A heatwave is a period of unusually hot weather with above normal temperatures that typically last three or more days. In India, heatwaves are generally experienced during March-June. On an average, two-three heatwave events are expected every season. Heatwaves are predominantly observed over two areas, central and northwest India and another over coastal Andhra Pradesh and Odisha, supported by favourable atmospheric conditions.
Total duration of heatwaves has increased by about three days during the last 30 years and a further increase of 12-18 days is expected by 2060. In future climate, heatwaves will be spread to new areas including southern parts of India. Climate change is causing heatwaves more frequently, and they are much stronger and can last for more days.
Heatwaves have multiple and cascading impact on human health, ecosystems, agriculture, energy, water and economy. The recent 2022 heatwave in India and Pakistan in March-April made devastating impacts. It is estimated to have led at least 90 deaths across India and Pakistan. It also triggered an extreme Glacial Lake Outburst Flood in northern Pakistan. The extreme heat reduced India’s wheat crop yields from Punjab and Uttar Pradesh. There have been power outages due to shortage of coal.
Adaptation to heatwaves can be effective to minimise the negative impacts, by developing a comprehensive heat response plan that includes early warnings, awareness rising and technology intervention. India has now a strong national framework for heat action plans involving the India Meteorological Department (IMD), the National and State disaster management authorities, and local bodies. Early warning systems are an integral part of this heat action plan.
How good is our early warning system for heatwaves? Research helped us to improve our understanding on the underlying mechanism of its genesis and intensity. Heatwaves are caused by large scale atmospheric circulation anomalies like high pressure areas, upper-tropospheric, jet streams, etc. The global forcing like the El Nino/Southern Oscillation (ENSO) and the Indian Ocean modulate the frequency and duration of Indian heatwaves. Heatwave can be further accentuated by local effects like depleted soil moisture and enhanced sensible heat flux.
Under the National Monsoon Mission, the Ministry of Earth Sciences (MoES) had established an advanced prediction system for early warnings of heatwaves. IMD has the capability to predict the genesis, duration and intensity of heatwave events with reasonable accuracy up to four-five days in advance. These predictions are widely disseminated to disaster management authorities.
Can we then predict heatwaves two weeks in advance and what about a season in advance? A recent study published in the Scientific Reports by the scientists at 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. They have used the hindcasts from the MoES Extended Range Prediction System (ERPS) that uses ensemble method combining four atmospheric general circulation models.
In another recent study published in the International Journal of Climatology of the Royal Meteorological Society last month, scientists from IMD, IITM and MoES have documented for the first time that Indian heatwaves can be predicted even one season in advance. They used 37 years (1981-2017) of hindcasts from the Monsoon Mission Coupled Climate Forecast Model (MMCFS) to document that seasonal predictions of frequency and duration of Indian heatwaves during April-June are very useful.
The model could reproduce the spatial distribution of heatwave frequency and duration very well. The model also showed useful skill in predicting the characteristics of heatwaves for different months (April to June) separately. The model skill in predicting heatwaves arises due to its fidelity in reproducing the impacts of ENSO and the Indian Ocean on atmospheric circulation anomalies over the Indian region.
Thus, we have an end-to-end seamless prediction system to predict heatwaves in all time scales, from short-range to seasonal. The seasonal forecast will provide an outlook or probability of frequency and duration of heatwaves, one season in advance. This early outlook can be further strengthened using the extended range (two weeks) and short range (four-five days) forecasts for more focused region-wise response strategies. However, there is a scope for further improvement of this prediction system.
Seasonal forecasts should use a multi-model ensemble (MME) forecasting strategy. Short- range ensemble forecasts should use higher resolution global models, initialized with observed soil moisture data, which are available from microwave satellites and IMD’s soil moisture network. We should then expect much more advanced forecasting system for heatwaves in the near future.
This is just one example how good science can help the society.
(Madhavan Nair Rajeevan is a former Secretary to the Government of India, Ministry of Earth Sciences. He currently works at the National Centre for Earth Science Studies)