Increasing frequency and lengthening season of western disturbances is linked to increasing strength and delayed northward migration of the subtropical jet

Abstract. Western disturbances (WDs) are cyclonic storms that travel along the subtropical jet, bringing the majority of seasonal and extreme precipitation to mountainous South Asia in the winter months. They are a vital component of the region's water security. Although typically most common in the winter, WDs can also occur during the summer monsoon with catastrophic consequences. This happened earlier this year, leading to fatal floods across North India, including Delhi. Preceded by an unusually harsh winter season, questions are now being asked about how climate change is affecting WD frequency and intensity in both summer and winter seasons.

An analysis of 17 previous studies assessing trends in WD frequency revealed no consensus, at least in part because they quantified trends in different regions, seasons, and time periods. In this study, a more robust approach is used, quantifying trends in WD frequency and intensity by region and month, using a track catalogue derived from seventy years of ERA5 reanalysis data. Winter WDs have increased significantly over the Western and Central Himalaya and Hindu Kush in the last 70 years. This trend is attributed to a strengthening of the subtropical jet. The WD season has also significantly lengthened with WDs becoming far more common in May, June and July. For example, WDs have been twice as common in June in the last twenty years than during the previous fifty. This is attributed to delayed northward retreat of the subtropical jet, which historically has occurred before the onset of the summer monsoon. The most important implication is that the frequency of `monsoonal' WDs is increasing significantly, and therefore, due to climate change, catastrophic events like the 2013 Uttarakhand floods and the 2023 North India floods are becoming much more frequent.