Falling Trend of Western Disturbances in Future Climate Simulations

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.

The impacts of climate change on the winter water cycle of the western Himalaya

“…This increase of 9% in the net flux is much smaller than the apparent contribution from increased humidity alone ( q ′̄ : 28%), which is due to mitigating effects from the dynamics ( q ′ : −20% ). For comparison, we note that Hunt et al (2019) found that WDs were likely to weaken by about 12% in an end-of-century RCP8.5 climate.…”

“…1.2) that proxy techniques are not reliable when it comes to counting WDs, we cannot be certain of the significance of these results in the context of changing WD frequency in a future climate. However, Hunt et al (2019) used a feature-tracking algorithm to detect WDs in the output of 24 CMIP5 models. Compared to a 1980-2005 baseline, they found that 2080-2100 WD frequency decreased significantly by an average 11% and 17% in RCP4.5 and RCP8.5 experiments respectively.…”

Central Himalayan tree-ring isotopes reveal increasing regional heterogeneity and enhancement in ice mass loss since the 1960s

“…Severe glacial mass loss (−0.437 ± 0.191 m w.e.) since the 1960s corresponds to a further and prominent shift towards drier conditions arising out of the weakening of both the ISM and westerlies (Qin et al, 2000;Hunt et al, 2019;Basistha et al, 2009;Singh et al, 2019;Sano et al, 2012Sano et al, , 2013Sano et al, , 2017. Increasing regional and global temperatures coupled with progressive regional industrialization may have exacerbated the retreat rates.…”

“…It broadly affected the Asian summer monsoon, and the region has progressively moved towards a drier phase, subsequent to a breakdown in the ITCZ and Northern Hemisphere temperature relation since the mid-19th century (Ridely et al, 2015;Xu et al, 2012). Nevertheless, the strength of the westerlies in the region remained unaffected until the mid-20th century (Joswaik et al, 2013;Hunt et al, 2019;Khan et al, 2019). Thus, depending upon the geographical location and proximity to the oceans, a distinct regionality developed, with a decline in summer monsoon precipitation.…”

“…Moreover, a weakening in the moisture-delivering systems (i.e. WD and ISM) since the mid-20th century have had a direct impact on the summeraccumulation type glaciers in the region (Qin et al, 2000;Hunt et al, 2019;Joswiak et al, 2013;Khan et al, 2019;Roxy et al, 2015Roxy et al, , 2017Sano et al, 2012Sano et al, , 2013Sano et al, , 2017Singh et al, 2019;Xu et al, 2018;Yadav, 2011).…”