Abstract
Abstract Western disturbances (WDs) are synoptic-scale cyclonic weather systems advected over Pakistan and northern India by the subtropical westerly jet stream. There, they are responsible for most of the winter precipitation, which is crucial for agriculture of the rabi crop as well for as more extreme precipitation events, which can lead to local flooding and avalanches. Despite their importance, there has not yet been an attempt to objectively determine the fate of WDs in future climate GCMs. Here, a tracking algorithm is used to build up a catalog of WDs in both CMIP5 historical and representative concentration pathway (RCP) experiments of the future. It is shown that in business-as-usual (RCP8.5) future climate simulations, WD frequency falls by around 15% by the end of the twenty-first century, with the largest relative changes coming in pre- and postmonsoon months. Meanwhile, mean WD intensity will decrease, with central vorticity expected to become less cyclonic by about 12% over the same period. Changes in WD frequency are attributed to the projected widening and weakening of the winter subtropical jet as well as decreasing meridional wind shear and midtropospheric baroclinic vorticity tendency, which also explain the changes in intensity. The impact of these changes on regional precipitation is explored. The decline in WD frequency and intensity will cause a decrease in mean winter rainfall over Pakistan and northern India amounting to about 15% of the mean—subject to the ability of the models to represent the responsible processes. The effect on extreme precipitation events, however, remains unclear.
Highlights
Western disturbances1 (WDs) are synoptic-scale cyclonic circulations embedded in the subtropical westerly jet that impact Pakistan and northern India, typically during the winter
We explore possible causes and consequences of these trends, subject to the caveat that evaluation of precipitation in CMIP5 models assumes that the responsible mechanisms are correctly simulated in the first place, and that these do not change hugely in future thermodynamic environments
We explore the trends of western disturbances in future climate scenarios
Summary
Western disturbances (WDs) are synoptic-scale cyclonic circulations embedded in the subtropical westerly jet that impact Pakistan and northern India, typically during the winter. There, they are an important agent of the hydrological cycle, being responsible for most nonmonsoonal precipitation Their presence plays a crucial role in supporting the rabi winter crop but is responsible for occasionally devastating precipitation, resulting in severe floods and avalanches (Mooley 1957; Rangachary and Bandyopadhyay 1987; Houze et al 2011; Dimri et al 2015; Houze et al 2017; Hunt et al 2018c). Several studies have attempted to infer whether a trend in the frequency of winter western disturbances already exists.
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