Heat Waves

February 27, 2023

Context:

The India Meteorological Department (IMD) warned that the maximum temperatures over northwest, west, and central India would be 35°C higher than the long term average.

On February 21, the national capital recorded its third hottest February day (33.6° C) in more than five decades.

What is a Heat Wave?

According to the IMD, a region has a heat wave if its ambient temperature deviates by at least 4.5-6.4°C from the long term average.
There is also a heat wave if the maximum temperature crosses 45°C (or 37°C at a hill station).
Heat waves are expected to become longer and more intense and frequent over the Indian subcontinent.
In 2022 itself, the heat waves started early and were more numerous. They also extended further south into peninsular India due to a northsouth pressure pattern set up by the La Niña, a world-affecting weather phenomenon in which a band of cool water spreads eastwest across the equatorial Pacific Ocean.
The last three years have been La Niña years, which has served as a precursor to 2023 likely being an El Niño year.
Heat waves tend to be confined to north and northwest India in El Niño years.

How do heat waves occur?

Heat waves are formed for one of two reasons — warmer air is flowing in from elsewhere or it is being produced locally.
It is a local phenomenon when the air is warmed by higher land surface temperature or because the air sinking down from above is compressed along the way, producing hot air near the surface.
A study published in Nature Geoscience offers explanations as to how different processes contribute to the formation of a heat wave.

1. In spring, India typically has air flowing in from the west- northwest. In the context of climate change, West Asia is warming faster than other regions in latitudes similarly close to the equator, and serves as a source of the warm air that blows into India.
2. Likewise, air flowing in from the northwest rolls in over the mountains of Afghanistan and Pakistan, so some of the compression also happens on the leeward side of these mountains, entering India with a bristling warmth.
3. While air flowing over the oceans is expected to bring cooler air, the Arabian Sea is unfortunately warming faster than most other ocean regions.
4. The strong upper atmospheric westerly winds, that come in from the Atlantic Ocean over to India during spring, control the near surface winds.
5. When winds blow from west to east, they move faster than the planet’s rotation, which creates energy that can only come from above.
  - This energy causes descending air to compress and warm up, which can generate heat waves.
6. Global warming is causing the lapse rate to decline, meaning that the upper atmosphere is warming faster than the air near the surface.
  - As a result, sinking air is warmer due to global warming, which causes it to produce heat waves as it sinks and compresses.

How does air mass contribute to heat waves?

The other factors that affect the formation of heat waves are the age of the air mass and how far it has travelled.
The north- northwestern heatwaves are typically formed with air masses that come from 800-1600 km away and are around two days old.
Heat waves over peninsular India on the other hand arrive from the oceans, which are closer (around 200-400 km) and are barely a day old. As a result, they are on average less intense.

Conclusion:

While mortality due to heat waves in India is lower than in other mid-latitude regions, there is still potential underreporting, and it’s important to improve forecast warnings and issue them promptly. This should be coupled with city-wide graded heat action plans to protect vulnerable populations.

News Source: The Hindu