The role of dry intrusions in breaks of the Indian summer monsoon

The Indian summer monsoon (ISM) is of great importance to over a billion people since it supplies over 75% of the country’s annual precipitation. Significant intraseasonal variability in rainfall affects people, with breaks responsible for causing water shortage. It is known that dry intrusions play a role in breaks; however, it is not well understood compared to the role dry intrusions play during progressions of the onset and withdrawal of the ISM. In this study, we use observations and the ERA5 reanalysis to understand the role of dry intrusions in breaks during 1940–2023. We develop an index based on moisture deficit to identify dry intrusions, and find that most breaks are associated with dry intrusions emanating from arid regions to the west and northwest of India. These dry intrusions begin to enter India around a week prior to the middle day of breaks, reaching their peak strength over north India three days prior to the middle day of breaks. Vertical profiles reveal that these are mid-level dry intrusions, which are similar to those driving the direction of the withdrawal of the ISM. As breaks evolve, these dry intrusions deepen throughout their horizontal extent and descend into the country, stabilising the troposphere and creating an unfavourable environment for deep convection. We also find that extended breaks have stronger dry intrusions as precursors. This work provides a new perspective on the causal relationship between mid-level dry intrusions and breaks. The results could help improve forecasts of breaks, ultimately benefiting stakeholders in improving long-term planning.