Four climate change scenarios for the Indian summer monsoon by the regional climate model COSMO-CLM

This paper discusses projections of the Indian summer monsoon (ISM) by the regional climate model COSMO-CLM, highlighting similarities to and differences from its driving model, the global atmosphere-ocean model ECHAM5/MPIOM. The ISM is quantified using the all-Indian monsoon rainfall (AIMR) index and two vertical wind shear indices. To investigate the impacts of greenhouse gas emissions on the ISM, four emission scenarios for the time period 1960-2100 (Special Report on Emissions Scenarios A2, A1B, B1, and commitment) are considered. The COSMO-CLM simulations show significantly weakening ISM trends in all indices for emission scenarios A2, A1B, and B1. Parts of northwestern India are projected to face a decrease in the monsoon rainfall amount of over 70% within this century. For the wind shear indices, the projected decreases are mainly due to changes in the upper troposphere winds. The weakening of the dynamics in the COSMO-CLM is in agreement with the weakening in the driving ECHAM5/MPIOM model. The two models further agree in significantly positive trends of atmospheric water vapor contents and rain day intensities. However, ECHAM5/MPIOM shows no decrease in AIMR. The different AIMR trends in the two models are found to be due to different changes in the residence time of water in the atmosphere: In the COSMO-CLM projections, the residence time is more prolonged than in ECHAM5/MPIOM. This again is the consequence of a decrease in the number of depressions moving toward the northwestern parts of India.