ITCZ

Navigating the Intertropical Convergence Zone (ITCZ): A Captain’s Comprehensive Guide

Introduction

As captains, we often find ourselves navigating through some of the most challenging and unpredictable waters on Earth. Among these, the Intertropical Convergence Zone (ITCZ) stands out as one of the most formidable and intriguing. This vast and ever-changing band of weather, located near the equator, where the northeast and southeast trade winds converge, presents unique challenges that test even the most seasoned mariners.

In this guide, we will explore the ITCZ in depth, drawing on both scientific understanding and practical experience. We will delve into the meteorological forces at play, examine historical accounts, discuss modern navigation strategies, and provide detailed advice on how to safely and efficiently traverse this complex region. Whether you're planning a passage from the Americas to the South Pacific or navigating the Atlantic, understanding the ITCZ is essential for a successful voyage.

Understanding the ITCZ: The Meteorological Perspective

The ITCZ is a critical component of the global climate system, driven by the convergence of the trade winds from the northern and southern hemispheres. This convergence causes air to rise, leading to the formation of extensive cloud systems, thunderstorms, and, often, torrential rainfall. The ITCZ is not a single, stationary line but rather a dynamic and fluctuating zone that can vary in width from a few miles to several hundred miles, depending on the time of year and local conditions.

At its core, the ITCZ is the result of differential heating of the Earth's surface. The equatorial region receives more direct sunlight, causing the air to warm and rise. As this air ascends, it cools and condenses, forming clouds and precipitation. The rising air creates a low-pressure zone that draws in the surrounding trade winds, which, upon meeting, further fuel the upward motion of air and the development of weather systems.

One of the most challenging aspects of the ITCZ is its unpredictability. The zone's location shifts seasonally, following the sun as it moves between the Tropic of Cancer and the Tropic of Capricorn. This seasonal movement, combined with the influence of ocean currents, local weather patterns, and larger atmospheric phenomena such as the El Niño-Southern Oscillation (ENSO), makes the ITCZ a constantly moving target for navigators.

Historical Encounters with the ITCZ: Lessons from the Past

Throughout history, mariners have had to contend with the challenges posed by the ITCZ. From the early explorers who first ventured into these tropical waters to modern-day sailors crossing vast oceanic distances, the ITCZ has been a region of both opportunity and peril.

Consider the experiences of the Spanish and Portuguese navigators during the Age of Discovery. Their voyages into the uncharted waters of the Atlantic and Pacific brought them into direct contact with the ITCZ, often leading to weeks of being becalmed or struggling through violent squalls. These early sailors quickly learned to respect the ITCZ, recognizing its potential to halt their progress or, worse, threaten their lives.

More recently, accounts from solo sailors and participants in transoceanic yacht races have highlighted the challenges of the ITCZ. For instance, in the 1968 Golden Globe Race, competitors faced the daunting task of navigating through the ITCZ while conserving limited resources and maintaining their mental and physical well-being. The lessons learned from these and other encounters emphasize the importance of preparation, patience, and adaptability when facing the ITCZ.

The Science of the ITCZ: Deep Dive into Meteorological Dynamics

To fully appreciate the complexities of the ITCZ, it's essential to understand the underlying meteorological dynamics that drive its behavior. The ITCZ is closely linked to the Hadley Cell, one of the Earth's primary atmospheric circulation patterns. The Hadley Cell is characterized by rising air near the equator, which then moves poleward at high altitudes before descending in the subtropics and returning to the equator as the trade winds.

Within the ITCZ, the rising air undergoes significant cooling as it ascends, leading to the condensation of moisture and the formation of large cumulonimbus clouds. These clouds can reach altitudes of up to 60,000 feet, producing intense thunderstorms, heavy rainfall, and sometimes even tropical cyclones. The vertical motion of air within the ITCZ also leads to the development of a distinct weather pattern known as the "tropical trough," characterized by a belt of low pressure that can spawn squalls and other hazardous conditions.

One of the key challenges in navigating the ITCZ is the unpredictability of these weather patterns. The convergence of the trade winds can create areas of relative calm, known as the doldrums, where winds are light and variable. These doldrums can trap vessels for days or even weeks, making progress difficult and testing the patience and endurance of the crew.

