Heatwaves: Causes, Effects, Prediction, Mitigation

Heatwaves: Causes, Effects, Prediction and Mitigation | UPSC

Context: The India Meteorological Department (IMD) has announced above-average heat wave days for India.
Total duration of heatwaves in India has increased by about three days during the last 30 years and a further increase of 12-18 days is expected by 2060.

Analysis

What is a Heatwave?

A period of abnormally high temperatures is called a heatwave. The India Meteorological Department (IMD) declares a heat wave if the maximum temperature of a weather station reaches at least 40 degrees C in the plains and at least 30 degrees C in hilly regions, with a departure of around 4.5-6.4 degrees C from the normal maximum temperature.
The IMD can also declare a heat wave if the actual maximum temperature crosses 45°C (or 37°C at a hill-station), and a ‘severe heat wave’ if it crosses 47°C.
Qualitatively, a heat wave can also occur when the temperature of the air becomes fatal to the human body.
Heat waves in India are typically recorded between March and June and tend to peak in May.

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Heatwaves in India

Heat waves in India are typically recorded between March and June and tend to peak in May.
Heatwaves are predominantly observed over two areas,
- central and northwest India and
- another over coastal Andhra Pradesh and Odisha.

Heatwaves are common over the Core Heatwave Zone (CHZ) — Rajasthan, Punjab, Haryana, Chandigarh, Delhi, West Madhya Pradesh, Uttar Pradesh, Chhattisgarh, Orissa, Vidarbha in Maharashtra, parts of Gangetic West Bengal, Coastal Andhra Pradesh and Telangana, as categorised by India Meteorological Department.

Causes of heatwaves in India

A study published on February 20, 2023, in Nature Geoscienceoffers 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. Source: The Hindu)
First of all, 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.
Likewise, air flowing in from the northwest after crossing the mountains of Afghanistan and Pakistan gets compressed and thus gets heated while descending the leeward side of these mountains and enters India as a warm wind.
While air flowing in over the oceans is expected to bring cooler air, the Arabian Sea is unfortunately warming faster than most other ocean regions.
Next, the strong upper atmospheric westerly winds, that come in from the Atlantic Ocean over to India during spring, descends, compresses, and warms up to generate some heatwaves.
Finally, the so-called lapse rate — the rate at which temperatures cool from the surface to the upper atmosphere — is declining under global warming.
In other words, global warming 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 heatwaves as it sinks and compresses.

a) Heatwaves and El Nino-La Nina phenomenon

In 2022 itself, the heatwaves 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 Nina.

La Nina is a world-affecting weather phenomenon in which a band of cool water spreads east-west across the equatorial Pacific Ocean.
The El Niño is a complementary phenomenon in which warmer water spreads west-east across the equatorial Pacific Ocean.
Heatwaves tend to be confined to north and northwest India in El Niño years.

b) Anticyclones and heatwaves

Anticyclones over western and north-western parts of India in March and the absence of western disturbances can also trigger the early and extreme heatwaves.
Anticyclones cause hot and dry weather by sinking winds around high-pressure systems in the atmosphere.

c) How does air mass contribute to heatwaves?

The north-northwestern heatwaves are typically formed with air masses that come from 800-1,600 km away and are around two days old.
Heatwaves over peninsular India arrive from the oceans, which are closer and are barely a day old. As a result, they are on average less intense.

Prediction of heatwaves in India

a) How good is our early warning system for heatwaves?

Under the National Monsoon Mission, the Ministry of Earth Sciences (MoES) had established an advanced prediction system for early warnings of heatwaves.
India Meteorological Department (IMD) has the capability to predict the genesis, duration and intensity of heatwave events with reasonable accuracy up to four-five days in advance.

b) Can we then predict heatwaves two weeks in advance and what about a season in advance?

The Indian Institute of Tropical Meteorology (IITM), Pune, has shown that heatwave genesis and duration in India can be predicted with good skill up to two weeks in advance.

c) Improved prediction

Scientists from IMD, IITM and MoES have documented for the first time that Indian heatwaves can be predicted even one season in advance.

Heat Wave Impacts

a) Water resources

Heat wave intensifies the water crises in several regions, especially in arid and semi-arid areas.

b) Wild/Forest fires

Wildfires occurs in uncommon areas such as the Arctic.
Forest Survey of India has estimated that 21.4 per cent area under forest is vulnerable to forest fires.

c) Energy consumption

Power outages are triggered by the disproportionally high electricity demand due to air-conditioning during heatwaves.

d) Crops

High temperatures beyond certain optimum level reduce plant growth.
In higher plants, heat stress significantly alters cell division and cell elongation rates which affect both leaf size and leaf weight.
As stress increases, closure of stomata slows down or stops CO2 diffusion.

Several districts of Punjab were affected with heat wave events due to increase in temperature which has resulted in yellowing and shriveling of wheat grain, wilting and forced maturity, lower grain weight, resulting in reduction in yields up to 25%.

e) Fisheries

With global warming, tropical and subtropical areas will experience more reduction in ecosystem productivity than temperate and polar ecosystems, with rapid poleward shifts in distributions of fish and plankton.
Sea level rise due to glacier melting will destroy the mangrove forest as well as destroy the marine fish nursery ground.
With rising temperature, the physiological activity of the fishes also increases with increase in oxygen demand, whereas the solubility of the oxygen in water is inversely related to temperature and salinity.
High temperature can cause stratification leading to algae blooms and reduced levels of dissolved oxygen.

References:

Practice Questions:

Q1. With reference to the occurrence of heatwaves in India, consider the following statements:

During La Nina years, heatwaves typically concentrate in the northern and northwestern regions of India, contrasting with El Nino years.
In El Nino years, heatwaves expand southward into peninsular India, contrasting with the pattern observed during La Nina years.

Which of the statements given above is/are correct?

(a) 1 only

(b) 2 only

(d) Neither 1 nor 2

Answer: d

Explanation:

A continuous spell of abnormally hot weather is called a heatwave.
According to the India Meteorological Department (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).
Heatwaves extend further south into peninsular India due to a north-south pressure pattern set up by the La Nina. Hence, statement 1 is not correct.

La Nina is a world-affecting weather phenomenon in which a band of cool water spreads east-west across the equatorial Pacific Ocean.
The El Nino is a complementary phenomenon in which warmer water spreads west-east across the equatorial Pacific Ocean.
Heatwaves tend to be confined to north and northwest India in El Nino years. Hence, statement 2 is not correct.

Hence, none of the statements given above are correct.

Therefore, option (d) is the correct answer.

Relevance:

Subject: Current Affairs | Geography

Tags: Analytical | Moderate Difficulty

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Q2. With reference to the phenomenon of heatwaves in India, consider the following statements:

Cyclonic conditions over western India in March are linked to early and extreme heatwaves.
Unlike cyclones, heatwaves cannot be predicted in advance with the present level of technology.