Bangor: 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.
The heatwave is strongest in the northern North Sea, northwest of Ireland, and the Celtic Sea between Cornwall and southern Ireland. However, in other areas, such as the southern North Sea, the English Channel and the southern Irish Sea, the surface temperatures are only a degree or so above normal.
The two regions are very different in oceanographic terms. The latter areas tend to be shallower (30-40 metres) with stronger tidal currents and so the water remains well mixed from the surface to the sea bed, all year around. In contrast, the regions where the heatwave is strongest are deeper (80-100 metres) with weaker tidal currents. As the mixing is weaker these seas “stratify” each summer, with a layer of warmer water overlying the cooler deeper layer.
In these seasonally stratifying regions the heat from the sun only warms the relatively shallow surface layer, while in the mixed regions the sun’s impact is diluted as its heat is mixed through the ocean from seabed to surface.
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