Subtropical Westerly Jet Influence on Occurrence of Western Disturbances and Tibetan Plateau Vortices

Western disturbances (WDs) are midtropospheric to upper-tropospheric mesoscale vortices, which typically propagate along the subtropical westerly jet stream and bring heavy rainfall to Pakistan and northern India during boreal winter. They are dynamically similar to Tibetan Plateau vortices (TPVs), which affect southwest China during spring and summer and emanate from the Tibetan Plateau. Here we propose that their similarity implies the existence of a more general group of upper-tropospheric vortices featuring interactions with the orography of the Hindu Kush-Himalaya-Tibetan Plateau region. Using existing track databases for WDs and TPVs derived from ERA-Interim reanalysis, we show that their respective occurrence frequencies are highly anticorrelated with each other through the seasonal cycle, yet both are strongly correlated with jet latitude. Our findings imply that the incidence of hazards due to WDs and TPVs is correlated on intra-annual and interannual time scales, particularly through upper-level baroclinicity. Plain Language Summary North India and south China (including Tibet) experience seasonal midlevel cyclonic systems, referred to as western disturbances (WDs) and Tibetan Plateau vortices (TPVs), respectively. These systems can be responsible for heavy, even catastrophic, rainfall in the areas associated with them, and thus improving prediction and risk correlation is vital. In this study, we show that: first, previous literature alludes to the two types of system bearing similar structures; second, the mean annual cycles of both are strongly affected by the subtropical westerly jet; third, the interannual populations of each are related; and fourth, this relationship can be explained by considering features of the jet, such as strength and position. We thus conclude that it is likely that WDs and TPVs are specific examples of a more general class of synoptic-scale vortex.