How interactions between tropical depressions and western disturbances affect heavy precipitation in South Asia

Abstract Interactions over South Asia between tropical depressions (TDs) and extratropical storms known as western disturbances (WDs) are to cause extreme precipitation events, including those responsible for the 2013 floods northern India. In this study, existing databases of WD TD tracks used identify potential WD–TD interactions from 1979–2015; these filtered according proximity intensity, leaving 59 cases which form basis paper. Synoptic charts, vorticity budgets, moisture trajectory analyses employed elucidate common interaction types among cases. Two broad families emerge. Firstly, a dynamical coupling TD, whereby either upper- lower-level vortices superpose (a vortex merger ), or is intensified it passes into entrance region jet streak associated with jet-streak excitation ). Secondly, exchange anomalous advected WD, resulting in near TD-to-WD WD-to-TD most post-monsoon period subtropical jet, brings WDs subcontinent, returns south; there smaller peak May June, driven by monsoon onset vortices. Precipitation heaviest dynamically-coupled interactions, particularly excitations. Criteria automated identification proposed, schematics each type presented highlight key mechanisms.