Crossing the Doldrums

Strategies for transiting the Intertropical Convergence Zone

For centuries sailors dreaded the aptly named Doldrums. This band of windless, hot, and humid weather near the equator could stall sailing ships for weeks, driving the crew to distraction with the monotony and sometimes even leading to the onset of scurvy as fresh supplies ran out. While sailors today needn’t fear scurvy, most of us still dislike this part of the ocean. 

Most voyagers try to minimize time spent in the Doldrums. This strategy starts with obtaining accurate weather forecasts, whether over single-sideband radio or satellite phone connection. Even in the last 15 years that I’ve been voyaging, the reliability, detail, and accuracy of these forecasts has markedly improved. When my husband Seth and I first started sailing offshore, all we had was voice communication over SSB and VHF. Halfway through our circumnavigation we began using a Pactor modem with the SSB to download SailMail GRIBs. Today, we use OCENS WeatherNet files downloaded over an Iridium phone paired with an OCENS Sidekick WiFi router.

The colorful maritime term “Doldrums” has been replaced by the less evocative but more accurate meteorological name of Intertropical Convergence Zone (ITCZ).

What is the ITCZ?

As its name implies, the ITCZ is a band of low pressure encircling the globe where the southeast and northeast trade winds meet. Heated by the intense sun and warm water around the equator, the air in the ITCZ rises, expands, and cools. This convective activity creates cumulonimbus cells, what most of us refer to as thunderheads or anvil clouds. Where the trade winds are strong, these cells can reach heights of 55,000 feet and the band of them known as the ITCZ can stretch more than 300 nautical miles.

Air from the north and south replacing the rising air in the ITCZ, combined with the Earth’s rotation, creates the northeast and southeast trade winds. 

The ITCZ shifts seasonally to the north and south, following the thermal equator. On average it lies around 6° N but shifts north in the Northern Hemisphere summer and south in the northern winter (Southern Hemisphere summer). This shift is the reason for rainy and dry seasons in the tropics.

Strategies

Bluewater sailors have to contend with the ITCZ on any trans-equatorial passage. Generally, voyagers want to minimize time spent in the convergence zone by crossing it as quickly as possible at its narrowest point and in an area with the fewest thunderstorms.

This starts with advance planning: choosing when to make a passage based on the most favorable time of year. This is much broader than ITCZ conditions, of course, as it is usually dependent upon avoiding cyclone seasons in each hemisphere and on the timing and route of the entire voyage. Once this is decided, attention turns to weather forecasts: aiming to set a course across the narrowest, most favorable point in the ITCZ. Once underway, most sailors like to check the forecasts regularly to track changes in the ITCZ and alter course as necessary. 

In the course of nearly 15 years of voyaging, Seth and I have crossed the ITCZ four times: once in the Atlantic and three times in the Pacific. Our experience might be of some value to sailors planning similar passages.

Panama to the Galapagos

We first encountered the ITCZ upon leaving Panama in early May 2007, bound for the Galapagos Islands. As I mentioned, at that time we had only voice radio communications, so we were careful to check the forecasts at an Internet cafe in Panama City before departure. The weather charts showed light headwinds and only moderate precipitation along our route and so we decided to head straight there. Nothing special, just a rhumb line route of about 1,000 nautical miles.

The passage was uneventful – something every sailor appreciates – and we sailed the entire way. We encountered mild ITCZ conditions. We saw only the occasional strong rain squall, no lightning at all, and light headwinds. We had just enough wind to sail rather than motor, but not so much as to kick up steep seas. The skies were consistently overcast, but fortunately this wasn’t a problem for our battery bank because our boat was very rudimentary and used little power. We needed only enough electricity to run our navigation lights and a small, black-and-white GPS. We had no refrigeration, autopilot, or anything else that draws significant power. The air was humid but not hot, since the route from Panama to the Galapagos lies mostly in the Humboldt Current, which carries cold water north from the Antarctic. I think we were in the ITCZ, or at least on its edges, for the entire passage, as we had solid cloud cover, occasional squalls, and shifty light winds on all 10 days we were at sea.

