Have you ever noticed how you get several successive swells from the same direction, from lows that seem to follow in each other’s tracks, one after the other? In the North Atlantic, this typically happens for around 10 days, until the situation suddenly changes; but sometimes it can get locked into a pattern for over a month.
It has to do with the fact that the behaviour of low pressures is strongly influenced by what is going on the in the upper atmosphere, and the fact that the high-altitude flows change less quickly than the surface flows (see my jet-stream article HERE).
The cyclic behaviour of the North Atlantic – where it locks itself into a pattern for a while then flips into another and back again – has been given a name: the North Atlantic Oscillation (NAO). Scientists have identified two major phases in the NAO, calling it a ‘climatological see-saw’, and have determined that it has a major influence on the weather in Europe. And, of course, the surf.
The North Atlantic oscillates between two different phases: a ‘blocking’ phase and a ‘fluid’ phase. The blocking phase is typically characterised by a large, stable anticyclone which hinders the formation of low pressures or sends them way northwards. The fluid or ‘mobile’ phase is when that high is absent or pushed way to the south; and low pressures are easily allowed to deepen and track quickly across the ocean, sending large pulses of swell to west- and northwest-facing spots.
The length of time each phase of the NAO lasts can vary from a couple of days to several months, but it is typically around one to two weeks. You might get a run of big swells or particularly stormy conditions lasting, say, ten days; and then, without warning, a blocking anticyclone once again establishes itself. Each one of these situations can be self-perpetuating. For example, once that blocking anticyclone appears, it will quickly become a very stable feature. Factors that work to destroy it are constantly amassing, but nothing seems to happen until they become strong enough to tip the balance. Then, one day, the system can no longer maintain itself in that state, and it collapses and flips into the other state.
The NAO has a kind of ‘see-saw’ effect on the weather and the waves in Europe. In the North Sea, for example, a blocking pattern is sometimes useful. A low over Scandinavia combined with the Atlantic anticyclone means a long passage of northerly gales feeding into the top of the North Sea, generating big northerly swells.
A very simple way to quantify whether the North Atlantic is in a fluid or blocking phase is by comparing the atmospheric pressure in the north with that in the south. A large north-south pressure difference means a fluid pattern and a strong westerly airstream, whereas a small or negative difference means a blocking pattern and a large high sitting in the middle.
A single number based on that pressure difference is called the NAO index. There are several different versions of the NAO index, some more complex than others, but the simplest one is just the difference between the pressure at two points, typically Iceland and the Azores.
The NAO index is commonly used as a proxy for storminess – the higher the index the stormier the North Atlantic
But what is the point of using a number to describe something we see fairly easily just by looking at the chart? Well, the NAO index is commonly used as a proxy for storminess – the higher the index the stormier the North Atlantic. This is very useful when analysing large data sets from the past, for example, to try to understand the long-term weather patterns over Europe.
We can also tentatively use it as a proxy for the amount of large surf likely to hit west- and northwest-facing coasts compared with the amount of likely surf in the North Sea or even the Mediterranean. We can look at this over entire winters, years or even decades, to help identify long-term trends in the surf.