Surface waters in Northwest European seas are currently experiencing moderate to severe marine heatwave conditions, with widespread areas of “strong” (Category 2) conditions and some locations reaching “severe” (Category 3).
This means that waters have reached temperatures usually expected in August and are on average 2°C warmer than usual, locally reaching +4-5°C anomalies offshore parts of the Welsh and English coasts.
These elevated sea-surface temperatures have developed rapidly partially because of last week’s record-breaking atmospheric heatwave and could intensify further in the coming week when settled and sunny weather is likely to return.
The provisional UK June temperature record was exceeded on three consecutive days last week. Typically lagging inland weather by around 3-5 days, sea-surface temperatures have responded quickly, with the Bay of Biscay reaching unprecedented values for June.
While this exceptional land heatwave was primarily driven by atmospheric conditions, the surrounding marine environment played a role in prolonging warmth on land as the heatwave declined, when light winds favoured the movement of warmer-than-average marine air inland.
Widespread marine heatwave conditions
Across UK waters, widespread Category 2 marine heatwave conditions are present around much of the English and Welsh coastlines, the English Channel, and the central and southern North Sea, meaning that sea-surface temperatures are about 1.5-4°C above normal (anomalies), with temperatures of between 15 and 19°C. In tidally-mixed regions such as the Irish Sea, English Channel and southern North Sea, temperatures are elevated not only at the surface but also at depth, with anomalies of around +2°C observed below the surface. Elsewhere, warming is largely confined to the upper ocean layers.
Looking ahead, inland temperatures are forecast to rise again next week, with parts of England potentially reaching low 30s°C in the southeast. While the likelihood of reaching the extreme temperatures and humidity levels observed last week is currently low, a renewed period of warm weather is in the forecast.
If this occurs, the sea surface will have limited opportunity to cool. This raises the possibility that the current marine heatwave could intensify further, potentially reaching Category 4, classified as “extreme” – a level rarely recorded in UK waters.
The English Channel has already experienced marine heatwave conditions for much of 2026, and in combination with continued sunny and calm atmospheric conditions, this region is particularly susceptible to further warming. Surface temperature anomalies in parts of the North Sea are forecast to peak at around +4 to +5°C above average in the coming days.
This week, the Copernicus Climate Change Service (C3S) and the Copernicus Marine Service (CMEMS) confirmed that global sea surface temperatures (SSTs) have surpassed the previous records for this time of year set in 2023 and 2024. This new seasonal high had been anticipated following the development of El Niño conditions in the Equatorial Pacific, which were formally announced by the World Meteorological Organization (WMO) on 2 June 2026, alongside persistently elevated SSTs observed across several ocean regions in recent months. These record temperatures are expected to have wide-ranging impacts on weather patterns, the global climate, and marine ecosystems
Copernicus have also reported above-average sea-surface temperatures across European seas at the end of June 2026.
UK marine heatwaves have developed
Dr Ségolène Berthou, Air-Sea Interaction Specialist at the Met Office, said: “Marine heatwaves around the UK have developed rapidly following the recent heat dome, and we are now seeing widespread strong to locally severe conditions.
“This is the third and most intense marine heatwave we have seen this year.
“While these warmer seas did not significantly increase peak temperatures on land, they reduced night-time cooling and helped sustain warmth, particularly in coastal areas.
“With further sunny and calm weather likely next week, there will be little opportunity for the ocean to release this excess heat. This means the surface marine heatwave could intensify further, potentially reaching extreme levels in the south. Such conditions would be highly unusual for UK waters.”
Shifts in fish populations
Prof. John K. Pinnegar is the Principal Scientist and Lead Advisor (Climate Change) at CEFAS. He said: “Marine heatwaves can have significant impacts on ecosystems and wildlife. Prolonged periods of unusually warm sea temperatures can lead to shifts in fish populations, damage important habitats such as seagrass and kelp, and increase the likelihood of harmful algal blooms. These changes can result in mass-mortality events for some marine species and alter the distribution of commercially-important fish and shellfish.
“Prolonged periods of elevated seawater temperatures can also encourage new species to visit UK waters, establish new populations, potentially shaking up UK ecosystems. A recent example includes the population bloom of common octopus we’ve been experiencing since last year, with serious negative consequences for crab and lobster fisheries in southwest England.”
Cold water shock still a factor in marine heatwaves
Holly Clements, Met Office Head of Warnings and Guidance, said: “It is vital for the public to understand that although sea-surface temperatures are currently well above average in many coastal locations, this does not avoid the risk of cold-water shock.
“With warmer weather approaching, and even with marine heatwave conditions, it’s important to remember that the water around the UK is still cold. Entering it unexpectedly can lead to cold-water shock.”
Where possible, always choose an RNLI lifeguarded beach, and swim between the red and yellow flags. In a coastal emergency remember to call 999 for the coastguard.
RNLI advice if you get into trouble, is to remember to Float to Live: tilt your head back so your ears are submerged, relax and control your breathing. You may need to gently use your hands to help you stay afloat, and it’s ok if your legs sink – everyone floats differently.
You can find water safety advice on our website and in the refreshed Met Office app, now featuring a dedicated beach forecast with tide times, wave heights and sea-surface temperatures.
The Met Office continues to monitor and forecast both atmospheric and oceanic conditions closely, as interactions between the two can influence the duration and impacts of heat events across the UK.
