This event, now lasting over two months, is significant due to its intensity and persistence. Current sea surface temperatures are warmer by up to 4°C west of Ireland, and by 1.5-2.5°C around the UK coastline: temperatures are what we would usually expect around mid-June.

The heatwave is the result of several overlapping factors. Prolonged high-pressure systems brought a dry, sunny spring and weak winds and waves, creating ideal conditions for warming. This means the sea started to warm mid-February, one month earlier than usual. Additionally, the waters around the UK were already warmer than usual coming out of winter, a trend that has been building over the past 40 years, with an average increase of 0.3°C per decade.

A key feature of marine heatwaves is the formation of a warm layer at the ocean’s surface. This layer acts like a lid, trapping heat near the surface and preventing it from mixing into deeper, cooler waters. It can be compared to a layer of olive oil floating on water, this thin surface layer allows heat from solar radiation to accumulate near the surface. When this layer forms, surface temperatures can increase fast: the last 9 days have seen a sudden additional 1°C warming.

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