Oddities: Why Did the Swell Period Suddenly Double in the UK on Monday?

Tony Butt

by on

Updated 69d ago

A little boy sends a postcard from Auckland, New Zealand to an address in San Francisco, California. The postal service is incredibly slow. Many years later, the postcard arrives in San Francisco, and the occupier of the address embarks on a mission to find out who sent it. He cannot find the sender, of course, because that little boy already grew up into a man, got old and died. The sending of the postcard and the boy himself have become nothing but past events, lost forever in time and space.

You might think that has nothing to do with swell and surf. However, when we look at a newly-arrived swell at the beach, we are looking into the past. That swell was produced by an event – a storm – that might have occurred five or six days ago in a part of the ocean thousands of kilometres away, and might already be just a memory. Much (not all) of the ‘information’ contained in the arriving swell – for example, the height and period and direction of the waves – was created during the formation of that storm. The characteristics etched into the swell by the original storm are like the words on the postcard written by the little boy.

Forecast: UK + Ireland

Chart for 18:00h on Friday 17th January 2020, showing the two storms that produced the swells that arrived on Sunday and Monday

Chart for 18:00h on Friday 17th January 2020, showing the two storms that produced the swells that arrived on Sunday and Monday

So, by looking at the ‘signature’ of a swell, we can get an idea where it came from. A few days ago, a clean, long-period swell hit Cornwall, England. It arrived in almost text-book fashion, with the long-period waves arriving first followed by the shorter ones. The wave period on the coast increased sharply as those first waves arrived, and the peak in wave height followed a short time afterwards. This is due to radial dispersion, which I talked about in a previous article (HERE).

The new swell came from a distant Atlantic storm that had developed off Newfoundland and tracked northeast towards Iceland a few days before. It arrived on top of another swell that had arrived the previous day from a different storm: a smaller, closer system that deepened north of the Azores and pumped out swell from there.

Note that both storms materialised around the same time, but the swell from the Newfoundland system arrived later because it had further to travel. Sometimes this isn’t the case. For example, if you have a much stronger storm further away, the swell might overtake the swell from a weaker storm closer by. It all depends on the speed of the longest-period waves in each swell.

On the MSW forecast page, which I have joined together for Sunday, Monday and Tuesday (see graphic), you can see how the different swells overlap each other. The blue swell is the one from the Azores low and the red and green swells are both from the Newfoundland system. You can clearly see the long periods arriving first. On the period chart animation you can check this against the arrival of the actual swells from each different storm.

The MSW forecast for Newquay, England, for Sunday 19th, Monday 20th and Tuesday 21st January 2020, stitched together and with each swell colour-coded

The MSW forecast for Newquay, England, for Sunday 19th, Monday 20th and Tuesday 21st January 2020, stitched together and with each swell colour-coded

In a following article I’ll have a deeper look into how you can determine which storm a particular swell came from, using a cunning technique based on the rate at which the period decreases as the swell arrives on the coast.

Cover pic, a West Cornwall gem from Cooper Surfboards