Evaluating the Indian Summer Monsoon Variability in the warm climate

Abstract

Indian Summer Monsoon Rainfall (ISMR) plays a critical role in agriculture and in turn affects the economy of India. Accurate and timely prediction of ISMR and its variability is necessary to mitigate its adverse effects. In the recent period, there is a large debate on ISMR variability in a warming climate. Many Coupled General Circulation Models (CGCM) are also showing a large spread in the Indian Summer Monsoon (ISM) Rainfall variability of the future projections (end of the 21st century). Understanding the variability of ISMR through past periods will help to predict the ISMR under the future warmer scenario with improved skill. To achieve this, we have selected the warm mid-Pliocene (3.0-3.3 Ma) period which has similar external forcing such as, carbon dioxide (CO2 ) concentration comparable to the end of the century and orbital parameters were the same as the present period. To evaluate the ISMR variability during the mid-Pliocene, we have used five Coupled Model Intercomparison Project phase 6 (CMIP6) models simulation datasets. All the model's skill in predicting the ISMR is evaluated using available observations and reanalysis datasets during the historical period (1914-2013). During the mid-Pliocene, we find an increase in the seasonal rainfall over most parts of India than the recent period. The enhanced CO2 forcing in the mid-Pliocene led to increase in both Surface air temperature (SAT) over the Indian landmass and SST over the Indian Ocean which favors more evaporation and increased convective activity over India. In combination, the enhanced Temperature Gradient between the Indian landmass and Arabian Sea (TGIA) during the recent period strengthened the ISM circulation. It is also observed that synoptic scale features like heat low, monsoon trough and the wind circulation at lower (850 hPa, cross equatorial jet observed along the Somalia coast and over the Arabian Sea) and upper levels (at 200 hPa, tropical easterly jet over the southern peninsula) are intensified and found to be consistent with the higher precipitation observed over the Indian subcontinent.