Remembering Hercules: The Gigantic North Atlantic Storm That Rocked Europe

Tony Butt

by on

Updated 53d ago

Quite a few of you are probably still stuck indoors, like me. So what better time to have a look back at some of the most classic storms and swells of the past few years.

And what better one to start with than a storm that ravaged the North Atlantic in January 2014; Hercules. The Hercules Storm was an extreme event of its own right, but it also occurred in the middle of one of the most extreme North Atlantic winters in living memory. The storm was truly outstanding because it generated a combination of wave heights and periods never before seen along the west coasts of Europe.

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For Ireland, it was kind of business as usual. A little over hyped, as some have said and there were probably three or four more sessions around this time that actually copped into the Emerald Isle better.

Hercules started off as a small disturbance off the east coast of north America around 2nd January. In then began to accelerate north-eastwards, moving into an area of extreme oceanic and atmospheric temperature gradients just east of Newfoundland. This, along with an abnormally-strong upper airstream, pumped enormous energy into the system. As it then moved out into the open Atlantic it became embedded in a strong westerly airflow, which fuelled the system even more and helped it accelerate eastwards. A phenomenon called dynamic fetch or captured fetch – the resulting swell was immense

Hercules then strengthened explosively, with its central pressure dropping 48 mb in 24 hours to around 936 mb, and winds on its southern flank exceeding hurricane force. As it moved eastwards, the windfield expanded and remained above storm force for the next 36 hours or so, generating a humungous swell.

Because the windfield moved at the same speed and in the same direction as the swell it was producing – a phenomenon called dynamic fetch or captured fetch (see HERE) – the resulting swell was immense.

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Next, as the storm arrived a few hundred miles west of Ireland around midnight on 5th January, it suddenly swung northwards and began to weaken. This meant that conditions on the coast never reached the storm-force winds that had been present in the storm centre.

How Hercules impacted Europe. The asterisk refers to significant height, which is the average of the highest third of all the wave heights. The max wave height would have been much larger.

How Hercules impacted Europe. The asterisk refers to significant height, which is the average of the highest third of all the wave heights. The max wave height would have been much larger.

The swell first arrived at the western extremes of Ireland and Galicia early on 6th January, with wave heights of around 30 feet and periods well over 20 secs.

In Southwest Ireland, Cornwall, Devon and Wales, conditions were practically unsurfable, with a huge, surging swell that inundated harbours and villages all along the coast, and fresh to strong southwest winds.

Further north in Ireland, the west swell didn’t hit with full force. But Mullaghmore still pumped, with sets exceeding 20 feet and fresh south or southwest winds.

MSW's animated swell chart of how the swell from Hercules drove into Europe.

MSW's animated swell chart of how the swell from Hercules drove into Europe.

In the Bay of Biscay, the west swell took a while to propagate along the coast. I paddled out at a northeast-facing spot half way along the northern Spanish coast – a spot that normally struggles to get big enough to break. I watched it go from virtually nothing to wildly out of control within the space of about two hours, with the horizon going white every time a 30-foot set broke on some outer reef that nobody had ever seen break. There was very little wind, but the sea looked angry, surging and boiling like a tsunami rather than anything resembling a normal swell. Miraculously, the five of us who paddled out made it back alive.

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The swell didn’t reach the France-Spain border until early evening, peaked overnight and then tapered off quickly the next day. Wind conditions were light enough for Twiggy Baker, Shane Dorian and Jamie Mitchell to paddle into a few bombs at Belharra.

Down into Portugal, the swell hit square-on at west-facing spots, with wave heights over 20 feet and periods up to 24 secs, accompanied by fresh southwest winds. Harbours and coastal towns were devastated, and very few waves were ridden.

Some winters this spot hardly gets big enough to break. Your author of this piece, Tony, and two others paddling on the far left of the photo, just about to get it on the head.

Some winters this spot hardly gets big enough to break. Your author of this piece, Tony, and two others paddling on the far left of the photo, just about to get it on the head.

The root causes behind the Hercules storm are complicated, and cannot be attributed to any one thing. What we can say is that in early 2014, North Atlantic was like a pressure cooker waiting to explode. The stage was already set for a storm like Hercules to form, with a large number of factors coming together to create the right conditions.

An important influence was the behaviour of the upper airstream. At the time, the jetstream was up to 30 percent stronger than average, allowing extra energy to filter down into the surface systems. There is also evidence that the surface lows help to maintain a strong jet, resulting in a self-perpetuating feedback loop between the surface and the upper airstream.

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One thing that also helped the North Atlantic to be more energetic than usual was a strong north-south temperature gradient. This was most striking in the northwest corner around Newfoundland, both in the ocean and in the atmosphere. Greater temperature differences lead to greater pressure differences, which, in turn, mean a greater chance of big storms.

And just to dig a little deeper, we can look upstream at the jet over the Northeast Pacific. In early 2014, a huge meander was present, deflecting the jet further north than usual. As it hit the North American continent it picked up a lot of cold air over Canada, which overflowed into the Atlantic. This cold air entering the North Atlantic contributed to the north-south temperature difference I mentioned above.

Jet-stream chart for Jan 3 2014, the red contours are windspeeds of 90 m/s (320 km/h) or more

Jet-stream chart for Jan 3 2014, the red contours are windspeeds of 90 m/s (320 km/h) or more

In summary, Hercules can still be considered the ‘gold standard’ by which North Atlantic storms can be judged, at least when it comes to swell. Even though the central pressure didn’t get as low as others in the past, the persistence and size of the windfield around Hercules and the resulting combination of wave heights and periods that arrived on the coast, were simply unprecedented for the North Atlantic.

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One unique feature of Hercules was that when it got to about 500 miles west of Ireland it veered north and started to weaken instead of blundering its way onto the coast. Normally, these systems don’t do that, and places that receive the bulk of the swell also receive strong onshore winds. Usually, only places some distance to the south that receive smaller, tangential swell, have clean conditions.

At the time of writing, we haven’t seen another storm as radical as Hercules. But global warming is pushing up storminess and wave heights, and making episodic events more extreme. So don’t be surprised if we get another one soon.

Corresponding surface pressure chart showing an embryonic Hercules in the bottom left-hand corner.

Corresponding surface pressure chart showing an embryonic Hercules in the bottom left-hand corner.

Forty-eight hours later the central pressure had dropped 72 mb and Hercules was in full swing.

Forty-eight hours later the central pressure had dropped 72 mb and Hercules was in full swing.

Cover shot; one of the most iconic images out of Hercules, Jamie Mitchell pre getting steam rolled at Belharra as shot by Alex Laurel.