Weak winds key factor in 2023 extreme North Atlantic heatwave – marine heatwaves set to worsen

The summer of 2023 saw the North Atlantic Ocean warm at an unprecedented rate, with surface temperatures exceeding previous records by more than 2°C in some regions. The marine heatwave contributed significantly to the record global mean temperature that year and has been linked to severe heatwaves and flooding across parts of Europe.

Using a combination of observational data, atmospheric hindcasts, and advanced ocean modelling, researchers from the University of New South Wales, PIK, the Australian National University, and the Australian Bureau of Meteorology identify weak winds and the resulting shallow uppermost layer of the ocean as key drivers of the marine heatwave.

The thickness of the ocean’s upper layer in summer depends on winds to stir and mix the water. In 2023, weaker winds made this layer thinner than usual – sometimes just 10 metres deep instead of the typical 20-40 metres – allowing heat to accumulate more quickly at the ocean surface. As the surface water warms, it becomes less dense and thus lighter, making it harder for winds to mix the ocean and resulting in a thinner upper layer.

“As global warming reduces ocean mixing, the upper layer becomes thinner and more prone to rapid heating — a trend that could lead to more frequent and intense marine heatwaves,” says co-author Stefan Rahmstorf, head of Earth System Analysis at PIK.

The study traces a steady trend over the past four decades toward a thinner upper layer in the North Atlantic, primarily driven by surface ocean warming. “This shows that climate change is also changing the structure of our oceans in ways that amplify climate extremes,” says Rahmstorf.

The research also highlights the broader risks of such events: the North Atlantic plays a central role in the global climate system, including the Atlantic Meridional Overturning Circulation, which helps regulate temperatures across the Northern Hemisphere. Continued warming and a thinning surface layer could have far-reaching effects, including feedbacks that affect ice melt in Greenland and long-term changes in ocean circulation.