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One of the most severe marine heatwaves on the planet is taking place in the shallow seas around the UK and Ireland. That’s according to the US National Oceanic and Atmospheric Administration (NOAA), which has labelled this a “Category 4” heatwave. Rarely used outside of the tropics, a cat 4 heatwave means “extreme” heat.
Marine heatwaves are classified as “prolonged periods of anomalously high sea surface temperature”, when compared to the long-term average for that time of year. And thanks to measurements made by satellites orbiting the earth we know that, in some areas around the UK, surface water temperatures are 4°C to 5°C above normal for mid June.
This is extremely unusual: buoys around Ireland and the UK have been recording sea surface temperature for over 20 years, and in that time it has never been this hot this early in the summer.
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At this time of year, these plankton are dependent on nutrients mixed up from the deep water into the surface layer. However, this year, this nutrient supply may be diminished, since the very high surface temperature means there is likely stronger stratification and less mixing.
A heatwave on the surface could potentially harm the deeper ocean too, and the fish that live there. These continental shelf seas are already suffering from a decline in deep water oxygen, which is partly offset by mixing oxygen-rich water from the surface. However, the fact that the surface temperatures are so high point to a lack of mixing between the layers, and in any case, warmer water contains less oxygen.
An ‘extreme’ heatwave has hit the seas around the UK and Ireland – here’s what’s going onhttps://theconversation.com/an-extreme-heatwave-has-hit-the-seas-around-the-uk-and-ireland-heres-whats-going-on-208052