Contribution of cross-equatorial dry intrusions to Indian summer monsoon rainfall

Daily-to-weekly variations between active and break monsoon phases critically control Indian summer monsoon (ISM) rainfall and directly influence society. This variability depends on the complex air flows from different origins; for example, dry intrusions (DIs) reaching the Indian region from western Asia limit ISM rainfall. This study documents DIs that originate in the southern hemisphere winter, cross the equator and reach the Arabian Sea during the ISM season. Being a global hot spot of such cross-equatorial intrusions, they have great potential to modify the moisture availability and hence the ISM rainfall.
Using 6-hourly ERA-Interim reanalysis data with a Lagrangian approach, we show that more than 95% of cross-equatorial DIs reach below 850 hPa in the Arabian Sea within five 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 and enhances the low-level Somali jet intensity. The result is compensation for their initial dryness and overall increasing the moisture transport towards India. Analysis based on 130 DI events during the ISM season from 1979-2018, shows that more than 64% of the DI events are associated with more rainfall, with a mean 12% enhancement in the rain compared to climatology. These DI events favor the central Indian Ocean as the major source of moisture during the ISM season, different from mean conditions. Indeed, 52% of known active spells are preceded by DI events. In summary, cross-equatorial DIs reaching the marine boundary layer over the tropical Indian ocean during ISM season is responsible for the development of anomalously moist air, which enhances the rainfall over the Indian region downstream, and are thus crucial for ISM rainfall predictability.