Official: This Was the Flattest It's Been in the Past 13 Years

Tony Butt

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Updated 83d ago

Well, the good news is, you're not going crazy. It has officially been one of the flattest spells in recent memory for the likes of the south west UK and other parts of Europe too -- and this is why the potential arrival of surf from Hurricane Larry has got us licking our chops.

The North Atlantic is highly seasonal, with huge storms in the winter and much calmer conditions in the summer. But even in summer, the most exposed spots don’t normally go for more than a few days or a week without something surfable, even if it is only two or three feet.

Forecast: Fistral

Chart for 19 August 2021 – almost completely devoid of swell anywhere in the North Atlantic. Even that low just west of Ireland didn’t have enough wind around it to generate surfable-sized waves.

Chart for 19 August 2021 – almost completely devoid of swell anywhere in the North Atlantic. Even that low just west of Ireland didn’t have enough wind around it to generate surfable-sized waves.

But for the last three or four weeks, even at highly-exposed beachbreaks in West Cornwall, Southwest France and Galicia – places that normally thrive on small-to-medium swells with relatively short periods – there has been almost a complete lack of surf. Wave heights have struggled to get above two feet, and even that has mostly been at spots exposed to local windseas generated by north or northeast trades.

So we thought it would be interesting to dig into some of the data archives and see how late August / early September 2021 stacks up against other years, and whether it really has been one of the longest flat spells in recent history. We used Fistral Beach, Cornwall as a reference point.

For the comparison we chose a 20-day period between August 19 and September 7. We then looked at the wave heights at Fistral over that period for the last 12 years, going back to 2009. We calculated the average wave height over that period, and also the number of days during that period that the surf got over 2 foot, which kind of gives you an idea how many surfable days there were. Those calculations were repeated for same period in each year, to allow a comparison to be made between all the years.

The results are shown in the charts. You can see that the average wave height for that 20-day period in 2021 – at a measly one and a half foot – is much smaller than in any other year. And the number of days over 2 foot for that period in 2021 is way, way below the other years.

Average wave heights for the same 20-day period.

Average wave heights for the same 20-day period.

So, why was it so flat? Well, the swells that reach exposed spots such as Fistral Beach are generated by storms in the North Atlantic. In the summer you don’t get anything like those huge low pressure systems covering the entire ocean.

But you do normally get the odd low developing, albeit a relatively weak one. If the southern flank of that low pushes down against the northern flank of the large anticyclone that tends to sit over the Azores, you will get enough pressure gradient to generate a passage of westerly winds. And if those winds produce three or four feet of swell, that’s enough to create rideable surf at exposed beachbreaks like Fistral.

But the last three weeks or so have seen a practically ‘empty’ North Atlantic, with no significant highs or lows. Therefore, very weak pressure gradients, not much wind and almost no swell.

But why was the North Atlantic like that? To answer that question properly we would have to trace it back and look for the ‘ultimate causes’ of that particular weather pattern. And that might not even be possible, because the ocean-atmosphere system consists of a vast number of mechanisms all linked together, each one affecting all the others in a complex web of feedback loops.

Number of days over 2 foot for the same 20-day period

Number of days over 2 foot for the same 20-day period

However, without getting into the realms of complex systems and chaos, we can look at some of the ‘proximate’ causes. These might include things like the strength and trajectory of the upper airstream. During the last few weeks the jet stream has been extremely weak to the point of being almost non-existent, meaning very little energy available to feed down from the upper atmosphere to the surface. Another factor might be the sea-surface temperature: smaller temperature gradients in the ocean mean smaller pressure gradients in the lower atmosphere.

These and other factors sometimes tend to form a self-perpetuating feedback loop, locking the North Atlantic into a ‘quasi-stable state’ which can persist for several days or even weeks. More about this sort of thing in my article on the North Atlantic Oscillation (here and here).