Doldrums triggered an Atlantic heatwave

Spektrum der Wissenschaft

14.6.2025

Translation: machine translated

In 2023, the Atlantic experienced extensive and previously unknown heating. It is now clear which factors led to the record heat in the ocean.

The summer of 2023 brought exceptionally high temperatures to large parts of the Atlantic: In some places, average surface temperatures were two degrees Celsius above the long-term average. The warm ocean was an article in the breaking of global temperature records; severe heat waves and extreme weather events in Europe are probably also linked to this. An international team led by Matthew England from the University of New South Wales has identified weak winds and the resulting poor mixing of the Atlantic surface water as one of the main causes of the record warmth.

The working group used various observational data to reconstruct the atmospheric conditions at the time and feed newly developed ocean models. The summer of 2023 was remarkably windless over large parts of the Atlantic, allowing a shallow layer of very warm water to form in the ocean, which was heated by the sun. Winds would instead cause the surface water to mix more with the areas below: heat transport into the depths would be the result, which would be prevented accordingly. Instead of the usual 20 to 40 metres, the top layer was only ten metres thick on average; and the water there heated up more and more.

This process was intensified regionally by clear skies: stricter regulations in shipping have reduced sulphur emissions, which has reduced the light-scattering sulphur aerosols in the atmosphere - and therefore cloud formation. More sunlight hits the water and heats it up. On the Atlantic, which is heavily frequented by ships, this effect had clear consequences. However, this was only an important factor for a few limited regions, the researchers write, otherwise the influence of a lack of winds clearly predominated.

The doldrums were apparently triggered by various climatic anomalies, the scientists write. For example, the high pressure system dominating parts of the North Atlantic had weakened, which led to the trade wind circulation falling asleep. The normally constant wind system drives the mixing of the water. It is possible that other anomalies caused the winds to blow only weakly or hardly at all - throughout the entire summer. There was only a regional revival, stronger mixing and cooling water layers, for example in July west of the British Isles.

In addition, even a region south-east of Greenland was affected by the warming, which has been cooling for 50 to 100 years, contrary to the global trend. The phenomenon known as the Atlantic «cold blob» is influenced by the Greenland ice melt: the cold water from the glaciers dampens the temperatures here. When the scientists realised that even this area was warming, they knew that something big must be going on.

The fact that huge areas of the Atlantic were overheated had a direct impact on the atmosphere and the environment: The warm water favoured severe heatwaves in Germany, France and Italy. In the Caribbean, coral reefs bleached over large areas. And the hurricane season was intense: The heat energy from the ocean drives the hurricanes. The duration of the marine heatwave was also astonishing: it lasted not just a few weeks or months, but extended well into 2024.

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