Natural and forced Asian monsoon rainfall – SST relationship

We examine the relative roles of natural variability and anthropogenic climate change in the Asian summer monsoon – El Nino-Southern Oscillation (ENSO) relationship during the 20th Century using observations and the Coupled Model Intercomparison Project - Phase 5 (CMIP5) models. By applying the signal to noise maximizing Empirical Orthogonal Function analysis (S/N EOF) to global sea surface temperature (SST) of the multi-model, multi-ensemble CMIP5 model simulations, we can extract a model-based best estimate of the radiatively forced component of the SST change. We then subtract the forced component from the total change to obtain the natural varying component of the Nino 3.4 region. The observed and model simulated monsoon rainfall changes are regressed onto the natural and forced SST components to identify the associated spatial patterns and relative contributions. When the radiatiely forced component is removed, the CMIP5 models and observations agree very well of the negative relationship between ENSO and monsoon throughout the 20th Century. The forced monsoon change associated with SST warming, however, is dominated by a drying signal in CMIP5 models, largely due to the aerosol forcing. Results also show that natural variability is the dominant factor of the 20th Century summer monsoon rainfall variability. We further contrast the mechanisms of the natural and forced changes using moisture budget analysis.