Western Disturbance
A western disturbance is an extratropical storm originating in the Mediterranean region that brings sudden winter rain to the northern parts of the Indian subcontinent, which extends as east as up to northern parts of Bangladesh and South eastern Nepal.
It is a non-monsoonal precipitation pattern driven by the westerlies.
The moisture in these storms usually originates over the Mediterranean Sea, the Caspian Sea and the Black Sea.
Extratropical storms are a global phenomena with moisture usually carried in the upper atmosphere, unlike their tropical counterparts where the moisture is carried in the lower atmosphere.
In the case of the Indian subcontinent, moisture is sometimes shed as rain when the storm system encounters the Himalayas.
Western disturbances are more frequent and stronger in the winter season.
Western disturbances are important for the development of the Rabi crop, which includes the locally important staple wheat.
Formation:
Western disturbances originate in the Mediterranean region.
A high-pressure area over Ukraine and neighbourhood consolidates, causing the intrusion of cold air from polar regions towards an area of relatively warmer air with high moisture.
This generates favorable conditions for cyclogenesis in the upper atmosphere, which promotes the formation of an eastward-moving extratropical depression.
Traveling at speeds up to 12 m/s (43 km/h; 27 mph), the disturbance moves towards the Indian subcontinent until the Himalayas inhibits its development, upon which the depression rapidly weakens.
The western disturbances are embedded in the mid-latitude subtropical westerly jet stream.
Significance and impact:
Western disturbances, specifically the ones in winter, bring moderate to heavy rain in low-lying areas and heavy snow to mountainous areas of the Indian Subcontinent.
They are the cause of most winter and post-monsoon season rainfall across northwest India.
Precipitation during the winter season has great importance in agriculture, particularly for the rabi crops.
An average of four to five western disturbances form during the winter season.
The rainfall distribution and amount varies with every western disturbance.
Western disturbances are usually associated with cloudy sky, higher night temperatures and unusual rain.
Excessive precipitation due to western disturbances can cause crop damage, landslides, floods and avalanches.
Over the Indo-Gangetic plains, they occasionally bring cold wave conditions and dense fog.
These conditions remain stable until disturbed by another western disturbance.
When western disturbances move across northwest India before the onset of monsoon, a temporary advancement of monsoon current appears over the region.
The strongest western disturbances usually occur in the northern parts of Pakistan, where flooding is reported number of times during the winter season.
Effects on monsoon:
Western disturbances start declining in numbers after winter.
During the summer months of April and May, they move across north India.
The southwest monsoon current generally progresses from east to west in the northern Himalayan region, unlike western disturbances which follow a west to east trend in north India with consequent rise in pressure carrying cold air.
This helps in the activation of monsoon in certain parts of northwest India.
It also causes pre-monsoon rainfall especially in northern India.
The interaction of the monsoon trough with western disturbances may occasionally cause dense clouding and heavy precipitation.
Impacts recently in India
Being an El Nino year, the expectation was that north India would see a sparse monsoon. But the opposite happened.
July saw record rainfall in many parts of Himachal Pradesh, Punjab and Uttarakhand.
Yamuna flooded.
Multiple floods and series of landslides in Himachal Pradesh and Uttarakhand.
The immediate explanation for the landslides is the unexpectedly copious rainfall that these States have received and that these in turn are due to excessive amount of Western Disturbances (WD).
However, right from the beginning of this year, the WD have been not regular in pattern.
There were too few of them in December and January and their absence was cause for the hottest February recorded in India in at least a century.
WD several of them incident over north India in the last two months — a time when they normally should not be around.
While a combination of WD and monsoon can be dismissed as ‘freak’ weather, climate scientists have been warning of the increasing probability of such high-risk events.
The away from the usual WD are in part due to a warming Arctic that causes the polar jet stream, which carries moisture, to deviate from its regular path and bring the disturbances to north India during the monsoon.
Concerns
It is in the context of these altered weather patterns that warnings by scientists and environmentalists of the perils of wanton construction in the Himalayas must be factored in.
The ongoing Char Dham road building project has led to large-scale altering of the mountains with significant chunks carved away, rendering them vulnerable to upheaval.
As recent fears of land subsidence in Joshimath, Uttarakhand demonstrated, ill-thought construction and haphazard building practices have magnified the risk to residents.
While State governments tend to search for short-term solutions such as demanding compensation from the Centre for ‘disaster relief’, it is time that more serious thought is given to the nature of infrastructure development.
If need be, restrictions imposed in the larger interest of minimising hazards and maintaining stability.
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