Most people think of a heatwave as something that happens on land. Something that brings scorching temperatures, dry spells, and cracked soil.
But oceans have heatwaves too, and when the sea overheats, the consequences can be just as dangerous for people living on shore, a new study shows.
The study published in Nature Communications shows how marine heat waves, which are prolonged periods of unusually warm ocean temperatures, can worsen extreme rainfall in coastal areas, potentially making floods more deadly.
Researchers found that between 1982 and 2022, between 5% and 25% of extreme rainfall events in coastal areas occurred right after nearby marine heatwaves.
The human cost of this destructive relationship between heatwave and rain was also painted starkly.
Rainfall events that were influenced by marine heat waves saw roughly 20–30% more rain on average, and a 30% increase in fatalities when compared to normally occurring rain.
HOW DO HEATWAVES MAKE RAIN?
When sea surface temperatures rise sharply during a marine heatwave, they push warm, moist air upward more forcefully. This drives stronger winds and upward air movement over the warm water, which then travels inland, dramatically increasing rainfall over land areas hundreds of kilometres away.
The data confirmed that the heaviest impacts were seen roughly one day after a marine heat wave.
To map this connection, the researchers combed through long-term satellite and climate databases, including data from the National Oceanic and Atmospheric Administration (NOAA)’s global sea surface temperature records.
With the data, researchers built worldwide maps of ocean temperatures, then linked marine heat waves to extreme rainfall events on nearby coastlines.
“This is a serious concern because marine heatwaves not only intensify general rainfall but also exacerbate extreme rainfall events,” said Zhengguang Zhang, the study’s author and a climate scientist at the Ocean University of China.
WHO IS AT RISK FROM RAIN AND HEATWAVES?
More than 2 billion people live within 50 kilometres of a coastline, making them highly vulnerable to exactly this kind of hazard. Crucially, almost all the marine heat wave-related flood events that killed over a hundred people occurred in developing countries.
Zhang urged coastal communities, especially in the developing world, to start factoring ocean conditions into their early warning and flood forecasting systems.
Marine heat waves in the tropical Indian Ocean have also been associated with increased rainfall over south peninsular India.
That puts India’s long coastline stretching from Gujarat to Tamil Nadu and up through Odisha and West Bengal at risk of frequent and intense natural disasters.
What’s without doubt is that marine heat waves are happening more often and lasting longer, meaning their influence over coastal rainfall could grow even more severe as the climate continues to change.
