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North Atlantic Winter Exposed

by on Monday 12th April, 2010   19180 Views

Upside-down winters have their .... Upsides © 2014 Lou Short

IF you live in Europe, you might have noticed that, this winter, things have been, shall we say, upside down.  The surf was completely different from any ‘normal’ winter, with places that usually get consistent surf either flat or onshore, and other spots that rarely work, pumping all winter. Because the surf is driven by the weather patterns, this tends to suggest that the weather patterns were upside down too. If you are a little more astute, you will have noticed that all the swells were generated from low pressure systems that developed right down in the south of the North Atlantic, where there is normally a large high pressure called the Azores Anticyclone.  It seemed to stay like that from early December all the way through till the end of March.
 
I’ll say before we begin that I don’t have the definitive answer as to exactly why it happened like that, but I’m going to dig deeper into the problem bit by bit and see how far we get.  First I’m going to look at what happened to the surf this year compared with last year (assuming last year was more ‘normal’ of course) in four different places: Northern Spain (where I live), Southwest England, Madeira and Cape Verde.

Northern Spain

In northern Spain, I can tell you first hand that we had an epic November with back-to-back swells and a couple of big-wave sessions as good as I can remember.  But then the swells started to come from the west.  It didn’t go flat, but it never really got much above six foot either.  At my local spot, which is surfable between around eight and twenty feet, we surfed about six times, most of them in November, whereas during a normal winter it would break at least 30 times.  You see, if the swell is too west it fails to reach most of northern Spain, tucked in behind two huge peninsulas called Estaca de Bares and Cabo Ortegal.  The approximate swell window is shown in Figure 1.
 
In true scientific style, just to reconfirm what my own eyes were telling me, I looked at some archived wave data from the beginning of November 2009 to the end of February 2010, and compared it with the corresponding data for 2008-2009.  For northern Spain, I compared the number of days of northwest, west-northwest and west swell directions.  It turned out that this winter there were about the same number of west-northwest swells as last winter, but about twice as many west swells and only half as many northwest swells (Table 1).

From this we can suggest that northern Spain was worse than normal because most of the swells were generated outside the swell window (below the blue patch on the diagram).

Table 1: Number of days of different swell directions for northern Spain

.......................................

W..WNW.NW

....................2008-2009…..18…...38…...31
....................2009-2010…..34…...41…...16

Figure 1: Approximate swell window for Northern Spain. The edges of the window are curved because of the particular type of chart used.

Figure 1: Approximate swell window for Northern Spain.  The edges of the window are curved because of the particular type of chart used.

Southwest England

Meanwhile, on the south coast of Cornwall, England, the tables were turned.  This stretch of coast has some world-class setups, but due to a fairly narrow swell window (Figure 2) and the rare chance of a big swell coinciding with offshore winds, days of good surf are few and far between.  However, this winter, instead of the usual mix of northwest swells that failed to wrap around to the south coast, or good swells hindered by strong westerlies, they had exceptionally good surf, particularly from December onwards.  When Spain was bad, south Cornwall was good, and vice-versa.  Steve England, a Porthleven legend who has hardly missed a session there since the mid 1970’s, reckons this winter was “definitely the most consistent in the last four years” and “the swell of Monday 11th [January] was the best high tide swell I’ve seen for ages”.

Again, just to confirm, I looked at the data archives for the same dates as above, but this time for south Cornwall.  I compared the number of potentially surfable days at Porthleven, this winter and last winter.  To get an approximate definition for ‘surfable’ I included any days with swell directions from the southwest and west-southwest, with peak periods longer than 10 s, and with winds from the east around to north.  You don’t have to be really finicky as long as you define it the same for this year and last year; compare apples with apples, so to speak.  It turned out that this year there were over five times as many potentially surfable days as last year (29 compared with a measly 5). 

So, we can conclude that the south coast of Cornwall was better than it normally is because more swells were generated from inside the swell window (inside the blue patch).

