Forced Response in the Mean State and Interannual Variability of the Indian Summer Monsoon in Future Projections

The Indian summer monsoon (ISM) is a complex system that plays a crucial role in the climate of South Asia. The Community Earth System Model 2-Large Ensemble (CESM2-LE) simulations are used to explore the forced response in the mean state and interannual variability of the ISM in future projections. This single-model, initial-condition large ensemble simulations are useful for studying the effect of natural variability as well as the forced response in a coupled ocean-atmosphere climate system. Simulations during historical periods (1980–2009) show that the model could fairly reproduce the mean state and interannual variability of the ISM in terms of excess and deficient rainfall years and its association with strengthening and weakening of monsoon circulation. It is also noticed that though low-level monsoon jet shows a weakening during deficient monsoon years, it has more eastward extension, up to the western Pacific Ocean, compared to excess monsoon years. In simulations for future periods, the rainfall pattern shows an overall enhancement in near future (2030–2059) and far future (2070–2099) periods. However, both the low-level jet and upper level tropical easterly jet stream are weaker in future periods than that of historical period. The future projections also exhibit stronger interannual variability, with wetter excess rainfall years and drier deficient years than corresponding historical years. Moreover, the likelihood of deficient years occurring in the future periods may increase with more dry conditions over southern peninsular India. Since the 100 ensemble mean suppresses most of the natural variabilities, changes exist in the projected mean values are mainly due to the induced forcing (SSP3.7) in the model. The difference between with ENSO and without ENSO cases in the interannual variability shows that ENSO amplifies the variability in future projections. Thus, the results obtained from the CESM2-LE simulations are crucial for developing more effective strategies for managing and policy-making efforts aimed at mitigating the impacts of monsoon variability in a warming world.