Sensitivity of Monsoon Onset to Idealised Volcanic Forcing

Large volcanic eruptions are a source of climate variability, affecting global temperatures and precipitation. The hydrology of the Indian monsoon region is particularly sensitive to volcanic forcing. Previous studies have focused on the seasonal mean response of the Indian monsoon to eruptions. Here, we investigate the changes to the onset of the monsoon, which is an important characteristic that impacts the water budget of the region. Using large Earth System ensemble simulations with idealised model eruptions that inject 40 Tg of sulphur into the stratosphere at varying latitudes, we observe changes in onset date by a few weeks compared to an unforced case. We find that the date of onset of the Indian summer monsoon is strongly dependent on the eruption latitude. Our results show a delayed (advanced) Indian monsoon onset for a Northern (Southern) Hemispheric eruption. However, the internal variability of the monsoon system also influences the onset. We find that existing mechanisms linking internal variability to monsoon onset are insufficient to explain the onset changes observed due to volcanic forcings. Based on the Low-Level Jet (LLJ) and ITCZ frameworks, we propose a new mechanism of monsoon onset variation due to volcanic eruptions. We show that not just a strengthening LLJ, but also an increased moisture flux into the Indian monsoon region triggers an earlier onset.