Western Disturbances: Origin, Spread, Effects and Climate Change | UPSC

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What are Western Disturbances?

  • Western disturbances (WDs) are eastward-moving extra tropical mid-tropospheric cyclonic circulations most often embedded in the subtropical westerly jetstream (STWJ).
  • A jet stream is a river-like current of air circulating across the globe at upper levels of the troposphere (near the altitude of 30,000 feet).

Why are they named so?

  • The word ‘disturbance’ is used because the air within low pressure systems (fronts, depressions and cyclones) tends to be unstable or disturbed.
  • And ‘western’ refers to the direction from which they originate vis-à-vis India.
  • “Extra-tropical” means outside the tropics. As the WD originates outside the tropical region, the word “extra-tropical” has been associated with them.

How do Western Disturbances Originate?

  • WDs are caused by pronounced temperature differences between higher and lower latitudes.
  • The transfer and interaction of warm and cold air creates an area of low pressure in the mid-latitudes, usually over the Mediterranean Sea.
  • Provided other conditions remain favourable, chances of WD intensification are higher when there is greater meridional (north versus south) temperature difference.
  • Other factors that determine the strength of WDs are:
      • the location and intensity of the jet stream and
      • the amount of moisture being carried by the low-pressure system.

Lost-in-transit

Where do Western Disturbances Originate From?

  • They typically originate over western Eurasia (Mediterranean sea, Black sea and Caspian sea) before propagating downstream across Pakistan and northern India.
  • They originate as perturbations (disturbances) in the subtropical jet, typically over the Mediterranean.
  • They are driven by westerlies, which are prevailing winds from the west toward the east in the middle latitudes between 30 and 60 degrees latitude.
  • After covering thousands of miles, these moisture-laden WDs eventually come up against the mighty Himalayas and are blocked.
      • As a consequence, the moisture gets trapped and precipitation is shed in the form of snow and rain over northwest India, and sometimes other parts of north India.

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When are Western Disturbances the Strongest?

  • They are most prominent during the winter, when the STWJ is situated over south Asia and as westerlies are stronger in winter, but can occur at any time of year.
  • Their effect is minimal during the monsoon months in India.

What are the Effects of Western Disturbances?

  • Western Disturbances along with their induced systems are the principle rain producing systems during non-monsoonal months over Northwest India including Punjab, Haryana, Uttar Pradesh and Delhi, as well as almost all extreme precipitation events in the region; but mostly rely on local sources of moisture (e.g. the Arabian Sea).
      • Induced systems are secondary low pressure areas or cyclonic circulations induced by the primary WD.
  • Their effect sometimes extends up to Gangetic plains and Northeast India.
  • They are also responsible for bringing snowfall in the higher reaches of Jammu & Kashmir, Himachal Pradesh and Uttarakhand.
  • Most of the moisture is shed between November and March.
      • The influence of WDs is strongest from December to February, when there can be as many as 5-6 WDs every month.
  • The formation of fog starts and slowly the cold wave occurs spreading to southwards in the country.
  • WDs are also associated with cloudy skies and an increase in night-time temperatures (due to trapping of heat reradiated by the earth’s surface by the clouds) in parts of north India.
  • They can also cause strong winds that help disperse suspended pollutants in the smog-filled cities of the region, including New Delhi.
  • It is worth mentioning here that although the effects of WDs are rarely seen after spring (March-April), due to the northward movement of the jet stream above India, in some cases (which we will read about ahead), WDs can persist even outside the traditional winter months.
  • Precipitation from western disturbances replenishes the Himalayan glaciers, preserves the natural ecosystem, helps rabi crops like wheat, and assists hydropower generation.
  • Light rain under the influence of western disturbance provides relief from the severe cold wave conditions in the north-western parts of India.

Do Western Disturbances Cause Extreme Weather?

  • WDs are not usually associated with disasters, since they are not high-intensity weather systems.
  • These are advective (horizontal movement of a mass of fluid such as air or an ocean current), not convective systems, so they don’t have a lot of energy and usually don’t cause heavy precipitation. However, anomalies do exist.
      • WDs had a role to play in the Leh cloudburst of 2010, Uttarakhand rains of 2013, and the J&K floods of 2014. 

WDs and Climate Change

  • The lack of data and understanding of the mechanics of WDs has also led to debates about the impact of global warming and climate change on western disturbance formation and intensity.
  • In an age where droughts, crop failure and melting glaciers are becoming all too common, studying these linkages more closely will allow scientists to get deeper insights into western disturbances, how they may have changed over the years, and most importantly, what we can do about it.

Answer Writing Practice for UPSC Mains

Topic: Salient features of World’s Physical Geography

  • Analyze the origin, trajectory, and impacts of Western Disturbances on the Indian subcontinent’s weather patterns. Discuss how climate change is influencing their frequency, intensity, and subsequent socio-economic effects. (Answer in 250 words)

Model Answer

Introduction:

  • Western Disturbances are extratropical storms originating in the Mediterranean region. They are characterized by low-pressure systems that travel eastward, gaining moisture from the Mediterranean Sea, the Caspian Sea, and the Persian Gulf. These disturbances enter the Indian subcontinent through Pakistan, predominantly affecting the northern states of India such as Jammu and Kashmir, Himachal Pradesh, Punjab, Haryana, and western Uttar Pradesh.

Impacts on Weather Patterns

  • The arrival of Western Disturbances in India significantly influences the weather patterns, especially during the winter months. These disturbances bring about several meteorological changes:
  • Precipitation: They cause widespread rainfall and snowfall in the northern regions, which is crucial for the Rabi crop season, particularly wheat.
  • Temperature: The cloud cover associated with these disturbances reduces diurnal temperature variation, leading to warmer nights and cooler days during winter.
  • Wind Patterns: They alter the wind patterns, often bringing cold winds from the north-western direction.

Climate Change and Western Disturbances

  • Climate change is increasingly influencing the dynamics of Western Disturbances in several ways:
  • Frequency: Studies suggest a potential increase in the frequency of these disturbances due to changes in atmospheric circulation patterns. However, this is still a subject of ongoing research with some studies indicating a decrease in their frequency.
  • Intensity: The intensity of Western Disturbances is likely to increase due to higher atmospheric moisture content driven by global warming. This could result in more intense rainfall and snowfall events.
  • Trajectory Shifts: Climate change could potentially alter the usual paths of these disturbances, causing them to affect areas that are not traditionally impacted.

Socio-Economic Effects

  • The socio-economic effects of Western Disturbances, influenced by climate change, are profound:
  • Agriculture: Changes in precipitation patterns can either benefit or harm agriculture. While timely rainfall is beneficial for Rabi crops, excessive or untimely rainfall can damage crops and lead to significant economic losses.
  • Water Resources: Snowfall in the Himalayan region is a critical source of freshwater. Changes in the snowfall patterns can affect the water availability during the summer months, impacting both agriculture and drinking water supplies.
  • Disaster Management: Increased intensity of Western Disturbances can lead to severe weather events like floods, landslides, and avalanches, posing significant challenges for disaster management and mitigation efforts.
  • Health: Fluctuations in weather patterns can influence the prevalence of respiratory and water-borne diseases, impacting public health.

Conclusion

  • Western Disturbances play a crucial role in shaping the climate of northern India, particularly during the winter season. The ongoing changes due to climate change add layers of complexity to their impacts, necessitating adaptive strategies in agriculture, water resource management, disaster preparedness, and public health policies. Understanding and mitigating these effects are essential for sustainable development and climate resilience in the region.

 

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