Eight things you need to know about the Inter Tropical Convergence Zone.
The actual location of the Doldrums varies with the seasons. Typically they said to lie five degrees either side of the equator, but they move and change shape subtly according to ocean and air currents.
This season the weather models have suggested they are stretching, in theory, as high as 7° North, with the best entry point - 27° 30 West – the focus for skippers positioning as much as raw speed on the line to the finish.
Taking the rough with the smooth – the Doldrums with the Trade Winds
The Doldrums is technically known as the Inter Tropical Convergence Zone (ITCZ), because it's where two sets of trade winds meet.
The trade winds that most of the fleet are currently enjoying and are at the heart of the history of the coffee trade that the Transat Jacques Vabre Normandie Le Havre is retracing exist either side of the Doldrums and the two phenomena are intimately connected. The convergence of weather fronts was only properly understood by the mid-20thcentury.
The Coriolis Effect
Between roughly 30 degrees north and 30 degrees south of the equator, the rotation of the Earth causes air to angle toward the equator in a south-westerly flow in the northern hemisphere and in a north-westerly flow in the southern hemisphere. This is what is known as the Coriolis Effect.
The Coriolis Effect (in combination with an area of high pressure) causes the trade winds - the prevailing winds - to move from east to west on both sides of the equator across this 60-degree “belt.”
The Pot au Noir
As the wind blows to about five degrees north and south of the equator, both air and ocean currents come to a halt in a band of hot, dry air; the Doldrums. The French call them the Pot au Noir – literally the black pot or pot of darkness.
These areas around the middle of the globe are burned in the collective conscience of all sea-faring nations, as they were a matter of life or death. Trapped for weeks, sailors would get scurvy, starve or mutiny.
There is wind!
The common misconception is that they are dead zone of no wind. The problem is they are horribly erratic, with violent squalls and electrical storms appearing from nowhere. That means boats flying to much canvas, and now Nylon, can be battered by sudden gusts. The new foiling IMOCAs, which accelerate so much more quickly, will have to beware suddenly being overpowered.
A watched pot boils
One way to think of the challenge facing the sailors looking out for the puffs and gusts of wind to keep them moving is to look at a boiling pot of water and try and predict where the bubbles will rise.
Hot air rises
The science behind the cycle is the intense solar heat in the Doldrums warms and moistens the trade winds causing hot air to rise into the atmosphere.
As this hot air air rises, it begins to cool, causing continual showers and storms in the tropics. The rising air masses move toward the North and South poles, then sink back toward the Earth's surface near 30 degrees either side of the equator. The sinking air triggers the trade winds with little precipitation, completing the cycle.
How does it affect the race?
The Doldrums are often a decisive section of the Route du Café. Once the skippers have crossed this area where the trade winds of each hemisphere form a large zone of uncertainty, the last stretch to Salvador de Bahia is certainly not a formality, but its harder to overtake those in front.
It could shuffle the front of the pack as it did memorably in 2015. And undoubtedly there will opportunities further back in the fleet, with skippers often completely stalling, as their pursuers go past them.
Multi50 – Now!
IMOCA 60 – overnight Tuesday to Wednesday
Class40 - TBC
- Matthew Pryor
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