Western disturbances (WDs) are upper-level synoptic-scale systems embedded in the subtropical westerly jet stream (STWJ), often associated with extreme rainfall events in north India and Pakistan during boreal winter. Here, a tracking algorithm is applied to the upper-tropospheric vorticity field for 37 years of ERA-Interim reanalysis data, giving a catalogue of over 3000 events.

More than one billion people live in regions affected by the South Asian summer monsoon. This Review provides an overview of our understanding of summer monsoon rainfall variability and its causes, and considers how the monsoon will change as a consequence of global warming.