Introduction
According to the India Meteorological Department (IMD), a heatwave is declared in the plains when surface air temperatures reach at least 40°C (30°C in hilly regions) with a departure of 4.5°C or more above the normal. Climate change has driven a notable spatial expansion in India's Core Heatwave Zone (CHZ)—stretching from Northwest to East-Central India—resulting in elevated frequency, duration, and severity of extreme temperature spells.
1. Synoptic Meteorological Mechanisms
- Anticyclonic Subsidence and Heat Domes: Large-scale high-pressure ridges over the subcontinent cause upper-tropospheric air to sink. As the air descends, it compresses and warms adiabatically, trapping hot surface air beneath a stagnant atmospheric lid known as a heat dome.
- Scorching Air Advection: Prevailing northwesterly winds transport dry, superheated air masses from arid regions across West Asia and the Thar Desert directly into the Indo-Gangetic Plains and Central India.
- Deficit in Western Disturbances: Suppressed frequency and southward tracking of Western Disturbances (WDs) during late winter and pre-monsoon months reduce cloud cover and unseasonal showers, fostering uninterrupted solar insolation.
2. Global Teleconnections
- El Niño Conditions: Abnormal sea surface warming in the equatorial central and eastern Pacific disrupts the Walker Circulation, weakening pre-monsoon convection, delaying onset mechanisms, and lengthening hot-and-dry conditions over the subcontinent.
- Rossby Wave Amplification: Extratropical atmospheric planetary waves moving from the North Atlantic trigger downstream wave-trains, inducing anomalous persistent ridges and intense air sinking over Northern and Central India.
3. Anthropogenic and Micro-Climatic Alterations
- Urban Heat Island (UHI) Dynamics: Unplanned urbanization, widespread concretization, and dark surfaces with low albedo absorb radiation during the day and re-radiate heat at night, leading to the dangerous amplification of night-time temperatures.
- Evapotranspiration Deficit: Land-use degradation, deforestation, and depleted soil moisture reduce the availability of moisture necessary for latent cooling, causing incoming solar energy to directly augment sensible surface heat.
Conclusion
Addressing the escalation of heatwaves as an insidious disaster necessitates mainstreaming localized Heat Action Plans (HAPs) overseen by the NDMA. Coupling early warning dissemination with climate-resilient urban spatial planning and green-blue infrastructure is vital to achieve the disaster risk reduction targets outlined in the Sendai Framework.