Evolution of the Indian summer monsoon rainfall simulations from CMIP3 to CMIP6 models

Systematic biases in simulation of the mean and variability of Indian summer monsoon rainfall (ISMR) and its teleconnection with the El Nino and Southern Oscillation (ENSO) by climate models have proven to be roadblocks for improving the seasonal prediction skill of the ISMR. Yet, the improvements of these biases in simulating the ISMR by coupled ocean–atmosphere general circulation models (CGCMs) have been slow. Here, we investigate the progress made by the latest CMIP6 models in simulating the mean and variability of the ISMR including its multi-decadal variability in comparison to the earlier CMIP3 and CMIP5 models and unravel key common and persisting biases. That the number of models simulating the pattern of climatological mean summer rainfall over the region with fidelity has increased from less than 10% in CMIP3 to more than 50% in CMIP6 is a notable progress. The CMIP6 models also indicate notable progress in simulating the inter-annual, multi-decadal variability and marginal improvement in simulating the ENSO-ISMR correlations. In addition to consolidating these advances in simulating the ISMR in more climate models, future developments must focus on improving the following three persisting biases namely; (1) most models, including those in CMIP6, tend to simulate the mean location of the continental ITCZ during June–September (JJAS) about 10° south of the observed location of ~ 22° N, (2) with the ensemble mean 21-year moving correlation between ISMR and JJAS mean Nino3.4 sea surface temperature for all models being ~ − 0.3 compared to observed ~ − 0.65, almost all models simulate significantly weaker ENSO-monsoon relationship and (3) the number of models simulating the observed variance and periodicity of the multi-decadal variability of ISMR remains small even in CMIP6. We argue that the position bias in simulating the continental ITCZ may be related to the large and significant biases in simulating the JJAS SST over the tropical Indian Ocean, persistent even in the best of CMIP6 CGCMs making the improvement of SST biases (and associated heat transport across the equator) another area where future model developments need to focus.