The sophisticated anatomy of heat waves

• Recently, the India Meteorological Department (IMD) warned that the maximum temperatures over northwest, west, and central India would be 3-5°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?

• 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 north-south pressure pattern set up by the La Niña.
• The last three years have been La Niña years, which has served as a precursor to 2023 likely being an El Niño year.
• As we eagerly await the likely birth of an El Niño this year, we have already had a heat wave occur over northwest India.
• Heat waves tend to be confined to north and northwest India in El Niño years.

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 800-1,600 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.
• In sum, heat waves have a sophisticated anatomy with important implications for how well we can predict them.

What can be done to address the Issue

• Nonetheless, early-warning systems can take advantage of the processes, modes of formation, location, and age of the air mass to improve the quality of warnings and also increase how soon they can be issued. Sizeable investments in human and computational resources have already increased India’s forecast skills in the last decade.
• For reasons that we are yet to fully understand, mortality over India due to heat waves are substantially lower than those in other mid-latitude regions.