Abstract
Western disturbances (WDs) are upper‐level synoptic‐scale systems embedded in the subtropical westerly jet stream (STWJ), often associated with extreme rainfall events in north India and Pakistan during boreal winter. Here, a tracking algorithm is applied to the upper‐tropospheric vorticity field for 37 years of ERA‐Interim reanalysis data, giving a catalogue of over 3000 events. These events are analysed in a composite framework: the vertical structure is explored across a large number of dynamic and thermodynamic fields, revealing a significant northwestward tilt with height, strong ascent ahead of the centre, which sits above the maximum surface precipitation, and a warm‐over‐cold, dry‐over‐moist structure, among other signatures of strong baroclinicity. Evolution of the structures of cloud cover and vertical wind speed are investigated as the composite WD passes across northern India. Cloud cover in particular is found to be particularly sensitive to the presence of the Himalayan foothills, with a significant maximum at 300 hPa approximately 1 day after the WD reaches peak intensity. k‐means clustering is used to classify WDs according to both dynamical structure and precipitation footprint and the relationship between the two sets of WDs is explored. Finally, the statistical relationship between the STWJ position and WDs on interannual time‐scales is explored, showing that WD frequency in north India is highly sensitive to the jet location over Eurasia. Years with a greater number of WDs feature a STWJ shifted to the south, a pattern that is substantially more coherent and reaches as far west as North America during boreal winter. This suggests that it may be possible to predict the statistics of western disturbance events on seasonal time‐scales if suitable indicators of jet position can also be predicted.
Highlights
Western disturbances (WDs) are formally defined by the India Meteorological Department (IMD) as: ‘[A] cyclonic circulation/trough in the mid and lower tropospheric levels or as a low pressure area on the surface, which occurs in middle latitude westerlies and originates over the Mediterranean Sea, Caspian Sea and Black Sea and moves eastwards across north India’.∗ WDs are, at the most fundamental level, synoptic-scale vortical perturbations embedded in the subtropical westerly jet stream (Dimri et al, 2016)
It is well known that the western disturbance frequency peaks during winter (e.g. Singh and Agnihotri, 1977; Dimri et al, 2015), when the subtropical westerly jet (STWJ) is at its most southerly position; we can use this as a consistency check for our database by ensuring the tracked WDs follow the expected seasonal cycle
Starting from the description given by the India Meteorological Department and a selection of case studies, we developed and applied a tracking algorithm to 36 years of global reanalysis data; applying a location filter over northern India and Pakistan, we recovered over 3000 upper-level disturbances in the subtropical westerly jet stream (STWJ) that fit previous definitions of western disturbances (WDs)
Summary
Western disturbances (WDs) are formally defined by the India Meteorological Department (IMD) as: ‘[A] cyclonic circulation/trough in the mid and lower tropospheric levels or as a low pressure area on the surface, which occurs in middle latitude westerlies and originates over the Mediterranean Sea, Caspian Sea and Black Sea and moves eastwards across north India’.∗ WDs are, at the most fundamental level, synoptic-scale vortical perturbations embedded in the subtropical westerly jet stream (Dimri et al, 2016). Cannon et al (2016) recently developed a wave-tracking algorithm based on 500 hPa geopotential height anomaly using daily NCEP–CFSR reanalysis data (Saha et al, 2010) Their computed frequency of 5–6 events passing through north India per year is roughly an order of magnitude less than most anecdotal climatologies suggest it should be, around 4–7 per month during boreal winter (Chattopadhyay, 1970; Mohanty et al, 1998; Dimri, 2006).
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