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Climate change exacerbated June 2023 marine heatwave

31 May 2024

New research has shown that while high-pressure weather conditions were the main driver behind the elevated temperatures, warming due to climate change made it reach category II (strong) instead of I (moderate).
The coastal waters of northwest Europe experienced unprecedented surface temperatures in June last year, with some locations experiencing sea-surface temperatures of up to 5°C higher than normal, classified as category 5 (extreme). This was followed by another marine heatwave in September 2023 and according to the UK Met Office, we are currently experiencing a category I (+1-2°C) marine heatwave with pockets of category II (+2-3°C). Moving towards the Norwegian part of the North Sea, it is reaching category III/IV (>+4°C) in areas.    

The study, led by Met Office scientists with a consortium of British and Irish institutions including Plymouth Marine Laboratory (PML), showed the UK experienced its longest recorded category II marine heatwave (16 days), with temperatures around the British Isles reaching a 16°C peak in June 2023 instead of the typical 13.5°C. 

The study showed the marine heatwave developed quickly due to high-pressure weather conditions including reduced levels of cloud cover, strong sunshine levels, weak winds and tropical air. Additionally, the high pressure suppressed wave activity resulting in little mixing through the water column, allowing the sea surface water to warm quickly.

The feedback from the warmer sea to the land contributed to record-breaking mean temperatures for the UK and heavier rainfall through stronger, warmer and more moist sea breezes. 

The 2023 findings come as UK seas experience another marine heatwave (since 16th May). 2024, like 2023, is still considerably warmer than average, and scientists have confirmed that any weather variability bringing prolonged anticyclonic conditions is generating a marine heatwave.

The authors have concluded that although the study shows climate change was not the direct driver, the warming trend for sea-surface temperatures over the last two decades exacerbated the scale of the marine heatwave, making it reach category II instead of I.

Prof. Tim Smyth, Head of Science for Marine Biogeochemistry and Observations at PML and co-author on the study, highlighted the importance of sustained long-term monitoring:

“Meteorological events, such a marine heatwaves and storms, highlight the importance of having autonomous instruments in the water measuring when we cannot get out to our sampling stations on boats, and at a high enough frequency (hourly), to capture data on these phenomena. For this study we provided data from the gliders we had deployed off NE Scotland and long-term data from our E1 and L4 sampling stations in the Western English Channel”.

“E1 has the longest hydrographical data series in the world, with depth-resolved measurements of temperature and salinity stretching back to 1903; a period of 120 years where we have seen considerable change to the marine environment. It is only with regular, high-frequency and high-quality data that we can begin to contextualise the marine heatwaves that are occurring”. 

Dr Juliane Wihsgott, Digital Oceanographer and co-author on the study, commented on the study:

“Using autonomous robots has enabled us to collect high-resolution in-situ data within of the hotspots of the June 2023 marine heatwave in northern North Sea. This has helped us look beyond the sea surface and understand the impacts of the marine heatwave on the whole water column. We saw how changes in surface mixing due to reduced wind and waves led to this extra heat being concentrated near the surface, which explains the record-breaking sea surface temperatures we observed.”

Looking at the future, the authors suggest that such high sea surface temperatures will become commonplace by the middle of the century without strong mitigation to slow the rise of greenhouse gas emissions.

The study involved a consortium of scientist from the Met Office, Plymouth Marine Laboratory, the University of Exeter, National Oceanography Centre, Scottish Association for Marine Science, Marine Institute (Ireland), the Marine Directorate of the Scottish Government and the University of Bristol. 

Related information

Full paper: Exceptional atmospheric conditions in June 2023 generated a Northwest European marine heatwave which contributed to breaking land temperature records

Prof. Tim Smyth explains sea surface #temperature data off the coast of Plymouth (UK) over the past 120 years, which one of the longest marine temperature records in the world.

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