All about the doldrums or Inter Tropical Convergence Zone
Chris Tibbs takes a look at the best options for navigating though the doldrums or Inter Tropical Convergence Zone (ITCZ)
Doldrums’: a state of stagnation or depression, or an equatorial region of the Atlantic Ocean with calms, sudden storms, and light unpredictable winds. Or so my dictionary puts it.
For sailors, the transition from the northern to southern hemispheres in the Atlantic can be a trying and often frustrating time. We leave the steady tradewinds of the northern hemisphere with our main aim to minimise the time taken before we progress into the South Atlantic trade wind belt – this is passing through the doldrums, or the ‘Pot-au-Noir’, as the French put it.
In meteorology we now use the more scientific term Inter Tropical Convergence Zone (ITCZ) where we can meet conditions we associate with the doldrums; calms, squalls, and thunderstorms. The ITCZ can be thought of as the thermal equator of the world that separates the northern and southern weather systems.
Why they’re tricky
The term ITCZ describes why we get the conditions we know as the doldrums. The tradewinds of the northern hemisphere, driven by the Azores High, are east- to north-easterlies, tending to be more easterly as we get further south towards the ITCZ. Meanwhile the tradewinds of the southern hemisphere mirror these, being driven by the South Atlantic High (often referred to as the St Helena High) and are east- to south-easterlies. Where they come together is the ITCZ.
Where we get convergence in airstreams we get an ascent of air: as the air rises clouds form and as this region is very warm and the air humid, when the air is forced upwards large cumulonimbus clouds form giving squalls and thunderstorms. In addition, where we get ascent of air, we get a lowering of the surface pressure so between the winds of the two hemispheres we get a trough of lower pressure.
Infra-red image from space showing the line of convective cloud along the ITCZ between Africa and the Americas
In this trough of low pressure we have light, predominantly easterly wind, however there is generally less wind to the east as the pressure gradient between the low pressure (of the ITCZ) and the semi-permanent high pressure is less. A greater pressure gradient exists in the central part of the Atlantic and, as the winds tend to be closer to parallel, there is less convergence.
Therefore we would expect the east to have a greater amount of squall and thunderstorm activity. This tends to be the case and close to the African coast is a large triangle which is usually considered a no-go zone for racing boats as it gives more extreme doldrum conditions.
Although the convergence may be less in the central part of the Atlantic we still see some large clouds and squall conditions as the water is generally warmer the further west we go, and the air holds more moisture. An unstable atmosphere and convergence generates the large convective clouds. We can safely say it’s unusual for there not to be a line of convective cloud defining where the doldrums are.
How to cross the doldrums
The challenge for navigators is getting through the doldrums without losing the wind and getting embroiled with squalls. Although squalls can give strong gusts of wind, the strong wind will be on the leading edge of the squall; once the gust front, with possible gale force winds, has passed through underneath the cloud itself, the wind will be light. As the tradewinds are light these squalls can be slow moving and escaping from the squall can be difficult.
A convergence of air in wet and warm regions results in the formation of cumulonimbus clouds bearing squalls. Photo: Brian Carlin/Team Vestas Wind
Even worse can be an old squall that has lost its energy and stopped raining. These large, dark clouds are very slow moving and what circulation they have gives ascending air beneath the cloud and descending air around the periphery. As the background wind is low, trying to get away from the cloud means being headed as you try to escape whichever edge of the cloud you are sailing towards!
Modern weather models are incredibly good but even they have limitations – navigators will be looking for where the steadiest tradewinds are expected to be, which is where the tradewinds are closest to being parallel. For this, stream line analysis is often used.
Holcim-PRB making her way through the Doldrums in The Ocean Race 2023. Photo: Georgia Schofield | polaRYSE / Holcim – PRB
Atlantic in January
Most around-the-world races leave Europe in the autumn, and we know where the climatological average for the best place to cross will be, but this year The Ocean Race started in January, making for quite a different Atlantic leg.
The ITCZ will be further south than in previous Volvo Ocean Races or Vendée Globes, and crews also have the added challenge of a stopover in the Cape Verde Islands.
The old adage is to get west while you can, as the tradewinds are more northerly further north. Making westing on starboard is much easier further north, followed by a long gybe south as the wind veers and eases as you get south. But this goes out the window as this no longer holds true if stopping at the Cape Verde Islands.
Restarting from the Cape Verde Islands it’ll be difficult for the fleet to get as far west as they’d like. How far west to go has always been a balance between the extra miles sailed, an easier crossing of the ITCZ, and not being too far west so that it’s a tight wind angle when you meet the south-easterly tradewinds. This will be determined by where the St Helena High is expected to be.
When leaving the Cape Verdes all the navigators will be aiming for the area which is historically the best place to cross the ITCZ, combined with the latest weather forecasts both for getting through and then south. Satellite images are very useful for identifying where the main squall activity is.
Once the models have been analysed, the satellite images studied, and all the homework done it is still not plain sailing.
The perfect place to cross may be difficult to get to. The more easterly the wind direction, the harder it is to get west. As the wind lightens, gybe angles get greater – so there will be compromises to be made.
The steadiest tradewinds are to be found where the northern and southern winds are closest to being parallel
Then there is the really tricky part: understanding the circulation of the clouds and how much wind will be on the leading edge. Is the cloud raining, giving a strong gust front? Or is it non-raining, giving light winds underneath? Is it building or subsiding?
There have been numerous occasions during the Whitbread and Volvo Ocean Races when one boat gets stuck on the other side of a cloud and at the next update is suddenly 20 miles ahead, so there is an element of luck involved here, but understanding the clouds is a good way to improve your luck. Closely following the direction of the squalls helps in avoiding (or using) the squall front. Radar is excellent for this.
So while navigators will have one eye on the weather models, results can come down to how individual boats negotiate the clouds and there’s always a potential for large gains and losses to be made.
I’ve raced across the ITCZ a number of times, and the last time heading south we were in the wrong place being too far east. Lower pressure over Africa had accelerated the trades so staying in the strongest wind band had forced us east of the Cape Verdes. As the wind was fixed in the east a starboard gybe would have had us heading north of west, so port gybe it was.
When you get to the point where you are close to the ITCZ the most important thing is to get south at maximum speed to minimise your exposure to the ITCZ. The ITCZ will move north and south in the short term and the most difficult time is to be heading south as the ITCZ also heads south.
We were lucky, the ITCZ moved north and we never stopped, giving us a better crossing than our competitors to the west – pure luck.
Our luck ran out later in the Southern Ocean when the mast came down, but that’s a whole different story!
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