While the recent heatwave over land is fading, the heat has not disappeared entirely.
Around the UK, sea surface temperatures remain significantly above average, continuing a trend of marine heatwave conditions that has become increasingly notable in recent years. These prolonged periods of unusually warm seas can influence marine ecosystems, coastal weather and even the severity of major weather events.
What is a marine heatwave?
Marine heatwaves are measured differently from heatwaves on land. Rather than relying on a fixed temperature threshold, they are defined relative to what is normal for a particular location and time of year.
A marine heatwave occurs when sea surface temperatures remain above the 90th percentile of a 30-year climate record for at least five consecutive days. Depending on how far temperatures exceed this threshold, events can be classified as strong, severe or extreme.
This approach reflects the fact that sea temperatures vary widely around the UK. A temperature that would be exceptionally warm in one area may be perfectly normal elsewhere.
A growing feature of UK waters
Recent years have seen an increasing number of marine heatwave events around the UK. According to Met Office scientists, 2025 was particularly exceptional, with marine heatwave conditions persisting for more than three-quarters of the year. Similar events were also recorded in 2022 and 2023, while 2026 has once again seen sea temperatures significantly above average.
At the same time, cold marine conditions have become increasingly rare. The last widespread cold spell affecting UK waters occurred during the Beast from the East in 2018. Since then, winters have tended to feature above-average sea temperatures.
The heat will start re-building across parts of England and Wales this week, with temperatures possibly approaching 30°C in places by Friday🌡️
— Met Office (@metoffice) July 21, 2026
However, temperatures won't rise as high as recent hot spells and northern areas of the UK will be somewhat cooler and cloudier🌤️ pic.twitter.com/FxMfTMlGWL
Impacts on marine ecosystems
Warmer seas can have important consequences for marine life. Species that are typically found further south can expand their range northwards when waters remain unusually warm for prolonged periods.
One example highlighted by Met Office researchers is the octopus. Traditionally more common in warmer waters further south, octopus populations have increasingly been observed around southwest UK waters.
Changes such as these demonstrate how marine heatwaves can reshape ecosystems and alter the distribution of species around UK coastlines.
How warmer seas affect our weather
Marine heatwaves can also influence weather conditions over land.
Sea breezes often provide cooling during hot summer weather, particularly close to the coast. However, when sea temperatures are unusually high, this cooling effect becomes weaker.
Forecast modelling has shown that marine heatwave conditions can lead to air temperatures more than 1.5°C warmer over surrounding seas, with the warmer air subsequently transported inland by sea breezes. This means coastal areas may receive less relief from hot conditions than would normally be expected.
Why UK seas are particularly vulnerable
The waters surrounding the UK are relatively shallow compared with many parts of the world's oceans. Much of the North Sea, for example, is shallower than the height of the Eiffel Tower.
Combined with strong tidal currents, this creates a unique environment where heat can become trapped and mixed throughout the entire water column. Rather than remaining solely at the surface, warmth can be distributed through much of the sea depth, allowing heat to persist for longer periods.
This is one reason why parts of the north-west European shelf, particularly the southern North Sea, have warmed faster than many other marine regions over recent decades.
Enhancing heavy showers and storms
Research has shown that marine heatwaves can influence rainfall intensity.
During a warm period in June 2023, scientists compared weather model simulations with and without marine heatwave conditions. When the warm sea anomaly was removed, the likelihood of heavy showers reduced substantially, demonstrating how warmer waters can help enhance convection and rainfall development.
The additional heat and moisture provided by the sea can create conditions more favourable for cloud growth and heavy showers, particularly when combined with sea-breeze circulations.
READ MORE: Will the heat return?
The case of Storm Babet
One of the most striking recent examples came during Storm Babet in October 2023.
The storm brought significant rainfall and flooding impacts, especially across eastern Scotland. At the time, the North Sea was around 1°C warmer than average, with the warmth extending throughout much of the water column rather than being confined to the surface.
Research led by scientists from the Met Office and the University of Reading examined how those unusually warm sea conditions influenced the storm. By rerunning weather simulations using cooler ocean conditions, researchers were able to compare the impacts directly.
The study found that marine heatwave conditions increased wave power by around 9%, river flows by between 12% and 18%, and storm surge by approximately 20%. These changes contributed to greater flooding and coastal impacts than would otherwise have occurred.
The findings demonstrate how relatively small increases in sea temperature can have significant downstream effects during major weather events.
Looking to the future
Climate projections suggest marine heatwaves are likely to become more common as the climate continues to warm. Events that are currently considered exceptional may become increasingly typical later this century if global temperatures continue to rise.
The greatest increases are projected during summer and early autumn, when UK seas naturally reach their warmest temperatures. This could have implications not only for marine ecosystems but also for weather patterns and the impacts of future storms.
Why monitoring marine heatwaves matters
Marine heatwaves may occur out of sight for most people, but their effects can be felt widely. They can influence wildlife, reduce coastal cooling during hot weather, increase the intensity of showers, and enhance the impacts of severe storms.
As our understanding of the links between the atmosphere and the oceans continues to improve, monitoring marine heatwaves will remain an increasingly important part of weather forecasting and climate research. Understanding what is happening beneath the surface helps provide a clearer picture of the conditions that shape the UK's weather both now and in the future.
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