What is the ITCZ and why does it matter?


We were warned about the Intertropical Convergence Zone (ITCZ) well before we left on our Pacific crossing. It was constantly brought up in discussions with other cruisers. We were given strategies for getting through as quickly as possible and cautioned by tales of voyagers from previous years who had not been so lucky. “Took years off my life,” was how one sailor described ghosting through thunderstorms.

So, it was with no small degree of trepidation that we began our approach to the ITCZ. We checked weather forecasts religiously. The ITCZ seemed to jump all over the place: One day it was at 8° N and the next 5° N. We stayed north and went farther west than we had originally intended in order to stay above it. At 10° N, 131° W, we saw it starting to drop and ducked south. We were lucky — the ITCZ stayed a few degrees ahead of us the whole way down to the equator, at which point it jumped right over us and set up at 6° N again. We were through without having to cross it. In a way I was disappointed, but in another and more honest way, I was very relieved. It was as much luck as it was route planning.

The experience raised a few questions: What exactly is the ITCZ, why are sailors so afraid of it and why is it so hard to predict?

In order to understand the ITCZ, it is important to understand a little about pressure and wind. Wind is nothing more than the movement of air from high pressure to low pressure.

Air pressure differences are primarily caused by differential heating of the Earth’s surface. Close to the equator, the sun is at or near its zenith and there is intense heating. As the sun heats the air, the air volume increases and it begins to rise. This zone of warm moist air is known as the ITCZ. As this huge column of air rises, air from surrounding areas must rush in to fill the void it leaves behind. This rush of air from the tropics toward the equator is what is known as the trade winds.

The ITCZ changes position during the course of the year.

However, due to the Coriolis effect (the result of the Earth’s spin on moving objects like masses of air), the trade winds don’t flow straight down to the equator but are bent to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. This results in the northeast trades and the southeast trades.

Why is there no wind in the ITCZ?
The ITCZ was first called the Equatorial Doldrums in 1855 due to its “calm and baffling winds.” The ITCZ is a wide band of constant low pressure due to heat from the sun. As we already know, wind is air moving from high pressure to low pressure, so where there is constant pressure there is no need for air to move around and hence there is no wind.

Why are there so many squalls in the ITCZ? 
The intense heating at the equator causes air to rise and wind to be sucked in from the tropics, or what we call the trade winds. As the equatorial air rises, it cools down and the air can no longer hold the moisture it carries. The cold air releases its moisture as ice and rain. The cold precipitation cools down the surrounding air as it falls and a convection cycle forms. The air moving in this convection loop is what we feel as wind at the ocean surface. It is for this reason that NOAA calls squalls “areas of localized moderate to strong convection.”

The South Pacific Convergence Zone (SPCZ) 
While the conditions for the ITCZ are a global phenomenon, it can be argued that the ITCZ as we know it forms only over oceans. Over the Pacific Ocean, the ITCZ is known as the South Pacific Convergence Zone (SPCZ) to differentiate it from where it forms over the Atlantic and Indian Oceans. The South Pacific Convergence Zone is the strongest of the ITCZ locations and the one most often crossed by sailors.

What to expect in the SPCZ
Conditions in the SPCZ are variable, but if you are going through it there are a few things you can come to expect.
Light wind: The wind will be very light, often in the range of 0 to 5 knots. It will generally come from the east, but depending on squall conditions it can be coming from any direction. Boats often conserve fuel so that they can motor quickly through the SPCZ, which in addition to the light wind also contains squalls with the chance of lightning. Boats departing from Mexico motor an average of 54 hours during a crossing, whereas boats departing from the Galapagos motor more than 80 hours.

A simplified diagram of how air warmed along the equator rises and produces a low pressure zone with variable winds.

Squalls: Expect to encounter frequent squalls, especially overnight and in the early morning when the air temperature cools. The cool temperatures cause the warm moist air to drop its moisture content and create localized convection. The wind speeds in the larger squalls are often in the 20- to 30-knot range and generally last 20 to 60 minutes. Squalls usually blow themselves out, but if they are large, they create a positive feedback loop and can last for a longer period of time. Riding the edge of a squall is sometimes a good way to sail through the otherwise light conditions, but generally it is safer to avoid them where possible. We used our radar to identify squalls at night and even to better understand their movement during the day. This helped us avoid the vast majority of squalls, especially larger ones.