Figure 2: Approximate swell window for the south coast of Cornwall

Figure 2: Approximate swell window for the south coast of Cornwall

Just one of 2010's south coast super swells ... Porthleven © 2014 Geoff Tydeman

Southwest coast of Madeira

Down in Madeira, particularly on that magical stretch of coastline near Jardim do Mar, things weren’t so good.  Instead of pristine, windless conditions and consistent, long-period swells wrapping down from the north, they had to put up with poor-quality west swells hitting the coast square-on, accompanied by strong southwest or west winds.  Jardim do Mar surfer Adriano Longueira confirms that they had a particularly bad winter for surf:  “From December till March, which is normally the perfect time to surf in Madeira, we only had good waves about five times.  I’ve never seen a winter like this one - nothing but rain and storms - really bad.”  Talking of rain and storms, this winter Madeira had the worst rains for 20 years, with serious flooding on 20th February that cost the lives of over 40 people. 

Normally, the problem with the southwest coast of Madeira is that the swells are often not quite big enough to make those epic pointbreaks fire, especially if it has a lot of north in it.  If the swell is more west, it will hit the coast more head-on and produce bigger waves, but the same weather patterns also lead to westerly winds.  A very large northwest swell accompanied by northeast trades is the best combination.

The archived wave data from Madeira showed that the southwest coast had a lot less rideable days this year than last, mostly due to bad winds and poor-quality swells.  This time I looked at the average windspeed for onshore winds (southeast around to northwest), the average wave height and the number of potentially rideable days (days with significant wave heights at 2 m or more, peak periods over 10 s and winds from the north-northeast around to the east).  You can see by looking at Table 3 that this year there were more than twice as many onshore days, the onshore winds were almost twice as strong, and there were about half the number of potentially rideable days compared with last year.  The wave heights were bigger this year, but that doesn’t mean the surf was any better.

From this data we can suggest that the southwest coast of Madeira was worse than last year principally because the storms came too close.

Table 3: Data for the southwest coast of Madeira

.................

Mean onshore wind [kts] ...Onshore days...Rideable days...Mean wave height [m]

2008-2009…...............5.1….........................31…....................35….....................2.4
2009-2010…...............9.2….........................70…....................21….....................3.2

Cape Verde

Lastly, it seems that the islands of Cape Verde, way to the south, got much more surf than normal this year.  Cape Verde isn’t known as a surfing paradise - probably due to the constant tradewinds it is more a windsurfing and kitesurfing destination.  Still, there are some class waves and many undiscovered spots.  Instead of fairly constant, but often small and windy surf, they got plenty of days of big, clean, Hawaiian-type conditions, with travelling windsurfers referring to it being “Just like Maui, but with better waves and no people”.  On 20th February they had the biggest swell in four years, with fifteen foot waves at Ponta Prieta. This is a southwest-facing spot that is not fully exposed to the main swell, so I’m assuming that other spots would have been bigger.

The islands of Cape Verde are a long way south of the usual position of the North Atlantic storms, so the swells usually arrive spread out and diminished in height - a phenomenon known as circumferential dispersion.  But not this year: the archived data shows an average wave height almost twice as high as last year (2.0 m compared with 1.1 m) and almost twice as many days of rideable swell (97 compared with 55).  Again, I defined ‘rideable swell’ as coming from the north around to the west, and with peak periods of more than 10 s.  The surf was extremely consistent, with up to 26 days of rideable surf at a time.

So, from this data we can probably say that Cape Verde had better surf this winter than last because the Atlantic storms were closer (but not too close like Madiera).

Digging a bit deeper

It is not difficult to guess the common denominator for those observations.  This winter, all the storms just tracked a lot further south than they normally do.  This can clearly be seen in two charts I have pulled out of the archives as typical representations of last year and this year (Figures 3 and 4).  The first one, from last winter, shows a low pressure centred just south of Iceland and a high pressure near the Azores (the famous Azores Anticyclone).  Northern areas are getting very strong westerly winds and southern areas will be getting big, clean surf.  The second chart, from this winter, shows a low pressure centred straddled across the Azores, with strong westerly winds hitting the far south.  The Azores Anticyclone has completely disappeared, and instead there is a strong westerly fetch.  This is producing nasty weather in southern Portugal, Madeira and northern Morocco, no swell for northern Spain and, if the fetch swings around to the southwest, some good long-period swell for southwest England.

