Enhancement of Indian summer monsoon rainfall by cross-equatorial dry intrusions

The Indian summer monsoon affects the lives of over 1/6 of the world's population. Precipitation extremes during summer monsoons have dire socioeconomic impacts. Yet, the mechanisms leading to these extremes are poorly understood, making their accurate forecasts and reliable future projections a longstanding challenge. Using a Lagrangian-based method, we show that precipitation extremes link to dry air intrusions from the southern midlatitudes upper troposphere, crossing the equator, and reaching the Arabian Sea. By triggering intense ocean evaporation, these dry intrusions are associated with modulated moisture transport patterns toward India and enhanced precipitation by >17% on average, often embedding local extremes. A notable example is the excessive rain that caused the devastating Kerala flood of 2018. However, depending on the wind pattern, these dry intrusions may, in some cases, decrease rainfall over land. The emerging connection of rainfall variability with midlatitude weather systems opens opportunities for improving the forecast of precipitation extremes and understanding their future projections.