Projection of the Indian Summer Monsoon onset using a regionally coupled atmosphere-ocean model

To examine the present and future mean variability of the Indian Summer Monsoon onset over Kerala, a high-resolution regional atmosphere-ocean coupled model (ROM) is deployed over the Coordinated Regional climate Downscaling Experiment (CORDEX) South Asia region. The model demonstrated its performance in simulating the onset and accompanying dynamical and thermodynamical characteristics. The model has shown reasonable skill in simulating the climatological onset with a slight delay when compared to observation (IMD) and reanalysis (NCEP), by 1 day in hindcast simulation (driven by ERA-40) and 4 days in historical simulation (driven by a global model of CMIP5; MPI-ESM), indicating the model bias in historical simulation is attributed to the driven forcing of parent MPI-ESM. Despite having abrupt transitional onset nature, its variability reflects strong teleconnections with the different modes of climate variability, such as El-Nino Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD). Additionally, the model's simulated onset dates have strong teleconnections with large-scale forcing such as sea surface temperature (SST). Under intermediate emission scenarios (RCP 4.5), ROM simulations are explored to assess future mean onset characteristics with possible deviations. The analysis indicates no shift in the projected mean onset with respect to historical simulation. The projected onset dates showed sizeable variability linked with future SST warming, suggesting that future ENSO and IOD events will substantially influence onset characteristics.