05.01.2011
From Gibraltar to the Cape of Good Hope: two hemispheres, two trades, two anticiclones
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The situation experienced during the Barcelona World Race 2007/08. The storm situated between 35º and 30º N cut off the flow of the trades and sent southerly wind to both the north and south of the Canary Islands. © MétéoStrategy |
In offshore meteorology things don’t always pan out according to the data, and even in the first leg across the Atlantic in a circumnavigation of the globe, the proof lies in the pudding, or in the sailors’ case – the crossing. In the previous edition of the Barcelona World Race the sailors came across the first exception when it comes to climate rules: the NE trades, the first objective in any sailor’s sights after exiting the straits of Gibraltar, were notable in their absence. The reason for this was straight out of a weather manual: a squall sitting to the west of the Canaries cut the flow of the trades and sent the fleet insidious and unstable winds from the SW.
The situation remained this way for seven long days and the exasperated sailors didn't catch the trades until some 250 miles south of Gran Canaria (see figs 1 and 2). A similar situation was experienced by the Mini Transat 6.50 fleet when, after setting off from Madeira, the boats were forced to sail against headwinds until the Canaries. The passage through the Canaries is a delicate balance anyway, with or without the trades. With a NE'ly wind blowing steadily there is an acceleration of the breeze between Gran Canaria and Fuerteventura, and again between Gran Canaria and Tenerife. Both routes must be considered as options depending on whether one reaches the islands from further east or west and according to the breeze forecast to the south of the islands.
Around mid January, when the Barcelona World Race entries cross the zone, the ITCZ will be positioned towards 5ºN, and the best “gate” to get through it may lie at around 30º W. However, climate forecasts are also famous for not fulfilling their promises. The boats may take up to four days to get through the equatorial zone: once they are through, they can look for some breeze in the numerous squalls ahead of them. Radars fitted to the boats may come into play here, to detect rain and the evolution of the squalls. The sailors will also receive data on winds in the area via satellite, but the evolution of the clouds on a vertical level tends to be very erratic. Another problem of the equatorial lulls is the intense heat and humidity in the air that can turn the Doldrum experience into a physical and psychological hell, where the only light at the end of the tunnel is the first gust of the trades from the SE in the southern hemisphere.
Facing the St Helena anticyclone
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| The ‘corridor’ created by the St Helena anticyclone in the previous edition of the Barcelona World Race.. © FNOB |
Once the entries reach the southern trade winds, the next concern is the St Helena anticyclone, lord and master of climatology in the South Atlantic just as the Azores is in the North Atlantic. In the southern summer, the St Helena anticyclone is disturbed by thermal lows produced in the South American and African continents and it changes position considerably, which means another headache for the skippers. In general, the sailors distance themselves considerably from the Great Circle line (the ideal course) to find the 'corridors’ of northerly wind that develop to the west of the anticyclone. In the last Barcelona World Race the boats descended the South Atlantic through a narrow corridor of N and NE winds that they stayed in for seven days before setting course for the Cape of Good Hope with westerly winds.
Before reaching latitudes to the extreme south of Africa, the sailors will have noticed some of the first effects of the high southern latitudes. The temperature drops drastically, the wave formation from the west grows threateningly and the Raging Forties usually means ‘goodbye Atlantic’, and hello to a new kingdom: the Antarctic Ocean
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The equatorial area between the trade systems of both the North and South hemispheres is scientifically known as the Inter-tropical Convergence Zone, the ITCZ, although sailors more commonly use the English expression “Doldrums” or the French phrase "pot au noir" to refer to the zone, one of the trickiest for sailing due to the extensive lulls only disturbed by sudden and violent storms that form and disappear rapidly. These form due to air that is overheated by the sun’s rays and heavily humid rising up to cooler layers of the atmosphere. Present all year round, the zone varies in location according to the seasons as it follows the position of the sun on its zenith, reaching its most northern position (8º N) during the northern hemisphere summer and the most southerly position (1º N) during the month of April. However, the ITCZ is less mobile in longitude, where it holds a more static position to the north of the Equator, with a width ranging between 20 and 400 miles, which may vary for short intervals every few days. The climate information for the equatorial zone is less than reliable when it comes to tracing a course to get through it and only daily weather reports can give any sort of reliable information. So, the boats with radar technology use the instrument to detect formations and storms and to foresee the winds they’ll generate. It’s the most trustworthy form of weather forecasting for the zone.
The trade winds
The NE trades flow during the northern summer through a strip in the Atlantic, whose northern and southern borders may be situated towards parallels of 30º and 10ºN; during the northern winter the borders of the strip are further south. The SE trades stretch out during the southern summer along a strip with northern limits positioned close to the Equator in the Atlantic; the southern limit of the strip is positioned during winter very near the Tropic of Capricorn, whilst in summer it moves to 30º or 35ºS. The average trade winds reach 15 to 20 knots, and as such have always been a welcome relief for sailors. The most favourable time for sailing with the trades is the second half of autumn in the northern hemisphere; and that’s why ports on the Canary Islands fill up in November with boats looking to sail across the Atlantic and to run the length of what is known in sailing slang as 'the trade wind highway'. As they reach the Antilles the trades lose intensity due to thermal variations originating on the islands’ land masses.
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