Unraveling cross-equatorial dry intrusion influence on Indian summer monsoon rainfall

The summer monsoon provides most of the rain in India. Any variability in rainfall, particularly on the daily-to-weekly scale, strongly impacts the environmental systems and socioeconomic activity. This study focuses on cross-equatorial dry intrusions (DIs) in which dry air masses descend slantwise from the southern midlatitudes upper troposphere towards lower levels in the Arabian Sea during monsoon season. The presence of dry air masses in the Arabian Sea, a hot spot for such DIs, has great potential to modify the moisture availability and rainfall over India.

Using the Lagrangian analysis tool, LAGRANTO, forced by ERA-interim reanalysis data for 40 monsoon seasons (1979-2018), we identify 130 DI events. We show that 95% of DI air masses reach below 800 hPa in the central Indian Ocean within two days of their initiation in the southern hemisphere. Though rare (<1% frequency in time), their presence in the marine boundary layer triggers intense ocean evaporation due to enhanced moisture gradient in the lower levels. These DIs enhance low-level flow that facilitates more moisture transport from the central Indian ocean towards India. Depending on the wind pattern, increased moisture transport changes rain over land: 60% of the DI events are associated with more than 25% enhancement, whereas 40% with about 10% decrease in the rain compared to climatology. In summary, cross-equatorial dry air masses reaching the marine boundary layer over the tropical Indian Ocean during the summer monsoon season are responsible for developing anomalously moist air and heavy precipitation over the Indian region downstream and thus crucial for summer monsoon rainfall predictability, especially its extremes.