Marine Heatwaves: Causes, Extreme Models & Climate Impact

Visualization of the 2023 ocean heatwave in the northeastern Atlantic (dark red spots). Temperatures were, in some cases, more than 3 degrees Celsius above the long-term average.
Image Credit: © Courtesy of Catherine Gregory

Scientific Frontline: Extended "At a Glance" Summary: Marine Heatwaves

The Core Concept: A marine heatwave is an extended period of abnormally high ocean temperatures that can severely disrupt marine ecosystems and regional climates.

Key Distinction/Mechanism: Unlike brief warming events, marine heatwaves occur when specific atmospheric conditions—such as high-pressure systems combined with weak winds and intense solar radiation—trap heat near the ocean's surface, preventing cooler, deeper waters from mixing upward.

Origin/History: In 2023, the Northeast Atlantic experienced a record-breaking marine heatwave lasting 240 days with temperatures exceeding the long-term average by over 3°C, prompting researchers at the University of Bern to analyze the limits of such events using current climate models.

Major Frameworks/Components:

• Ensemble Boosting: A climate modeling technique used by researchers to simulate extreme weather events by introducing slight random variations to initial atmospheric conditions, generating numerous plausible trajectories for heatwave duration and intensity.
• Atmospheric Stagnation: The role of high-pressure systems and weak winds in suppressing ocean heat loss to the atmosphere and preventing vertical water column mixing.
• Global Warming Thresholds: Projections indicate that if global warming exceeds 3°C, extreme marine heatwaves (e.g., lasting up to 1,500 days or reaching 5°C above average) will transition from rare anomalies to increasingly common occurrences.

Branch of Science: Marine Climatology, Oceanography, Environmental Physics.

Future Application: Advanced climate modeling via ensemble boosting can improve early warning systems for marine heatwaves, allowing fisheries and coastal economies to prepare for prolonged ecological disruptions.

Why It Matters: Marine heatwaves increase mortality among marine life, trigger harmful algal blooms, devastate global fisheries, and can even amplify heatwaves over adjacent continental landmasses, posing severe threats to both marine biodiversity and human economies.

In 2023, the Northeast Atlantic experienced a marine heatwave of unprecedented intensity and duration. From November 2022 onward, sea temperatures continued to rise in the waters between the British Isles, the Iberian Peninsula, and northwestern Africa. By June 2023, temperatures across a large area were more than 3°C above the long-term average. The heatwave lasted a total of 240 days. Ocean heatwaves can increase mortality among birds, fish, and marine mammals, trigger harmful algal blooms, and disrupt marine food supplies, with increased fish mortality reducing fishery catches. The ocean heatwave in the northeastern Atlantic also intensified the heatwave that was affecting the European continent at the time.

Researchers at the University of Bern have now investigated whether an even more extreme marine heatwave could occur under today’s climate conditions. Their findings suggest that small changes in atmospheric conditions could substantially intensify and prolong marine heatwaves in this region. In the most extreme case, such an event could last as long as 579 days, with sea temperatures reaching up to 4.5°C above the long-term average. The results have just been published in the journal Nature Communications.

Dr. Catherine Gregory, postdoctoral researcher with the Ocean Modeling research group at the Climate and Environmental Physics (CEP) division of the Physics Institute and the Oeschger Centre for Climate Change Research of the University of Bern.
Photo Credit: © Courtesy of Catherine Gregory

“Boosted” heatwaves

“High-pressure systems combined with weak winds and strong solar radiation favor the development of marine heatwaves,” explains lead author Catherine Gregory, a postdoctoral researcher with the Ocean Modeling research group at the Climate and Environmental Physics (CEP) division of the Physics Institute and the Oeschger Center for Climate Change Research. “Strong solar radiation warms the ocean surface, while weak winds reduce mixing with cooler water below and limit heat loss to the atmosphere. Together, these conditions allow heat to accumulate near the surface, intensifying the marine heatwave.”

To explore different possible scenarios for the heatwave, the researchers used a method known as ensemble boosting. This modeling approach simulates different possible evolutions of a selected extreme weather or climate event. To do so, the researchers used a climate model to make small, random changes to the initial atmospheric conditions seen during the 2023 marine heatwave event and then allowed the model to run freely to generate numerous different versions of the event. The simulated trajectories remain physically plausible but can differ considerably in intensity and duration of the resulting marine heatwaves. “With this method, we can investigate which weather conditions make a marine heatwave particularly intense and long-lasting, and how extreme it could become in the worst case,” explains Catherine Gregory.

Prof. Dr. Thomas Frölicher, head of the Ocean Modeling research group at the Climate and Environmental Physics (CEP) division of the Physics Institute and the Oeschger Centre for Climate Change Research of the University of Bern.
Photo Credit: © Courtesy of Thomas Frölicher

The 2023 record still stands—for now

The 2023 heatwave set a record in the Northeast Atlantic, and that record still stands despite the extreme heat across much of Europe in the summer of 2026, says climate researcher Thomas Frölicher, the study’s senior author and head of the Ocean Modeling research group. “Ocean temperatures worldwide did indeed reach record highs in 2026. In the Northeast Atlantic, however, temperatures remained slightly below the levels recorded in 2023.” Thomas Frölicher stresses that this is no reason for complacency: “Our study shows that the 2023 marine heatwave did not represent the upper limit of what is physically possible. Even more extreme events could occur in the future, with far-reaching consequences for marine ecosystems, fisheries, and heat conditions on land—particularly as human-induced climate change continues to warm the oceans.”

Global warming of 3°C could make extreme marine heatwaves more common

In the second part of the study, the researchers investigated how the duration and intensity of marine heatwaves in the Northeast Atlantic could change under different climate change scenarios. With global warming of less than 2°C, severe heatwaves comparable to the event in the summer of 2023 remain relatively rare. However, once global warming exceeds 3°C, such events become increasingly common, with the hottest event reaching almost 5°C above the long-term mean and the longest-lasting event persisting for 1,500 days. “The 2023 marine heatwave would no longer be exceptionally intense in a world with 3°C of warming. This shows that we need to prepare for events that go beyond anything we have experienced historically,” says Thomas Frölicher.

Published in journal: Nature Communications

Title: The 2023 North Atlantic marine heatwave could have evolved into a far more extreme event

Authors: Catherine H. Gregory, Urs Hofmann Elizondo, Thibault Guinaldo, and Thomas L. Frölicher

Source/Credit: University of Bern

Edited by: Scientific Frontline

Reference Number: as102026_01

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