Abstract/Summary

The Indian summer monsoon supplies over 75% of the coun- try’s annual precipitation, profoundly impacting over a bil- lion people. Variability in the timing of its onset and withdrawal has a direct impact on the agricultural sector and other users of water resources. Previous studies have shown that a wedge of mid-tropospheric dry air emanating from the midlatitudes is present over India during the pre-monsoon, which gradually retreats toward the northwest as the onset progresses. The withdrawal of the monsoon is observed to progress in a southeast direction during September–October, but there is a lack of a conceptual model. In this study, we use observations and the ERA5 reanalysis to understand the dynamics and thermodynamics of the withdrawal. We find that the climatological mid-level dry intrusion appears over northwest India around mid-September. Vertical profiles associated with this dry air show how the most unfavourable environment for deep convection occurs in the northwest, where the withdrawal occurs first. As the withdrawal progresses, the wedge of dry air deepens through- out its horizontal extent and descends, as well as pushing further across the country. This stabilises the troposphere, suppressing deep convection and ultimately driving the withdrawal toward the southeast. By mid-October, the dry air engulfs most of India, causing the monsoon to withdraw from the entire country. Thus the strengthening of the mid-level dry advection from the midlatitudes can explain the withdrawal of the monsoon and its direction, in a reversal of the processes at work during progression of the onset. This work establishes a new paradigm for the withdrawal of the Indian summer monsoon in terms of midlatitude interactions, which could be tested for other monsoon regions.