A new approach for seasonal prediction using the coupled model CFSv2 with special emphasis onIndian Summer Monsoon Rainfall

Predicting the Indian summer monsoon (ISM) is a challenging task due to the complexity of the climate system. Any improvement in the prediction skill of ISM in general circulation models would highly benefit the country as a whole due to its close linkage with the economy. In this study, we have adopted a new strategy to improve the ISM rainfall (ISMR) bias and prediction using the National Centers for Environmental Prediction-Climate Forecast System version 2 (NCEP-CFSv2). This model is currently used for the seasonal prediction in many countries including India but is known to have persistent dry bias over the Indian landmass. Three sets of hindcast experiments are carried out for 9 months each, for the period 2005-2019. The experiments differ from each other in the way they are initialized. Significant reduction in dry bias over the Indian landmass in the summer season with improved representation of tropical Indo Pacific sea surface temperature is reported from the new initialization  strategy. It is found that enhanced moisture transport to Indian landmass from the Arabian Sea,  improved representation of mean cyclonic circulation over north India, weak southeasterlies from Bay of Bengal and western Pacific together with enhanced Walker circulation contributed to the reduction  in dry bias over the Indian landmass. In addition to the above, the midlatitude circulation contribution by enhancing the strength of Subtropical High in the North Pacific resulted enhanced precipitation over the Indian landmass. The initialization strategy used here would be highly useful for improving the seasonal monsoon forecast.