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Dynamically Downscaled Climate Change Projections for the Indian Monsoon

The complex orography and the extreme concentration of population of South Asia combine to make the future evolution of the South Asian monsoon both challenging to project technically and critical to accurately understand insofar as the global warming impact is concerned. Here we describe a series of climate projections that have been constructed using the Weather Research and Forecasting Model (WRF V3.9) for South Asia to downscale a global warming simulation constructed using the Community Earth System Model (CESM) under the representative concentration pathway 8.5 (RCP8.5) scenario. A physics-based mini ensemble is employed to investigate the sensitivity of the projected change of the Indian monsoon to the implementation of different parameterization schemes for processes that are not explicitly resolved at the spatial resolutions at which WRF has been employed using a two-step nesting procedure. We analyze not only the changes in mean precipitation but also the impact of the warming process on precipitation extremes, characterizing the latter using Extreme Value Analysis. All projections are characterized by a consistent increase in summer average precipitation and a fattening of the tail of the daily rainfall distribution. Both the average rainfall intensity changes and the extreme precipitation increases are projected to be slightly larger than expectation based upon the Clausius–Clapeyron thermodynamic reference of 7%/°C of surface warming in most parts of India. This further increase can be primarily explained by the fact that surface warming is projected to be smaller than the warming in the mid-troposphere, where a significant portion of rain originates, while dynamical effects play only a secondary role.