degreesC) and in monsoon precipitation (less than 10%) over India, relative to the 1950-1999 climatology. However, the model does not show a clear strengthening of the monsoon circulation, but rather a northward shift of the westerly monsoon flow. The increase in monsoon precipitation is therefore partly due to a 'non-dynamical' response to global warming, namely a large increase in precipitable water over India. While the transient response of the model shows a qualitative agreement with the surface warming observed over recent decades, neither the observations nor the model indicate significant trends in All India monsoon rainfall in the late 20th century. A long-term increase in simulated monsoon precipitation does appear from 1950 to 2099, but is superimposed onto relatively large multi-decadal fluctuations. The simulated ENSO-monsoon teleconnection also shows a strong modulation on multi-decadal time scales, but no systematic change with increasing amounts of greenhouse gases.