Western Disturbance
Kartavya Desk Staff
Source: DD News
Context: The India Meteorological Department (IMD) has forecast that a fresh Western Disturbance will influence weather conditions over northwest India, including Delhi-NCR.
About Western Disturbance:
What it is?
• A Western Disturbance (WD) is an extratropical storm system originating from the Mediterranean region, which brings non-monsoonal winter rain and snow to northwestern India, Pakistan, and the Himalayas. It is driven by the westerly jet streams in the mid-latitudes.
How it Forms?
• Origin: It begins in the Mediterranean Sea region due to a clash between cold polar air from Europe and warm, moist subtropical air.
• Cyclogenesis: This temperature contrast triggers cyclonic circulation in the upper atmosphere, creating a low-pressure system.
• Movement: The system moves eastward with the subtropical westerly jet stream, gaining moisture from the Mediterranean, Caspian, and Black Seas.
• Dissipation: When it encounters the Himalayas, it releases moisture as rain or snow and weakens rapidly thereafter.
Factors Influencing Western Disturbances:
• Jet Stream Dynamics: The position and strength of the subtropical westerly jet stream determine WD frequency and intensity.
• Topography: The Himalayas act as a barrier, forcing uplift and precipitation from moisture-laden air masses.
• Temperature Gradient: Strong contrasts between polar and tropical air enhance WD formation.
• Oceanic Conditions: Variations in Mediterranean and Eurasian sea surface temperatures affect WD development and track.
Impacts on India:
• Agriculture: Crucial for Rabi crops, especially wheat and mustard, as they provide much-needed winter moisture across north India.
• Precipitation: Cause rain and snow in northwest and Himalayan states, replenishing rivers and groundwater.
• Weather Conditions: Lead to cloudy skies, warmer nights, and cold days; sometimes bring fog and cold waves post-passage.
• Disasters: Excessive WDs can trigger floods, avalanches, landslides, and crop damage due to heavy precipitation.
• Air Quality: The associated rain and wind often improve air quality temporarily by dispersing pollutants in north India.
Impact on Monsoon:
• Pre-Monsoon Role: During April–May, WDs contribute to pre-monsoon rainfall, moderating heat buildup in north India.
• Interaction with Monsoon: Occasionally, WDs interact with the monsoon trough, leading to heavy precipitation events—as seen in the 2013 Uttarakhand floods.
• Transition Effect: Decline of WDs in late spring allows smooth onset of the southwest monsoon from the Bay of Bengal side.