WEATHER BRIEFING

CHRIS TIBBS THE NORTH ATLANTIC OSCILLATION

Although we often hear about the expected changes to the weather from the El Niño/ La Niña Southern Oscillation (ENSO) it does not have a large effect on the weather in Europe. What does affect us to a greater extent in Europe – most noticeably in the autumn and winter months – are the different phases of the North Atlantic Oscillation (NAO).

If we measure the pressure differences between general low pressure near Iceland, known as the Icelandic Low, and the sub-tropical Azores High, we see fluctuations in their respective strengths. This is important as it controls not only the strength of the westerly winds but also how far north or south winter storms are likely to track.

We say that there is a high NAO index when there is a stronger than ‘normal’ Azores High and deeper Icelandic low. This increased pressure gradient gives us stronger winds and generally wilder storms crossing the Atlantic, which will tend to track further north. As the storms track further north it gives warmer and wetter weather in Europe keeping the UK (and most of Europe) on the south side of lows in the warmer and wetter south-westerly tropical maritime air.

A negative index is indicated by a less well developed high to the south or low pressure to the north. This weaker pressure gradient and more southerly position of the Azores High means that the winter storms will tend to track further south. This tends to bring unsettled weather into the Mediterranean and southern Europe while the colder, drier air will affect the UK giving us less stormy but drier and colder weather.

Both the phases of the oscillation affect the whole of the North Atlantic and into the Mediterranean as well as into central Europe. In the extremes it will affect the weather into the Middle East and Russia.

Jet Stream

Although we tend to measure the NAO in terms of pressure fields, what it is also telling us is the position and intensity of the Jet Stream. This high altitude ‘river’ of wind drives the depressions across the Atlantic and as it moves position slowly we tend to get locked into patterns of weather which can last for most of a season. In summer months this can dictate whether we have a good settled summer, or one where lows passing close by keep the weather very changeable. Calling them ‘good’ or ‘bad’ is rather subjective – this year may not have been very good for barbecues but produced a great Cowes Week!

The NAO exhibits variability over long periods meaning we commonly experience a number of years of positive or negative features. Exactly what causes these long-term patterns, however, remains unclear. In the winter of 2009-10 when it was unusually cold in the UK (the coldest in 30 years) the year before and the year after were also negative NAO phases. Incidentally, 2009-10 was the warmest recorded winter in Canada, indicating how the NAO affects beyond the Atlantic basin.

This is all very well in theory but can we use it when sailing? The tradewinds are driven by the Azores high so with a positive NAO we generally get good tradewinds for an Atlantic crossing and a windy Caribbean season. On a negative year we need to head further south to pick up generally lighter tradewinds. Bear in mind that there is always variability within the season so it is not guaranteed, but the pattern will be there.

Last winter was generally a strongly positive phase as Cumbria recorded one of the wettest winters on record. I was very fortunate in crossing the Atlantic with the ARC, and my passage to the Canary Islands during September and October was generally good with just a wait in Bayona to allow a storm to pass through.

With the well-established Azores High the Portuguese trades were strong and we enjoyed a fast ride south (if a bit too exciting at times!). This pattern stayed with us through the winter months giving good strong tradewinds for the crossing.

Departing from Las Palmas the Trades were established quite far north. This meant there was no advantage in dipping south towards the Cape Verde Islands, giving a relatively straightforward crossing only a little south of the rhumb line. We enjoyed a 16-day passage, of which all but three were in moderate tradewinds. Of the three light wind days, we hardly dropped below 5 knots, teasing the last bit of speed out of the spinnaker, and did not have to use the engine at all.

Global warming

Scientists have suggested that with less Arctic ice in the summer, water temperatures will increase, due to the fact that ice reflects the sun whilst the water absorbs it. The warmer water will also tend to change pressure fields, but how this will affect the NAO and our weather is not certain and research looking at this is currently underway.