Western disturbances are low-pressure areas embedded in the Westerlies, the planetary winds that flow from west to east between 30°-60° latitude. They usually bring mild rain during January-February, which is beneficial to the rabi crop. But in the past few years western disturbances have been linked to disasters.
The cloud burst in Leh in 2010, the floods and landslide in Uttarakhand in 2013 and the excessive rain in Jammu and Kashmir in 2014 were all linked to these disturbances. This year, as per the India Meteorological Department (IMD), the average rain received between March 1 and March 18 was 49.2 mm—197 per cent above normal. This caused severe damage to crops in several states of the country. -crops in over 5 million hectares have been damaged.
Scientists agree that western disturbances are formed naturally. They originate in the Mediterranean region and travel over Iran, Iraq, Afghanistan and Pakistan to enter India loaded with moisture, where the Himalayas obstruct them, causing rain and snow in western Himalayas. The snow adds to the glaciers which provide water to India’s major perennial rivers. But we need to understand why this beneficial weather phenomenon increasingly disastrous.
Following views are significant
1. According to IMD, the severe rain this year is the result of the confluence of western disturbance and easterly wave from the Bay of Bengal. Easterly wave, or Easterlies, blow throughout the year from east to west. The confluence of the two winds happens throughout the year, but the results vary. They generally bring rain only to the northern part of the country but this year states in central and south India also received rain. Western parts of Madhya Pradesh, for instance, received over 2,025 times more than usual rainfall during March 1-18, while the rainfall in central Maharashtra was 3,671 times above normal(IMD data).
2. A phenomenon called Pacific Decadal Oscillation (PDO) is said to have contributed to the severity of this year’s rainfall. PDO is the name given to long-term fluctuations in the surface temperature of the Pacific Ocean.
3. Widely used weather models, such as the Global Forecast System, are consistently showing the movement of new upper air troughs into India. Such troughs in the jet streams (narrow bands of strong winds flowing in the upper troposphere) could be affecting the western disturbances which, IMD says, are present in the lower and middle troposphere. One such trough started forming in the upper troposphere over Iran, Afghanistan and Pakistan on February 26 and intensified and moved towards north-western parts of India on February 28. This led to the formation of a low-pressure region in the lower troposphere over northwest India, causing an incursion of moisture from Arabian Sea, and produced heavy rains. This shows how problematic the combination of western disturbances and upper air troughs can be for India.
4. Climate change induced: A study by the Indian Institute of Tropical Meteorology (IITM), Pune, has directly linked western disturbances to global warming. Researchers say global warming is impacting air currents and causing freak weather events. Pronounced warming over the Tibetan plateau in recent decades has increased the instability of the Westerlies and this has increased the variability of the western disturbances. According to the study, the western Himalayan region has seen a significant rise in surface temperatures since the 1950s. Observations from the area show a significant increase in precipitation in recent decades.