India’s prolonged heatwave linked to record poor summer rains

An early and prolonged heat wave in the Indian subcontinent since March 2022 has adversely impacted life, health and availability of power and water. It has also resulted in loss of work hours.

Data from the India Meteorological Department (IMD) suggests two major causes behind this spell — the absence of rainfall and convective activities such as thunderstorms for a long period, absence of active western disturbances (an extratropical storm originating in the Mediterranean region that brings rain to northern parts of the Indian subcontinent) and anti-cyclonic subsidence of warmer dry air at lower and middle levels over north Arabian Sea and adjacent south Pakistan and Gujarat.

Heat waves are not new. There is evidence of large-scale heat waves in other parts of the world including Europe and the United States. When the base temperature of the earth goes up, disturbances such as heat waves are most likely to occur. However, what is concerning is the evidence of increase in their frequency and intensity¹ and numerical models suggesting possible future rise in temperature. Globally, the last seven years have been the warmest on record.

In India, the summer of 2022 unfolded in an unusual way as early as 11 March mostly over the plains of northwest, central India, Gujarat and the western Himalayan region. The 'major' and 'severe' heat waves came in six spells: 11-23 March, 27 March-12 April, 17 April-20 April, 23 April-2 May, 7-16 May and 19-21 May. Most of these spells were over the plains of northwest and central India, Gujarat and parts of western Himalayan region, southern parts of Uttar Pradesh, Jharkhand and the national capital region of Delhi. The period of 24-30 April is the only one when it spread further east to some parts of West Bengal and Odisha.

Comparison of heat waves of the past 13 years (2010-2022) shows record highs. For the month of March, for instance, 2022 had the highest number of meteorological sub-division days (MSD) at 93. MSD is the sum of the heat wave days in different meteorological sub-divisions of the country. For April, 2010 had the highest number of MSDs of heat waves at 404 followed by this year (2022) at 209 MSDs.

Temperatures recorded during March and April surpassed earlier recorded values in the last 122 years^2,3. In March 2022, the average maximum temperature over India was 33.1ºC, highest for March on record. In April 2022, the average temperature over India rose to 35.3ºC against a normal of 33.94ºC (normal calculated based on the period 1981-2010). It was the third highest on record for April in 122 years. The average minimum temperature in April also shot up to 23.51ºC, second highest for the same period.

In northwest India alone, average maximum temperature in April 2022 was the highest with 36.32ºC, breaking the earlier record of 35.4ºC in 2010. Over central India, the average maximum temperature in April was the highest ever on record with 38.04ºC.

Normally March and April in India see rain and thunderstorms at regular intervals with both the Bay of Bengal and Arabian Sea providing ample moisture under favourable wind conditions, while the temperature follows a regular ‘high rise and less fall’ pattern in the same period. Although the temperature rises in value seasonally, the number of heat waves are not at an alarming level.

There have been cases of a sharp fall in temperature by up to 5-10ºC during thunderstorm days over various parts of India. All convective systems formed during April and May provide relief from the continuous rise in temperature. In 2022 (1 March-18 May), this rain was highly subdued, mainly in March and April, especially over northwest plains and central parts of India (see Tables 1 and 2). The country received only 8.9 mm rains in March 2022, which is 71% less than its long period average (LPA) of 30.4 mm and third lowest since 19014.

The monthly rainfall for April 2022 (see Table 1) highlights the tremendous deficiencies in northwest and central India⁴. Though it improved slightly in May 2022, the deficiency continued in the main heat wave areas of northwest and central India comprising almost 60% of the country, while northeast and peninsular India recorded excess rainfall⁵.

The absence of active western disturbances (WDs) in March and April 2022 over north India led to severely subdued rainfall and scant thunderstorm activities over northwest and central India. Five western disturbances moved across north India in March, but were dry without any moisture from the Arabian Sea or Bay of Bengal except one that caused isolated rainfall over northwest and central India for a couple of days early March. April was similar with only six WDs in north India, mostly feeble and dry and moving across the higher ridges of the Himalayas. The last three caused gusty winds and dusty and dry thunderstorms at some places including Delhi and Rajasthan on 14, 22 and 25 April.

In contrast, northeast India, southern peninsular states of Kerala and Tamil Nadu, sub Himalayan Bengal and Sikkim had thunderstorms and received scattered to widespread rain in some days of March 2022. Rains improved significantly in April 2022 and more in May due to favorable synoptic systems, which coupled with winds saved northeast and peninsular India from heat waves.

IMD has issued heat wave warnings 3 to 5 days in advance across the country. In step, the National Disaster Management Authority (NDMA) and and heat prone states are trying to reduce mortality due to heat waves. Early warning systems need to be improved for more accurate forecasts and better implementation of heat action plans by the states and other authorities.

IMD is looking at improving early warning lead time and impact assessment by factoring in relative humidity, night temperature and wind, especially for days when very high maximum temperatures of 45-49ºC are reported over large areas of plains of northwest and central India including in Delhi. This will not only help save more people from the devastating effects of severe heat waves but also local administrations to take heat-action measures.