Role of tropical Indian Ocean and Pacific Ocean dynamics on Indian summer monsoon rainfall simulation and prediction in CFSv2 coupled model
Sensitivity experiments have clearly highlighted that the prediction skill of ISMR is basically coming from the ENSO-monsoon relationship and it is reasonably captured in the CFSv2 model. As a result of cold SST bias in the tropical Indian Ocean, the seasonal mean rainfall over the Indian landmass is underestimated in the CFSv2 control (CTL) run. The enhanced convection over the eastern equatorial Indian Ocean modulates the local Hadley circulation and forces subsidence over the Indian landmass, which also enhances the dry bias over the Indian landmass. Further, teleconnections of ISMR and Indian Ocean SST are opposite to that of observed teleconnection. In order to understand the cause for these discrepancies we have carried out sensitivity experiments. Once we reduce the cold SST bias in the tropical Indian Ocean (by suppressing Indian Ocean dynamics), the magnitude of rainfall over the Indian landmass is enhanced. However, the prediction skill of ISMR has reduced due to absence of coupled ocean-atmosphere dynamics in the Indian Ocean slab (ISLAB) run. It is concluded that the cold SST bias in the tropical Indian Ocean is to be minimized to improve the magnitude of the ISMR simulation whereas the Indian Ocean SST- monsoon teleconnection is to be corrected in the model to improve the ISMR prediction skill. Therefore, this study highlights the need to improve the Indian Ocean dynamics in CFSv2 for the further improvement of the prediction skill of ISMR.