The sophisticated anatomy of heat waves

Source: The post is based on the article “The sophisticated anatomy of heat waves” published in The Hindu on 27th February 2023

What is the News?

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.

What are Heat Waves?

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).

How do Heat Waves originate in India?

A study published in Nature Geoscience offers explanations as to how different processes contribute to the formation of a heat wave. (The study’s findings have been adapted here to the Indian context):

In spring, India typically has air flowing in from the west-northwest. The Middle East 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.

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.

The air flowing over the oceans is also expected to bring cooler air, but the Arabian Sea is warming faster than most other ocean regions.

The strong upper atmospheric westerly winds that come in from the Atlantic Ocean over to India during spring control the near-surface winds. The energy to run past the earth near the surface, against the surface friction, can only come from above. This descending air compresses and warms up to generate some heat waves.

Global warming also tends to warm the upper atmosphere faster than the air near the surface. This in turn means that the sinking air is warmer due to global warming and thus produces heat waves as it sinks and compresses.

How do La Nina and El Nino impact Heat waves?

Heat waves are expected to become longer and more intense and more 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 north-south pressure pattern set up by the La Ni-ña. 

2023 is expected to be an El Nino year. Heat waves tend to be confined to north and north-west India in El Niño years.

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