Meteorologist Chris Tibbs explains the Atlantic tradewinds and how to use them to ensure a smooth transatlantic crossing
A transatlantic tradewind crossing from the Canary Islands to the Caribbean is on many a sailor’s bucket list. Endless sunny days of running before the wind followed by nights under a sky full of stars with dolphins playing alongside…
The tradewinds are the driving force for sailing across the Atlantic and with the clockwise circulation of air around the Azores High, it appears quite obvious which way we should go – skirt the high without getting too close and losing the wind. With today’s good forecasts and communications this should be easy enough, however reality shows that the day-to-day weather can be quite different from the climatological averages.
How far north or south the Azores High is established, and where any low pressure over Africa is, will determine just where the band of strongest winds will be found. There are large initial gains to be made if we can head on a direct route at the beginning of the passage, but at some stage we will have to make a dive south to stay in the tradewinds.
In phase with the shift
The gains made by heading west first are that it is in phase with the expected wind swing. We start in a more northerly wind that will tend to veer towards the east as we progress. Going west first helps avoid a dead run all the way. But sailing and weather is not that simple; the wind shadows and acceleration zones can extend a long way from the Canary Islands.
My rule of thumb is to get at least 100 miles south of Tenerife but we have to get clear of the Gran Canaria wind shadow first. If we can slip through during the afternoon, when the wind shadow is at a minimum, gains can be made but there is a risk of getting caught in light wind.
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We may also find a band of stronger wind along the African coast where a heat low increases the pressure gradient. Heading south in this wind band can give some great speeds, but on port gybe in a veering wind gybe angles can be large.
There are other considerations to be taken into account; often in November there will be the tail end of a cold front splitting the Azores High. This can give a band of light and variable wind, and with the trades generally steadier to the south, a dive south may be called for.
It is often said that a more northerly route following the great circle is faster but has a greater chance of beating or encountering some strong wind. If you are racing across, it may well be worth the risk, but at times it has proved to be punishing.
Over the last few years the record for the ARC has been broken a number of times and interestingly the fastest routes have been both to the north and the more traditional southern route, which just goes to prove that the weather is not quite as settled as averages show.
The majority of yachts follow the traditional route heading towards the Cape Verde Islands before tracking west. There are a number of good reasons for this: it’s the most well trodden path and gives a good average, while staying as far away from the tiresome swell created by North Atlantic storms. It is therefore considered a more comfortable route with consistent tradewinds at the cost of a few extra miles.
With the Azores High well established to the north there were strong tradewinds through the Canary Islands for the 2015 ARC (left). In 2016 (right) the pattern was completely different.
The last time I sailed the ARC was in 2015. It was a pretty straightforward year; a fast start and a rhumbline course for the first week, then a dive south to avoid light winds. We were downwind the whole way with a poled out headsail for about 70 per cent of the time and spinnaker the rest.
We were lucky and had a fast passage of 16 days. In 2016 the wind gods were not so kind and it was a year of either going way south or following a northerly course. The middle route was particularly slow.
Having been forecasting for yachts in the ARC for over 15 years I have seen a great deal of variability. Following the traditional route will usually give a very pleasant sail, however for the racing division and more performance orientated crews, a more northerly option may be faster (but not always).
On a long passage, extended forecasts are useful, but look for consistency in weather routing solutions. It is easy to be tempted to take an extreme route only to find that the forecast changes radically over time.
Stick or twist?
Some sailors will choose a route well ahead of time and stick to it. However, large gains can be made by using the available forecasts and being more flexible. For me, this is a large part of an ocean passage but over 15-20 days at sea many may feel there is no harm in an extra day or so of great sailing!
About the author
About the authorChris Tibbs is a meteorologist and weather router, as well as a professional sailor and navigator, forecasting for Olympic teams and the ARC rally.
First publish in the December 2017 edition of Yachting World.
