All concerning the doldrums or Inter Tropical Convergence Zone

Doldrums’: a state of stagnation or melancholy, or an equatorial area of the Atlantic Ocean with calms, sudden storms, and light-weight unpredictable winds. Or so my dictionary places it.

For sailors, the transition from the northern to southern hemispheres within the Atlantic could be a making an attempt and infrequently irritating time. We depart the regular tradewinds of the northern hemisphere with our fundamental goal to minimise the time taken earlier than we progress into the South Atlantic commerce wind belt – that is passing via the doldrums, or the ‘Pot-au-Noir’, because the French put it.

In meteorology we now use the extra scientific time period Inter Tropical Convergence Zone (ITCZ) the place we will meet situations we affiliate with the doldrums; calms, squalls, and thunderstorms. The ITCZ may be considered the thermal equator of the world that separates the northern and southern climate methods.

Why they’re difficult

The time period ITCZ describes why we get the situations we all know because the doldrums. The tradewinds of the northern hemisphere, pushed by the Azores Excessive, are east- to north-easterlies, tending to be extra easterly as we get additional south in the direction of the ITCZ. In the meantime the tradewinds of the southern hemisphere mirror these, being pushed by the South Atlantic Excessive (also known as the St Helena Excessive) and are east- to south-easterlies. The place they arrive collectively is the ITCZ.

The place we get convergence in airstreams we get an ascent of air: because the air rises clouds kind and as this area may be very heat and the air humid, when the air is pressured upwards massive cumulonimbus clouds kind giving squalls and thunderstorms. As well as, the place we get ascent of air, we get a decreasing of the floor strain so between the winds of the 2 hemispheres we get a trough of decrease strain.

Infra-red picture from area displaying the road of convective cloud alongside the ITCZ between Africa and the Americas

On this trough of low strain we’ve got mild, predominantly easterly wind, nevertheless there may be usually much less wind to the east because the strain gradient between the low strain (of the ITCZ) and the semi-permanent excessive strain is much less. A higher strain gradient exists within the central a part of the Atlantic and, because the winds are typically nearer to parallel, there may be much less convergence.

Subsequently we might anticipate the east to have a higher quantity of squall and thunderstorm exercise. This tends to be the case and near the African coast is a big triangle which is often thought-about a no-go zone for racing boats because it offers extra excessive doldrum situations.

Though the convergence could also be much less within the central a part of the Atlantic we nonetheless see some massive clouds and squall situations because the water is usually hotter the additional west we go, and the air holds extra moisture. An unstable ambiance and convergence generates the big convective clouds. We are able to safely say it’s uncommon for there to not be a line of convective cloud defining the place the doldrums are.

Methods to cross the doldrums

The problem for navigators is getting via the doldrums with out shedding the wind and getting embroiled with squalls. Though squalls may give robust gusts of wind, the robust wind can be on the vanguard of the squall; as soon as the gust entrance, with potential gale power winds, has handed via beneath the cloud itself, the wind can be mild. Because the tradewinds are mild these squalls may be gradual transferring and escaping from the squall may be troublesome.

A convergence of air in moist and heat areas leads to the formation of cumulonimbus clouds bearing squalls. Picture: Brian Carlin/Crew Vestas Wind

Even worse may be an outdated squall that has misplaced its power and stopped raining. These massive, darkish clouds are very gradual transferring and what circulation they’ve offers ascending air beneath the cloud and descending air across the periphery. Because the background wind is low, making an attempt to get away from the cloud means being headed as you attempt to escape whichever fringe of the cloud you might be crusing in the direction of!

Trendy climate fashions are extremely good however even they’ve limitations – navigators can be on the lookout for the place the steadiest tradewinds are anticipated to be, which is the place the tradewinds are closest to being parallel. For this, stream line evaluation is usually used.

Holcim-PRB making her approach via the Doldrums in The Ocean Race 2023. Picture: Georgia Schofield | polaRYSE / Holcim – PRB

Atlantic in January

Most around-the-world races depart Europe within the autumn, and we all know the place the climatological common for the very best place to cross can be, however this yr The Ocean Race began in January, making for fairly a special Atlantic leg.

The ITCZ can be additional south than in earlier Volvo Ocean Races or Vendée Globes, and crews even have the added problem of a stopover within the Cape Verde Islands.

The outdated adage is to get west when you can, because the tradewinds are extra northerly additional north. Making westing on starboard is far simpler additional north, adopted by an extended gybe south because the wind veers and eases as you get south. However this goes out the window as this now not holds true if stopping on the Cape Verde Islands.

Restarting from the Cape Verde Islands it’ll be troublesome for the fleet to get as far west as they’d like. How far west to go has at all times been a stability between the additional miles sailed, a neater crossing of the ITCZ, and never being too far west in order that it’s a good wind angle while you meet the south-easterly tradewinds. This can be decided by the place the St Helena Excessive is anticipated to be.

When leaving the Cape Verdes all of the navigators can be aiming for the world which is traditionally the very best place to cross the ITCZ, mixed with the newest climate forecasts each for getting via after which south. Satellite tv for pc photos are very helpful for figuring out the place the principle squall exercise is.

As soon as the fashions have been analysed, the satellite tv for pc photos studied, and all of the homework executed it’s nonetheless not plain crusing.

The proper place to cross could also be troublesome to get to. The extra easterly the wind path, the more durable it’s to get west. Because the wind lightens, gybe angles get higher – so there can be compromises to be made.

The steadiest tradewinds are to be discovered the place the northern and southern winds are closest to being parallel

Then there may be the actually difficult half: understanding the circulation of the clouds and the way a lot wind can be on the vanguard. Is the cloud raining, giving a robust gust entrance? Or is it non-raining, giving mild winds beneath? Is it constructing or subsiding?

There have been quite a few events in the course of the Whitbread and Volvo Ocean Races when one boat will get caught on the opposite facet of a cloud and on the subsequent replace is all of the sudden 20 miles forward, so there is a component of luck concerned right here, however understanding the clouds is an efficient approach to enhance your luck. Carefully following the path of the squalls helps in avoiding (or utilizing) the squall entrance. Radar is great for this.

So whereas navigators can have one eye on the climate fashions, outcomes can come right down to how particular person boats negotiate the clouds and there’s at all times a possible for big positive aspects and losses to be made.

I’ve raced throughout the ITCZ various occasions, and the final time heading south we have been within the incorrect place being too far east. Decrease strain over Africa had accelerated the trades so staying within the strongest wind band had pressured us east of the Cape Verdes. Because the wind was fastened within the east a starboard gybe would have had us heading north of west, so port gybe it was.

While you get to the purpose the place you might be near the ITCZ a very powerful factor is to get south at most pace to minimise your publicity to the ITCZ. The ITCZ will transfer north and south within the brief time period and essentially the most troublesome time is to be heading south because the ITCZ additionally heads south.

We have been fortunate, the ITCZ moved north and we by no means stopped, giving us a greater crossing than our rivals to the west – pure luck.

Our luck ran out later within the Southern Ocean when the mast got here down, however that’s a complete completely different story!

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