Based on the last five years of data published by Pacific Puddle Jump, the average highest wind speed encountered on the passage is a little more than 32 knots and is almost always experienced in squalls. The highest reported wind speed in a squall from the last five years was 50 knots, though 82 percent of boats reported maximum encountered wind speeds of less than 40 knots. Maximum reported wind speeds are usually higher than experienced wind anyway, as the transducer is at the top of the mast and is subject to the sway of the boat in swells, which can add 5 knots of speed. The highest wind speed we saw on the crossing was a little more than 30 knots in a squall and it lasted for five minutes.

Thunder and lightning: Large cumulonimbus clouds can generate thunder and lightning. We saw quite a bit of sheet lightning (cloud to cloud) in the distance when we were close to the SPCZ, but nothing touching the water. It is uncommon for lightning to touch down on the open ocean and even more rare for a boat to get struck, but it can happen. It is mainly for this reason that the SPCZ is feared by sailors. Avoiding the larger convection cells will reduce the likelihood of encountering lightning.

How to cross the SPCZ
Timing: Certain times of year are better than others to cross the SPCZ. The tropical cyclone season in the North Pacific is generally accepted to extend from May to November. While crossing during cyclone season is possible, your insurance premiums will skyrocket — if you can manage to get insurance at all. Most sailors choose to cross during the Northern Hemisphere’s spring/summer, with the best time usually somewhere between mid-March and the end of April. According to Pacific Puddle Jump data, 67 percent of boats leave between March 15 and April 15, with another 20 percent leaving before the end of April.

Local conditions: Each departure port has its own specific weather conditions, and it is important to check these before planning a passage. In our case, we departed from Puerto Vallarta, Mexico. The Baja peninsula causes a lot of wind shadow, so it’s important to wait for a low pressure system above the peninsula to create enough north wind down the Sea of Cortez before sailing 300 miles off the coast until you hit the trade winds. Otherwise it can be 72 hours of motoring. San Diego, Panama and the Galapagos are also common hopping-off points that have their own conditions to take into account.

Screenshot of a grib file. The author found this type of weather data can be useful for predicting squalls in the ITCZ.

Weather forecasting: Once into the trade winds, the sailing is very consistent until you approach the SPCZ location. The location changes from day to day, so it only makes sense to really pay attention to it when you are getting close to typical latitudes. The SPCZ rarely goes above 10° N in the spring/summer months, so we began checking at 12° N. Pacific Puddle Jump data from the last five years indicates that boats leaving from Mexico cross the equator at an average of 126° W, whereas boats leaving from points farther north cross at an average of 133° W. Boats that leave from Panama and the Galapagos typically cross around 92° W and 87° W, respectively.

NOAA’s National Hurricane Center provides an Eastern Pacific Tropical Weather Discussion, which lists the location of the ITCZ and weather conditions around Mexico and South America. This was the most valuable source we had and the one we checked most often. We also looked at 12-, 24-, 48- and 72-hour surface analysis charts from NOAA, though we found anything beyond 24 hours to be highly changeable. These products are available via weatherfax from the Pt. Reyes and Honolulu stations or more commonly accessed through an email request to Saildocs.

We found grib files to be surprisingly accurate, especially when sourced from the GFS. The gribs we requested listed pressure, wind, precipitation and swell. While we had been warned not to rely on the gribs for accurate precipitation, we found that they were quite good at predicting when and where we would start seeing squalls up to two days out.

Many cruisers also bring a large-scale Admiralty chart of the Eastern Pacific on which to mark the location and movement of the ITCZ. Using colored pencils, we drew the axis of the ITCZ based on the NOAA tropical weather discussion every 12 hours. We could see the movement over time of the ITCZ, which we hoped would help predict its future location. The movement seemed almost random and convection could be encountered anywhere from 30 miles to 180 miles on either side of the axis. This method was interesting, but it didn’t prove very useful.

The ITCZ is not a mythical beast bent on harming transoceanic sailors. It is a complex weather system that, when properly understood and accounted for, does not need to be feared. Squalls are common and most passagemakers receive a boat wash or two. Picking the right time of year to go, checking weather along the way and altering course to accommodate the location of the ITCZ will help make a crossing less stressful and more comfortable.

Robin Urquhart and his partner Fiona are on a multi-year voyage aboard their Dufour 35, Monark. Urquhart has a Master’s in building science engineering. 

By Ocean Navigator