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The United Kingdom (UK) is experiencing an unusually warm spring in April and May due to abnormally warm North Atlantic Ocean close to the region. Such transboundary heatwaves are missing from the general scientific and popular understanding of the weather phenomenon.
While UK’s Met Office has stated that the temperatures have not yet breached their heatwave thresholds, the hotter than usual temperatures do portend the outlook for the coming season for the country which starts in June, according to some experts.
“The sea surface temperature (SST) anomaly pattern in the North Atlantic is pretty dramatic at the moment, with anomalous warmth strongly focused around the UK, tied to the much sunnier than usual spring to-date,” said James Peacock, head meteorologist at MetSwift, a private company working on weather communication on social media platform X.
“In the UK, a heatwave is officially defined when a location records at least three consecutive days with daily maximum temperatures meeting or exceeding the heatwave temperature threshold. This threshold varies by county, reflecting the diverse climate across the UK,” wrote the Met Office in a blog on its website on April 26.
“While temperatures could peak at 27°C next week, with the highest temperatures expected in the south of England and only possibly lasting a couple of days, next week’s conditions are likely to fall just outside of official Met Office heatwave thresholds,” the agency added.
Though not officially a heatwave, the abnormally high land temperatures in the UK, especially in its southern parts, are closely tied to the warmer than usual SSTs in the North Atlantic Ocean, which makes the heat event transboundary.
Such transboundary heatwaves that occur over both land and ocean and move from one to the other are more extensive, last longer and are more severe than heatwaves that occur only on land or over ocean, according to a new study published in the journal Nature Communications on April 10 2025.
The study showcased a major gap in the understanding of heatwaves that needs to be fixed, especially in the tropical regions of the planet.
The report defined a heatwave as a period when mean surface air temperature exceeds the 90th percentile threshold during the warm seasons. This means that the recorded temperature values are higher than 90 per cent of all temperature values in a particular reference period.
Transboundary heatwaves are rarer than land-only or ocean-only heatwaves but are going to increase in frequency with global warming and consequent climate change, according to the study.
The study conducted by scientists from the China University of Geosciences, Sun Yat-sen University and The Chinese University of Hong Kong found that there were a total of 14,830 heat wave events in between 1981 and 2020.
Out of these, the maximum number of events — 8,145 — occurred only on the ocean and 3,198 events occurred only on land, according to the study. As many as 2,133 heatwaves travelled from ocean to land, like the heat event in the UK; those that travelled from land to ocean were the rarest. occurring 1,354 times.
Some land and ocean regions were more prone to transboundary heat waves. Ocean-to-land heatwaves (OTLH) occurred with higher frequency, larger spatial coverage and greater intensity in western South America, central Africa, southern Europe and eastern Asia.
The land-to-ocean heatwaves (LTOH) increased with latitude in the northern hemisphere and are severe and extensive in the Atlantic and Southern Oceans.
OTLHs and LTOHs are generally more persistent, extensive and severe than land-only and ocean-only heatwaves. OTLHs can last up to nine days on an average whole LTOHs can last for eight days on an average.
OTLHs can occur on average over an area of 6.33 million square kilometres which is almost double the size of India and LTOHs can occur on average over an area of 4.24 million square kilometres which is 1.3 times the size of India.
The Pacific Northwest Heatwave in 2021 was partially fuelled by heat from the tropical Pacific Ocean and led to many all time temperature records in Canada and the United States.
During the heatwave the city of Lytton in southwest Canada had recorded a staggering maximum temperature of 49.6°C which is the highest temperature ever recorded in the country.
In 2013, the South Atlantic marine heat wave evolved from the eastward migrating high pressure area from the land mass of South America. But generally the focus for scientists has been studying land only or ocean only heat waves.
The study showed that such a focus “may fail to capture the full spatiotemporal evolution of heatwaves, which can span across both land and ocean from initiation to termination. A comprehensive understanding requires treating the land-ocean transboundary migration of heatwaves as a unified process, based on near-surface air temperature that bridges both spatial domains”.
During 1981-2020, oceanic heatwave migration to land has increased by 30.9 per cent and the migration of land based heat waves to the ocean has increased by 24 per cent, as per the study.
Global warming due to the emissions of greenhouse gases (GHG) and consequent climate change is also altering atmospheric circulations that connect land and ocean. Such as the intensification and movement of sub tropical high pressure areas which trigger more heat wave occurrences over both land and ocean in eastern Asia and western Pacific regions, according to the study.
GHGs have caused 39.3 percent rise in OTLH frequency over land and 33.7 per cent increase in LTOH frequency over the oceans between 1981-2020. Under the most likely future GHG emission scenarios OTLH frequency could increase by 19.5 per cent by 2041-2080 and LTOH frequency by 36.3 per cent.