Study of the jet stream over India and to its north in winter: Part I

Singh, Sqn. Ldr. M. S.

doi:10.54302/mausam.v22i1.3978

Cited by 5 publications

(3 citation statements)

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“…We now explore potential causes of these trends. It is known that both interannual and intraseasonal variability of WD frequency is strongly controlled by the latitude of the subtropical jet (Singh, 1971;Hunt et al, 2018a), so we can be fairly confident in a hypothesis that explains trends in WD frequency in terms of trends in upper-level zonal winds. Indeed, there has been a significant increase in the wind global warming (Hewitt, 2005) and against the trend of glaciers in neighbouring ranges such as the western Himalaya.…”

Section: Resultsmentioning

confidence: 63%

“…Western disturbances (WDs) are upper-level troughs embedded in the subtropical westerly jet. Dynamical instabilities in the jet, often arising over the Mediterranean, grow baroclinically as they move eastwards, arriving as cyclonic winter storms over the Hindu Kush, Karakoram, and western Himalaya several days later (Singh, 1971;Dimri et al, 2015). On average, six to seven strong WDs pass over the Indian subcontinent each month during winter (Rao and Srinivasan, 1969;Hunt et al, 2018a).…”

Section: Introductionmentioning

confidence: 99%

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

Hunt

2024

Weather Clim. Dynam.

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Abstract. Western disturbances (WDs) are cyclonic storms that travel along the subtropical jet, bringing the majority of seasonal and extreme precipitation to the Hindu Kush, Karakoram, and western Himalaya 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 interact with the summer monsoon, leading to catastrophic consequences. These seem to be happening more frequently, and along with increasingly harsh winter seasons, 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 70 years of European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) data. Winter WDs have increased significantly over the western and central Himalaya and the 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, months where they were previously rare. For example, WDs have been twice as common in June in the last 20 years than during the previous 50. This is attributed to a 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.

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Section: Resultsmentioning

confidence: 63%

Section: Introductionmentioning

confidence: 99%

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

Hunt

2024

Weather Clim. Dynam.

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“…WD cyclogenesis occurs to the west of the Hindu Kush and Himalayas (Mull and Desai, 1947;Pisharoty and Desai, 1956), over regions such as the Mediterranean Sea (Madhura et al, 2015). They then propagate as troughs embedded within the subtropical westerly jet (Singh, 1971) until they reach South Asia. Early studies established that WDs move eastward at speeds of 6-12 m s −1 , although they disagreed on their lifetime-usually because of inconsistent definitions of genesis and decaythey typically fall in the range of 2 to 12 days (Datta and Gupta, 1967;Rao and Srinivasan, 1969;Chattopadhyay, 1970;Rao and Rao, 1971;Subbaramayya and Raju, 1982).…”

Section: What Are Western Disturbances?mentioning

confidence: 99%

Western disturbances and climate variability: a review of recent developments

Hunt,

Baudouin,

Turner

et al. 2024

Preprint

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Abstract. Western disturbances (WDs) are synoptic-scale weather systems embedded within the subtropical westerly jet. Manifesting as upper-level troughs often associated with a lower-tropospheric low over Western India, they share some dynamical features with extratropical cyclones. WDs are most common during the boreal winter (December to March), during which they bring the majority of precipitation – both rain and snow – to the Western Himalaya, as well as to surrounding areas of north India, Pakistan and the Tibetan Plateau. WDs are also associated with weather hazards such as heavy snowfall, hailstorms, fog, cloudbursts, avalanches, frost, and coldwaves. In this paper, we review the recent understanding and development on WDs. Recent studies have collectively made use of novel data, novel analysis techniques, and the increasing availability of high-resolution weather and climate models. This review is separated into six main sections – structure and thermodynamics, precipitation and impacts, teleconnections, modelling experiments, forecasting at a range of scales, and paleoclimate and climate change – each motivated with a brief discussion of the accomplishments and limitations of previous research. A number of step changes in understanding are synthesised. Use of new modelling frameworks and tracking algorithms has significantly improved knowledge of WD structure and variability, and a more frequentist approach can now be taken. Improved observation systems have helped quantification of water security over the Western Himalaya. Convection-permitting models have improved our understanding of how WDs interact with the Himalayas to trigger natural hazards. Improvements in paleoclimate and future climate modelling experiments have helped to explain how WDs and their impacts over the Himalaya respond to large-scale natural and anthropogenic forcings. We end by summarising unresolved questions and outlining key future WD research topics.

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