Role of tropical Indian Ocean and Pacific Ocean dynamics on Indian summer monsoon rainfall simulation and prediction in CFSv2 coupled model

The El Niņo Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) are the two most dominant tropical coupled modes of climate variability driven by ocean dynamics. These two forcing factors not only drive the majority of interannual variability of Indian Summer Monsoon Rainfall (ISMR), but also interact with each other through atmospheric teleconnections, which acts as a bridge between the two ocean basins. The present study investigates the relative role of the Indian Ocean and Pacific Ocean dynamics on the simulation and prediction of ISMR using a set of sensitivity experiments with CFSv2 coupled model. The analysis reveals that the Indian Ocean dynamics are misrepresented in CFSv2 model, which can lead to the summer monsoon dry bias over the Indian land region.

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.