The Response of Onset and Withdrawal of the Indian Summer Monsoon to Volcanic Aerosols

Large volcanic eruptions are a source of climate variability, affecting the seasonal mean precipitation of the Indian summer monsoon. However, the extent to which changes in seasonal precipitation can be attributed to variations in monsoon length vs. monsoon intensity has remained unclear. Using large ensemble simulations of idealised volcanic eruptions at varying latitudes, we find that the monsoon onset and withdrawal change by a few weeks compared to an unforced case. Southern (Northern) hemispheric eruptions extend (shorten) the Indian summer monsoon in the year following the eruption. We show that these changes in the length of the monsoon season are more important in controlling the response of total monsoon rainfall to eruptions than changes to the intensity of rainfall. We explain our results using the low-level jet and Inter-tropical Convergence Zone (ITCZ) frameworks, which have been used to explain the internal variability of the monsoon onset.