A nice simple way to express the state of the charts without actually having to look at a chart itself is to use the atmospheric pressure difference between Iceland and the Azores.  This is called the NAO Index, and it is related to a climatic cycle called the North Atlantic Oscillation (NAO).  A chart like Figure 3 will produce a large positive index, i.e. the pressure over Iceland is much lower than that over the Azores; and a chart like Figure 4 will produce a zero or negative NAO index, i.e. the pressure over the Azores is equal to or lower than that over Iceland.  A zero or negative index can also occur if you get a large high filling up the North Atlantic, called a blocking anticyclone, which obstructs the formation of low pressures.  The blocking anticyclone is actually a much more common occurrence than the strange patterns we had this winter.

The NOA index is shown as a graph in Figure 5.  You can see at a glance how different it is this year from last year.  You can even see how it changed radically from the end of November onwards, which was basically when the surf on the North Coast of Spain switched right off.

Figure 3: Atlantic chart for January 2009: NAO index ≈ +60 mb. The thick black line indicates the approximate position of the jet stream

Figure 3: Atlantic chart for January 2009: NAO index ? +60 mb.  The thick black line indicates the approximate position of the jet stream

Figure 4: Atlantic chart for December 2009: NAO index ≈ -20 mb 2010: NAO index ≈ -20 mb

Figure 4: Atlantic chart for December 2009: NAO index ? -20 mb

Figure 5: NAO index for 2008-2009

Figure 5: NAO index for 2008-2009

Figure 6: NAO index for 2009-2010

Figure 6: NAO index for 2009-2010

Ultimate causes

Looking at the charts again, you can see that the jet stream, indicated by the thick black line, was also much further south than normal.  The jet stream is the flow of air in the upper atmosphere which determines the trajectory of the storms in the lower atmosphere.  A south-running jet means that all the low pressures will form in the south of the North Atlantic.  A jet with a large meander in it means that the atmosphere contains that large blocking anticyclone I have just mentioned.

So, the jet stream controls the formation and trajectory of the low pressure systems.  But what controls the jet stream?  Well, that is a very good question, which scientists are still trying to answer.  The behaviour of the atmosphere over the North Atlantic, whether we are talking about the lower or upper atmosphere, is extremely difficult to predict, particularly in the short term such as weeks or months.  However, it seems that the long-term patterns might be more heavily dependent on the circulation patterns of the ocean than on most other things.  The sea surface communicates its temperature to the overlying air, which then changes its pressure, temperature and movement, which, in turn, feeds back to change the water temperature.  In other words, the atmosphere and ocean are in a constant feedback loop, passing information from one to the other, each one changing the other’s behaviour. 

A recent study by Arnaud Czaja and Claude Frankignoul shows that changes in the sea surface temperature and changes in the upper air stream are strongly related to each other in the North Atlantic.  A significant correlation was found between the changes in the behaviour of the atmosphere and changes in the sea surface temperature patterns, with the latter leading the former by up to several months.  In other words, it suggests that the water temperature has an influence on what the atmosphere will be like in several months time.  They state in their paper that this shouldn’t be taken as evidence of any cause-and-effect relationship, and (as always) that a lot more work needs to be done on this subject.

Lastly, the El Niño Southern Oscillation (ENSO) is thought to have an influence on the storm tracks, but in the North Pacific.  ENSO is a major climatic cycle mostly involving water temperatures, but which has a whole series of knock-on effects.  In the North Pacific, the jet stream sometimes takes a more southerly track during the El Niño part of the cycle, which means you’ll get epic big surf in Hawaii, Central and Southern California but stormier than normal conditions in Northern California.  In the North Atlantic, however, it is not quite so certain that this effect spills over. 

Whatever the case, it’s all still bewilderingly complicated and we are only just beginning to understand it.

Steve Voyzey at the end of the motorway ... and yes it is secret © 2014 Nick Turley/www.nickturleyphotography.co.uk

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