Ascension Island to Barbados

Our next experience with the ITCZ was in the Atlantic in 2010, on the final ocean crossing of our circumnavigation. At that time we were obtaining GRIB files over SSB radio and Pactor modem, but we made our big decision on routing for that passage before we departed Ascension Island. Ascension is a tiny dot in the middle of the tropical South Atlantic. A long way from anywhere, with few civilian inhabitants and without any civilian air links at all, it is a very remote and isolated rock. In 2010 it did have a couple of boxy old computers with which we could connect to a very slow Internet and look at weather charts. The weather images showed a huge band of ITCZ in the western Atlantic: big red and purple swirls of precipitation crashing into the Amazon rainforest. No surprise there. However, on the far eastern side of the Atlantic there seemed to be almost no ITCZ at all. Our plan was clear: head due north from Ascension in order to stay as far to the east as possible, and only fall off to the west once we reached the northeast trades. This was the first time in our voyaging that we had added significant mileage to a passage in order to avoid a weather system. Our decision added 300 miles to the great circle route, bringing the total distance we would sail from 2,870 to just under 3,200 nautical miles.

It worked beautifully. We had one of the best passages in all our years of voyaging. Light but consistent southeast trades wafted us north for about a week and then began to back until we were sailing close-hauled to a northeast wind. We continued north for another day, hardly daring to believe that we had crossed right from the southern trades to the northern ones without even a hint of cloud, let alone any real ITCZ weather. But it was true, and we soon fell off onto our course for Barbados.

This positive experience with routing ourselves around the ITCZ has certainly informed our subsequent decisions on trans-equatorial passages. I think it is why we are not shy to add miles to a trip in exchange for better weather and more pleasant sailing. While we haven’t had as much success with this approach in the Pacific, I think this way of thinking about the ITCZ has allowed us to avoid potentially worse conditions than we’ve in fact encountered.

Baja to the Marquesas

In the middle of May 2018, 11 years after our first Pacific crossing, Seth and I cast off from the Cabo San Lucas fuel dock, bound for French Polynesia’s Marquesas Islands. This time we were equipped with a satellite phone, Sidekick WiFi router and firewall, and OCENS WeatherNet software. This was not because of any increased risk aversion since our circumnavigation, nor because we’d run into any trouble with our former radio-only approach, but simply because we preferred the OCENS set-up. It’s easier and faster and not that much more expensive than the SSB and Pactor modem, and WeatherNet also provides a range of forecast products, including Arctic ice charts, which we’d used on a voyage to the far north.

For this trip, our passage planning involved two main considerations: where to cross the ITCZ, and staying far enough east at the outset in order to have a decent angle on the wind when we reached the southeast trades.

We’d done our homework studying the pilot charts, and we knew that the best time of the year would have been March, when the southeast trades hadn’t yet gathered their winter strength and when there seemed to be a bit more east in the trade winds than south. The course from the Baja peninsula to the Marquesas is much more southerly, of course, than the course from the Galapagos, and thus it would be favorable to encounter more easterly trade winds. Work, however, had gotten in the way of sailing, as it’s wont to do, and we were unable to leave Baja until the middle of May, when the trades were strong and southerly. To keep our easting, the best plan would be to sail due south until we reached the SE trades. This would add about 400 miles to the great circle route – making the trip 3,000 instead of 2,600 nautical miles – but it would significantly improve our angle on the wind.

The ITCZ in the eastern Pacific at that time looked thick, about 300 miles wide, and the precipitation charts showed red and purple blobs throughout it, all along our route. It did seem to get a little narrower further west, so we faced a compromise between keeping our easting and crossing the Doldrums at the most favorable point. It was an easy decision, however, as the ITCZ was only marginally better further west, whereas the improvement to our sailing angle by staying east would be significant.

So we prioritized our wind angle. We would sail due south and keep a close eye on the forecasts. Once in the Doldrums, we would swallow our pride, motor through the calms, and get across as quickly as possible. With that in mind, we carried a number of extra jerry cans on deck. We also carefully checked our lightning grounds: we both have a healthy respect for lightning after a close call once aboard a wooden ketch.

