Isolated tropical Indo-Pacific SSTA Impacts on ISMR Variability in Observations and CMIP6 Historical Simulations

Year-to-year variability in Indian summer monsoon rainfall (ISMR) can greatly impact the densely populated Indian subcontinent. While it has been demonstrated previously that tropical Pacific and Indian Ocean SSTAs can influence ISMR variability, the relative contribution from each basin has been difficult to determine due to the complicated inter-basin interactions. Using observational data and the historical simulations (1950-2014) from seven CMIP6 models with large ensemble sizes, we apply a cyclo-stationary linear inverse model (CS-LIM) to assess the isolated contributions from tropical Pacific SSTAs, Indian Ocean SSTAs, and their interaction to ISMR interannual variability. Observational results indicate that Pacific SSTAs enhance precipitation variability over northeastern and southern India, while Indian Ocean SSTAs and the Indo-Pacific interaction reduce the variability, with the Indo-Pacific interaction strongly damping the precipitation variability over central India. In CMIP6 models, Pacific SSTAs typically increase ISMR variability, but their spatial patterns largely differ from observations. For the impacts from Indian Ocean SSTA and the Indo-Pacific interaction, all models capture the observed reduction in precipitation variability, but the magnitude and spatial patterns vary considerably, with most models failing to simulate the stronger damping effect due to the Indo-Pacific interaction.