Storm Breakdown: Hercules Vs Ciara Vs Dennis

Tony Butt

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Updated 43d ago

The last couple of weeks in the North Atlantic have been, let’s say, fairly active. A continuous stream of deep low pressures have brought storm-force winds, torrential rain and flooding over Britain and Ireland, and, of course, giant swells to west and southwest-facing spots, with wave heights barely dropping below ten feet. Among the many storms, the most notable have been Ciara (see HERE) that crossed the North Atlantic during the first week of February, and Dennis, (see HERE) that formed about a week later.

The low that developed during the first week of January 2014, called Hercules, has become the ‘gold standard’ for comparing recent storms in the North Atlantic, particularly when it comes to wave heights. That storm generated a humungous, long-period swell that was unprecedented. So how do Ciara and Dennis compare with Hercules?

The storms were similar in some ways but very different in others. For example, they were all extremely deep depressions and covered large areas; and they all contained areas of gale-force winds stretching over huge swathes of the North Atlantic, which generated a lot of swell. But Hercules maintained an area of even stronger winds – up to hurricane-force – that tracked across the ocean in tune with the swell it was producing. This dynamic fetch (see my article HERE) was a major contributor to the exceptionally large wave heights generated by Hercules.

The biggest difference was in the trajectory and evolution of each storm. Hercules was more or less a single, giant low pressure that tracked from west to east across the middle of the North Atlantic, and weakened before it actually hit Europe.

Ciara and Dennis, on the other hand, were peripheral systems that tracked around the southern flank of a complex ‘mother’ system, stationed in the far north. These peripheral systems deepened as they tracked form southwest to northeast, reaching peak intensity further north than Hercules. As a result, the swells reaching most areas were smaller than those from Hercules, but local wind and weather conditions were much worse, particularly in northern areas.

All these storms caused a lot of upheaval to society and expensive damage to human structures. But the main damage from Hercules was from the size and period of the swell itself, not the local wind and rain, as was the case for Ciara and Dennis.

The Hercules swell was so big and so powerful that it arrived at many spots more like a tsunami than a normal swell, destroying harbours, flooding coastal towns and causing severe erosion along most of the western European coast. Ciara and Dennis, on the other hand, smashed straight into Ireland and Britain, carrying storm-force westerly winds and torrential rain. Of course, the waves were huge as well, but not as destructive as Hercules.

Our pressure charts comparing the three storms.

Our pressure charts comparing the three storms.

You might be wondering if there is a root cause for these intense storms; something that perhaps triggered off a chain of events that led to favourable conditions.

Certainly one of the things we can trace it back to is the behaviour of the upper airstream – the jetstream (article HERE). This is largely responsible for the steering and intensification of surface low pressure systems. Basically, a strong, meanderless jet means a lot of energy which filters down to the surface and helps to fuel those lows. Just before Hercules formed in early 2014 the jetstream on the far side of the North Atlantic was about 30 per cent stronger than average

Just before Hercules formed in early 2014 the jetstream on the far side of the North Atlantic was about 30 per cent stronger than average. It has also been very strong these last couple of weeks, with that famous case of an aeroplane breaking the New York to London speed record.

Our swell chart for Hercules...

Our swell chart for Hercules...

But why was the jetstream so strong? Well, to get a strong jetstream you have to have a strong north-south pressure gradient in the atmosphere, which, in turn, needs a strong north-south temperature gradient. Just before storm Ciara, near Newfoundland, that temperature difference was over 30°C over a distance of less than the length of Britain.

And to go even further ‘upstream’ we can find out what might have caused that temperature gradient. In early 2014, the jetstream over North America was further north than usual, pushed northwards by a large meander in the northeast Pacific. This meant that it picked up a lot of really cold air over Canada, which then flowed up against the warmer air at the entrance to the North Atlantic jet.

Ciara...

Ciara...

You could carry on doing this – trying to trace it back, find the root cause. But in the end you would just start getting tied in knots. You see, there isn’t really one, single, ultimate cause. Everything is connected to everything else – more like a tangled web of wire wool than a nice neat chain of events. In such a complicated system like the Earth’s ocean and atmosphere, every cause is an effect of every other cause and every effect is a cause of every other effect.

and Dennis. See our North Atlantic swell charts, HERE.

and Dennis. See our North Atlantic swell charts, HERE.