Rising sea surface temperatures in coastal waters are leading to a 50% to 64% increase in large-scale, humid heatwaves on land, a new paper published in the journal Nature Geoscience on Tuesday suggests.
The study, prepared by researchers at the Potsdam Institute for Climate Impact Research (PIK), Princeton University and Sun Yat-sen University, said that warming Indian Ocean waters is closely linked to higher risks of humid heat in South and West Asia.
Likewise, warming of the tropical North Atlantic increases the risk of humid heat in northern South America. The study also shows that ocean impacts are stronger in large-scale events than in isolated, local heatwaves.
Humid heatwaves have intensified rapidly in recent decades and are expected to worsen, increasing the risk of heat-related deaths, the Potsdam Institute for Climate Impact Research said in a note.
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Previous research has suggested that even healthy people can succumb to humid temperatures – a measure that combines heat and humidity – above 31.5°C, when the body is no longer able to cool itself by sweating. HT reported on April 17 that the widely accepted wet-bulb temperature survival threshold of 35 degrees Celsius has come into question after recent physiological studies suggested that the limit may actually be closer to 31 degrees Celsius, researchers from Harvard University revealed following a recent multidisciplinary conference with the Union environment ministry to discuss heat stress in India and adaptation interventions.
However, the widespread occurrence of humid heatwaves over large areas, such as the 2023 heatwave in Asia, is not yet understood.
The authors used a complex grid approach applied to climate reanalysis data to show that the observed intensification of humid heat waves is closely linked to coastal ocean warming over the period 1982-2023.
“This association is more pronounced for large-scale clustering of extreme humid heatwaves than for locally confined events. In particular, approximately 50% and 64% of the upward trends in the frequency of humid heatwaves and the strength of spatial clustering over hot spots are associated with their surrounding oceans, respectively.”
These land-ocean connections arise largely from the transport of moisture driven by tropical oceans toward land areas. Ocean-atmosphere coupling patterns, such as El Niño and the Indian Ocean Dipole, have been found to exacerbate humid heat hazards by stimulating Rossby wave trains in the atmosphere. Rossby waves are huge undulating movements of the atmosphere that extend horizontally across the planet for hundreds of kilometers to the west. They are so big and massive that they could change Earth’s climate conditions, according to the National Oceanic and Atmospheric Administration.
Compared with mid- to high-latitudes, the tropics include the most vulnerable areas and show stronger connections between land and oceans, highlighting the crucial role of the tropical oceans, the researchers said.
“We see a strong link between warming coastal waters and hot and humid extreme events, especially in the tropics, where the oceans provide more moisture to the atmosphere, which is then transported to land, amplifying the heat,” lead author and PIK scientist Fenying Cai said in a statement. “In regions farther from the equator, the combined warming of land and oceans, associated with large-scale atmospheric wave patterns, also plays an important role,” he added.
Network analysis revealed that significant warming in the tropical Indian Ocean coincides with regional humid heatwaves in both South and West Asia.
“Enhanced evaporation from warmer oceans typically leads to higher atmospheric humidity. At the same time, warming in the Indian Ocean induces low-level anomalies in the cyclonic circulation over the Arabian Sea, enhancing moisture convergence and intensification of humid heatwaves in neighboring regions, including South and West Asia,” the paper said.
“Better understanding these interactions between land and oceans could help improve climate adaptation strategies. It is crucial that coastal sea surface temperatures emerge as a potential early warning indicator of large-scale humid extreme heatwaves,” said co-author and PIK scientist Jürgen Kurths.
HT reported on March 19 last year that the Indian Ocean has been warming rapidly over the past few decades, with the sea surface temperature estimate recording an increase of 0.15 degrees Celsius per decade between 1951 and 2015, the Ministry of Earth Sciences (MoES) said in Lok Sabha.
HT reported on April 28, 2024 The Indian Ocean Basin, the world’s fastest warming basin, will see temperatures accelerate at a rate of 1.7°C to 3.8°C per century during the period 2020-2100, a new research paper has predicted, warning that this will lead to an increase in extreme weather events, prolonged marine heat waves and extreme dipole events in the Indian Ocean that will affect the development of monsoons and hurricanes.
The authors used observed data for the historical period and IPCC climate simulations under low to high emissions scenarios to simulate future conditions in the Indian Ocean. Based on the current trajectory of emissions, the ocean would be somewhere in between those two scenarios, said Roxy Mathew Cole, a climate scientist at the Indian Institute of Tropical Meteorology, who led the research.
