The UK has just had the hottest summer ever. Unimaginable temperatures were recorded last month of up to 40 degrees C around London and more than 30 degrees C in Cornwall. Despite this, Cornwall's had some surfable waves during those periods of extreme heat – not epic, but not completely flat either. And yeah, you could even surf in boardshorts.
So what is going on here? In the North Atlantic, you would normally associate hot weather with small surf. After all, the biggest waves occur in the winter and the smallest ones in the summer, right?
But does that mean a hotter-than-normal summer will have flatter-than-normal surf, and a cooler-than-normal summer will have bigger-than-normal surf? Or perhaps they are not really connected.
To get an idea, I decided to do a simple test using some data. I had a look over the last 12 summers to see if there was any correlation between the wave height at Fistral Beach and the temperature in Camborne, some 20-miles west and further inland.
If the two things are correlated (‘correlated’ means there is a definite relationship between the two), it means that they are probably both related to a third factor. This could be some feature in the atmosphere – something that has an effect on both the surf at Fistral and the temperature in Camborne.
The test was very basic, using temperature data from the UK Met Office, and wave height data from the MSW wave model. I looked at the data for July-Aug, for the years 2011 to 2022.
To find out if the temperature or wave height was higher or lower than average, and by how much, I compared each summer’s data with the average over the all the summers, to obtain what is called the anomaly.
By plotting the wave-height anomaly against the temperature anomaly on a graph, I could then see if there was any relationship between the two (see graph below).
I suspected that the two things probably wouldn’t be very well correlated (spoiler, they are! Read on...) Even though the local atmospheric features that control the temperatures over Cornwall are no doubt connected to the Atlantic storms that generate the surf (it’s all linked in some way), the two are probably not that well related to get a really strong, direct correlation.
It turns out that the two are actually quite well correlated. Which is not what I was expecting.
To be extra sure, I did a bit of simple statistical analysis. That’s the diagonal line going through the dots, and those two numbers pointing to the line. Higher-than-average summer temperatures correspond with lower-than-average summer wave heights. The values of those two numbers pointing to the line mean that the correlation between the two is what is called statistically significant. I mean, I’m not just guessing.
I got curious, so I dug a bit deeper. I found a recent study in the prestigious journal Nature.
It showed how extreme heat waves in the UK and Europe are related to the jet stream (the westerly winds that blow at five to ten kilometres above the surface). Over the last few years, heat waves have coincided with periods during which the jet has become split into two, with one branch across the north and another across the south.
The jet stream over the North Atlantic spills over into the European continent and hence affects the weather there. If the jet is split in two, any surface weather systems track across the far north, ending up north of Scotland. Weaker ones track across the far south, affecting the Mediterranean. Running through the middle, there is a band of high pressure, which means calm conditions and hot weather in the UK. Those storms that get pushed further north generate swells, but these swells tend to reach northwest Ireland and Scotland, but struggle to get into Cornwall.
In the opposite situation, with a jet stream that is not split in two, Atlantic weather systems can develop at latitudes corresponding to mid-areas. These can generate surf for places such as Cornwall, and also cause windier, cooler weather over the UK.
Therefore, through the behaviour of the jet stream, summer temperatures are connected to summer wave heights. Summers with a split jet stream mean high temperatures over the UK and small surf in Cornwall; summers without a split jet stream mean not-so-high temperatures over the UK and not-so-small surf in Cornwall.
According to that study, the first situation – a split jet stream – is becoming more and more common every summer as the effects of global warming take hold.
So how is this going to affect us in the future? Well, summers are getting hotter and hotter. No surprises there. Phenomena like the split jet stream, in addition to other factors, are being made more and more likely by the general increase in temperature over the whole planet, due to the burning of fossil fuels. If we all carry on with ‘business as usual’ we should expect more radical heatwaves, droughts and crazier weather every year. Things that we never thought would be possible in England, like 40-degree temperatures, melted roads, buckled railway lines and people dying from heat stroke, will probably not get any better in the near future. Travelling could be very difficult in the future, if not impossible.
In summary, hotter summers in the future might coincide with slightly smaller surf, particularly if you live in Cornwall. But is that really all we should be worried about?