In Europe, whenever we talk to competitors or indeed amateur sailors, they immediately think of the tropics and the route to the West Indies, such as in the Route du Rhum. Or downwind delivery trips wearing T-shirts out on deck with blue skies and light floating clouds. These are the winds that enabled the Portuguese in their square-rigged vessels to discover the American continent, then the passage to India. The winds of discovery…
It all begins between the two tropics (Cancer in the north and Capricorn in the south). It is there that the sun, at its highest point directly heats the Earth the most. The Equatorial zone is a melting pot of hot, damp air, which rises to create the inter tropical convergence zone (ITCZ) or Doldrums, where you find thunderstorms and light winds.
This hot air rises up 12-15 km in the air before coming up against the tropopause, the limit between the troposphere and the stratosphere.
The air then moves outwards, towards the north in our hemisphere and the south in the Southern hemisphere.
This air cools moving away from the Equator and comes back down to Earth. The circle is complete, when on the surface of the sea or on land, this air moves off towards the Equator to rejoin the ITCZ.
The wind that results from this movement should blow from north to south in our hemisphere and the contrary in the south. However, we need to consider the rotation of the planet and the Coriolis effect, which means that hot air moves to the right in the Northern Hemisphere and towards the left in the Southern Hemisphere.
Consequently, these winds in the north and in the south are diverted westwards. Therefore, the circle is more like twisting a bicycle wheel.
These NE’ly winds in the Northern Hemisphere and SE’ly winds in the South, are referred to as trade winds and depend on the strength of the Azores high in the north and the St. Helena high in the south.
This year, the Vendée Globe competitors have had hardly any trade winds in the South Atlantic on the way down or on the way back up. However, back in the North Atlantic after the Doldrums, Michel Desjoyeaux has had some NE’ly trade winds. That explains why he is currently heading off towards Newfoundland. As the winds are clockwise around a high in the Northern Hemisphere, there is little point in battling against the trade winds. It is better to take the long way around to take advantage of more favourable winds.
Even when they are well established, the trade winds can vary in strength and direction. They generally blow between 15 and 25 knots with an average variation of 5 knots a day and an angle that changes by 10 to 30° . Racing in the trade winds is therefore not a time for resting, as you need to watch your course and adjust the boat to wind changes.
In the Météo France forecasts for this evening, we can see the Azores high is well developed and the NE’ly trade winds are well established. On the other hand, in the South Atlantic, anarchy rules. The St. Helena high is cut in two by a low, meaning the SE’ly trade winds are absent, which is a great pity for Armel Le Cléac'h, who has not been able to take advantage of them to climb back up the Brazilian coast...
Loïc Le Bras