A new study has revealed that the North Atlantic marine heatwave of 2023 was exacerbated by climate change
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The UK is blessed with an exceptionally varied and beautiful coastline, which means that from seaside piers to tiny rocky coves and the long wind-whipped stretches of sand, there’s a beach for every mood in the UK. However, as beautiful as our beaches are, when it comes to a beach holiday there’s a problem. As anyone who has splashed into the British seas will have noticed, the water, even in high summer, can be a little bracing. Except that is, for last summer. With sea temperatures on the more sheltered beaches being several degrees warmer than average it meant that bathing was a pleasure rather than merely a feat of endurance.
The reason for those added degrees was a very strong marine heatwave created, as was widely reported at the time, by strong high-pressure systems in the North Atlantic and over the UK. Now though, new research has shown that while high-pressure weather conditions were the main driver behind the unusually high sea temperatures, warming due to climate change exacerbated the situation.
Like hurricanes, marine heatwaves are categorised from 1-5 with 5 being an extreme marine heatwave. In June 2023 some coastal waters in northwest Europe experienced sea-surface temperatures of up to 5°C higher than normal, which meant that they were classed as a category 5 marine heatwave. This was followed by another marine heatwave in September 2023 and, according to the UK Met Office, we are currently experiencing a category 1 (+1-2°C) marine heatwave with pockets of category 2 (+2-3°C). Moving towards the Norwegian part of the North Sea, it is reaching category 3/4 (>+4°C) in areas.
The study, led by UK Met Office scientists and a consortium of British and Irish institutions, including Plymouth Marine Laboratory (PML), showed that the UK experienced its longest-ever category 2 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 that the marine heatwave developed quickly due to high-pressure weather conditions, including reduced levels of cloud cover, strong sunshine, weak winds and tropical air. Additionally, the high-pressure dominant over the North Atlantic made for minimal swell activity which resulted in little mixing of warmer surface waters with colder deep-sea waters. This allowed the sea surface water to warm unusually quickly.
The study also found that the influence of the warmer sea water led to warmer land temperatures than average, and heavier rainfall through stronger, warmer and more moist sea breezes.
The authors of the report say 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 2 instead of 1. Worryingly, the authors also 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.
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