The first few days of the passage brought beautiful sailing with moderate quartering winds. Soon the anvil clouds of the ITCZ were looming on the horizon, however, and the GRIBs were still showing the same persistently wide band of Doldrums. As we slipped from the bright blue world of the northern trades into the leaden skies and low atmospheric pressure of the ITCZ, the wind died and we fired up the Yanmar. Whenever we had wind we sailed, of course, but our primary aim was to get through unscathed, as fast as possible, so we motored whenever our boat speed dropped below four knots. We kept a close watch – both visually and with the radar – for bad squalls, and altered course to avoid them. We were inevitably caught in a number of stiff rain squalls, but fortunately we successfully avoided thunderstorms. We saw plenty of lightning nearby, however, and were thankful that it was mostly cloud-to-cloud rather than grounding fork lightning.

In a little less than three days, we were through. A stiff southerly wind hit us, accompanied by brilliant blue skies. There wasn’t a hint of east in the wind, so we’d be close-hauled or close-reaching the rest of the way. Nonetheless, we were thrilled to be out of the Doldrums.

Tuamotus to Hawaii

Last year, 2020, Seth and I once again crossed the ITCZ in the Pacific. This time we were sailing from the Tuamotu archipelago of French Polynesia north to Hawaii. This passage is shorter than either of the classic Galapagos-to-Marquesas or Mexico-to-Marquesas passages, but we found it to be tougher. Once again, we weren’t doing it at the ideal time of year. From looking at the pilot charts, and from having lived in Hawaii and observed the weather patterns, April and May would probably be best. In those months, cyclone season is over in the South Pacific but not yet begun in the North Pacific and the trade winds are not especially strong in either hemisphere. Seth and I, however, due to various extenuating circumstances, made the trip in November. By then, the strong northeast winter trades were well established north of the ITCZ and the southern hemisphere trade winds had actually shifted into the northeast, as is quite common in the southern summer. So we encountered headwinds for the entire 2,400 nautical miles we sailed.

Once again, we had two considerations for the passage. One was to make enough easting to have a better angle on the northeast trades once above the ITCZ. The other was to cross the ITCZ at its softest, narrowest spot. Sound familiar?

Because of the northeast winds we encountered even in the southern hemisphere, making that easting was tough, but we managed it through attentive sailing and daily scrutiny of our OCENS weather forecasts. Contending with the ITCZ was more complicated. It seemed to shift quickly: one day it would be narrow and relatively free of precipitation, the next it would have widened to nearly 300 miles in exactly the same spot. As we approached it, our forecasts were showing a heavy band of squalls and potential thunderstorms. However, that band was predicted to narrow considerably about three days later. In fact, the GRIBs showed an opening entirely free of precipitation which we might be able to sail through if we timed it right.

No sailor likes to slow down and wait, adding days to his passage time. Seth and I were anxious to keep going: we were tired of beating to windward; we had hoped to be in Hawaii for Thanksgiving; and we’re mildly competitive and secretly wanted to reach Hawaii in under 20 days, thereby beating by several days the records of all our friends who had made the passage before us. On the other hand, after sailing over 55,000 miles all over the world and encountering our fair share of lightning, we’re very cautious when it comes to thunderstorms. Not only did we have good forecasts, we could actually see the thunderheads piled up on the horizon ahead of us. We slowed down and waited.

Two and a half days of sitting around grated on us, so we were thrilled when we checked the weather on the third day and found it was time to press on and grab our ITCZ window. It worked exactly as we had hoped. We saw no lightning at all and sailed through only one strong rain squall and a number of lesser ones. We had wind the whole way through: Celeste flew along at seven knots with only her triple-reefed main, staysail, and jib set. We came out on the other side – into the northeast trade winds – in only 24 hours.

We didn’t make it to Hawaii for Thanksgiving, but we made it in just under 20 days nonetheless, sailing close-hauled the whole way. Not bad, we thought. Good enough to treat ourselves to a burger on the Hilo waterfront.

Conclusions

The ITCZ can be a difficult obstacle on any trans-equatorial passage. But modern technology has made the Doldrums much less of an obstacle than they used to be. Nevertheless, the Doldrums are still a major global weather feature for which it’s well advised to prepare. Even with all our contemporary technology, the best preparations are the oldest ones: good seamanship, sound judgment, and a seaworthy boat. n

Contributing editor Ellen Massey Leonard is a circumnavigator and with her husband Seth has sailed nearly 60,000 ocean miles.

Categories: Ocean Voyaging, Weather