Projected seasonal mean summer monsoon over India and adjoining regions for the twenty-first century

Abstract

In this study, we present the projected seasonal mean summer monsoon over India and adjoining regions for the twenty-first century under the representative concentration pathway (RCP) 4.5 and RCP 8.5 scenarios using the regional model RegCM4 driven by the global model GFDL-ESM2M. RegCM4 is integrated from 1970 to 2099 at 50 km horizontal resolution over the South Asia CORDEX domain. The simulated mean summer monsoon circulation and associated rainfall by RegCM4 are validated against observations in the reference period 1975 to 2004 based on the Global Precipitation Climatology Project (GPCP) and India Meteorological Department (IMD) data sets. Regional model results are also compared with those of the global model GFDL which forces the RegCM4, showing that the regional model in particular improves the simulation of precipitation trends during the reference period. Future projections are categorized as near future (2010–2039), mid future (2040–2069), and far future (2070–2099). Comparison of projected seasonal (June–September) mean rainfall from the different time slices indicate a gradual increase in the intensity of changes over some of the regions under both the RCP4.5 and RCP8.5 scenarios. RegCM4 projected rainfall decreases over most of the Indian land mass and the equatorial and northern Indian Ocean, while it increases over the Arabian Sea, northern Bay of Bengal, and the Himalayas. Results show that the monsoon circulation may become weaker in the future associated with a decrease in rainfall over Indian land points. The RegCM4 projected decrease in June, July, August, September (JJAS) rainfall under the RCP8.5 scenario over the central, eastern, and peninsular India by the end of the century is in the range of 25–40 % of their mean reference period values; it is significant at the 95 % confidence level and it is broadly in line with patterns of observed change in recent decades. Surface evaporation is projected to increase over the Indian Ocean, thereby supplying more moisture into the atmosphere. As per the RegCM4 projection, the northward flank of the southwesterly winds (i.e., over the central and north India) may become stronger and veer towards the north over the Arabian Sea, traverse trans-India over the foothills of the Himalayas and northern Bay of Bengal, and reach up to Burma. The changes in circulation lead to a change in the moisture distribution and result in a decrease of moisture convergence over central and peninsular India, the Indian Ocean, and the southern part of Bay of Bengal and an increase over the Arabian Sea, northern Bay of Bengal, and the Himalayas.