ASIA is in the grip of record heat. It began in April in the east, with historic heatwaves in places like the Philippines, Malaysia, Thailand, Bangladesh and eastern India. Since then, high temps moved west to western India and Pakistan. This comes amid a year of scorching temperatures worldwide, with every month since June 2023 breaking temperature records. The latest bout of Asian heatwaves is one scientists say is unlike anything they’ve seen before, so what explains it and what can Pakistan expect, since summer has just begun? An obvious cause, besides long-term climate change, is the powerful El Nino that began in early 2023. During an El Nino, warm water normally buried deep in the western Pacific is released and spreads eastwards across the Pacific’s surface, exposing itself to air. The Earth’s entire atmosphere warms up, dramatically altering worldwide weather patterns.
The El Nino of 2023 is the strongest since 2016 and one of the strongest on record. It also directly follows a La Nina that lasted an unusually long three years. La Nina is El Nino’s opposite, in which the western Pacific accumulates more heat while the rest of the planet cools. During those three years, a lot of heat must have gathered in the ocean depths, so the consequences of its sudden release by the recent El Nino should be obvious. One such consequence is an outbreak of unprecedented ocean heatwaves worldwide. Average ocean temperatures have been rising for a decade, but in March 2023, temperatures started spiking in several regions, staying that way since. A hotter atmosphere must have warmed ocean surfaces everywhere.
The recent El Nino ended in early 2024 and meteorologists say another La Nina is on the way later this year, yet record high ocean temperatures still persist in many places, including the Indian Ocean, and are rising further during current boreal summer. This makes sense because water takes longer than air to cool. But it also indicates a prolonged influence of the last El Nino. Marine heatwaves like those of the last year will have profound effects on weather on land, and Asia’s current spate of heatwaves is likely an example. Obviously, extra heat can pass from sea into air. But other factors must be considered. Hotter oceans can mean less rainfall and clouds overland, because the temperature differential between ocean and land is weaker, reducing the pressure that moves moisture-laden air currents towards land. But heated ocean air that does reach land is hot, moist, and initially stays near the ground. For humans, this creates a dangerous combination of heat and humidity.
This may be the reason Sindh has been suffering so this June and could mean less rainfall in northern Pakistan in coming months. The Indian Ocean is highly susceptible to warming and is where current heatwaves are especially severe. But the Pacific Ocean also matters. El Nino generally weakens the South Asian monsoon, making us more susceptible to drought, while La Nina strengthens it, making flooding more likely. That explains the underwhelming monsoon rainfall India and Pakistan received in 2023, a situation likely to be reversed by the coming of La Nina in 2024. That El Nino makes heatwaves in Asia more likely by reducing rain and cloud cover, while La Nina does the opposite, makes up for the fact that the western Pacific is cooled by El Nino and warmed by La Nina. So a country like the Philippines can swelter while the surrounding ocean is cooler than normal. Less simple to explain is how heatwaves within that same stretch of ocean happen.
El Nino, which warms the entire world, may be responsible for marine heatwaves worldwide. But La Nina, which warms the western Pacific by concentrating heat there, somehow does not make marine heatwaves in the western Pacific more likely. The likely explanation is that the trade winds, which strengthen during La Nina, are cool and dry from blowing over a cold eastern Pacific and produce high evaporation in the western Pacific, cooling the surface while most of the Pacific’s heat sinks to the dark depths. Said winds then power Asia’s monsoon. But when oceans everywhere are hotter, like after this latest El Nino, the entire atmosphere may get too hot and humid to provide relief anywhere.
In fact, during an El Nino, even the western Pacific may get heatwaves, because the surface temperature is involved here. A sea surface heatwave basically signifies that heat transfer from sea to air has stalled, which in turn can stall transfer of moisture from sea to land. But the eastern Pacific during an El Nino releases so much heat that it may become a vast, low-pressure zone that raises air pressure across the rest of the world. Heatwaves are a common result of high pressure, explaining their abundance in marine and terrestrial settings the past year.
A pressing question is what is in store when La Nina returns later this year. A combination of La Nina conditions with conditions currently persisting could have tumultuous effects. A hot Indian Ocean and a hot Pacific Ocean may coexist, supercharging the monsoon and creating extreme flood risk for Pakistan. This may be preceded by severe heatwaves in June. There are countless possibilities regarding how Earth’s immensely complex climate system will behave next, so our nation’s scientists must scramble to better understand the risks up ahead.
—The writer is a meteorological analyst and director at Pakistan’s People-Led Disaster Management.