In contrast, the ITCZ is also known for its sudden and violent squalls, which can bring winds of 30 knots or more, along with heavy rain and lightning. These squalls can develop rapidly, often with little warning, and can pose a significant threat to the safety of the vessel and crew. Understanding the signs of an approaching squall, such as the buildup of towering clouds and the sudden drop in pressure, is crucial for any captain navigating the ITCZ.

Modern Navigation Strategies: Leveraging Technology and Experience

In today's world, captains have access to a wealth of information and technology that can help them navigate the ITCZ more effectively. Satellite imagery, weather forecasts, and real-time data from weather buoys and ships allow for a more accurate assessment of current conditions and the likely behavior of the ITCZ in the coming days.

One of the most valuable tools for navigating the ITCZ is the use of satellite-based weather forecasting systems. These systems provide detailed information on the location and movement of the ITCZ, as well as the development of weather patterns within the zone. By analyzing this data, captains can make informed decisions about the best course to take, avoiding the most severe weather and minimizing the time spent in the ITCZ.

In addition to satellite data, captains can also benefit from the experience and knowledge of other mariners who have recently navigated the ITCZ. Reports from other vessels, shared through maritime communication networks or online forums, can provide valuable insights into current conditions and potential hazards.

Despite the advances in technology, navigating the ITCZ remains a challenge that requires skill, experience, and a deep understanding of the ocean and atmosphere. Captains must be prepared to adjust their plans as conditions change, using their judgment and intuition to make the best decisions for the safety and success of the voyage.

Practical Advice for Navigating the ITCZ

When planning a passage through the ITCZ, there are several key factors to consider. First and foremost, timing is crucial. The width and intensity of the ITCZ can vary significantly depending on the time of year and the specific location. Generally, the ITCZ is narrower and less active during the winter and early spring, making these the preferred times for crossing. However, local conditions and long-term weather patterns, such as El Niño or La Niña, can also influence the behavior of the ITCZ, so it's important to monitor the latest forecasts and adjust your plans accordingly.

Another important consideration is the choice of route. The ITCZ can be more active in certain regions, such as the Atlantic and eastern Pacific, while being less intense in others. For example, the ITCZ in the central Pacific tends to be narrower and more predictable, making it a popular choice for transoceanic passages. However, even within a specific region, the ITCZ can vary in intensity and location, so it's essential to gather as much information as possible before setting out.

One strategy for minimizing the time spent in the ITCZ is to stay north or south of the zone for as long as possible, taking advantage of the more stable trade winds before making the crossing. This approach can reduce the risk of encountering severe weather and increase the likelihood of a smoother passage. However, it's important to avoid getting too close to the equator, where the trade winds can weaken, and the doldrums can become more persistent.

When crossing the ITCZ, it's important to be prepared for a wide range of conditions. This means ensuring that your vessel is in top condition, with all systems and equipment fully operational. It's also essential to have a well-trained crew who are familiar with the challenges of navigating in tropical waters. In particular, be prepared for the possibility of squalls and thunderstorms, which can develop rapidly and with little warning. Having a plan in place for dealing with these situations, including reefing sails, securing loose gear, and maintaining a proper lookout, can make all the difference in ensuring a safe passage.

Finally, it's important to remain flexible and patient when navigating the ITCZ. The weather in this region can change rapidly, and it's not uncommon to experience periods of calm followed by intense storms. By staying alert and being ready to adjust your plans as conditions change, you can increase your chances of a successful crossing.

Case Studies: Navigating the ITCZ in the Pacific and Atlantic

To illustrate the challenges and strategies involved in navigating the ITCZ, let's examine two case studies: one in the Pacific and one in the Atlantic.

Case Study 1: Pacific Crossing from Mexico to the Marquesas

One of the most popular routes for long-distance cruisers is the passage from Mexico to the Marquesas Islands in French Polynesia. This route typically involves crossing the ITCZ somewhere between 05N and 10N, depending on the time of year and the specific conditions.

For many captains, the key to a successful Pacific crossing is careful planning and timing. Most cruisers aim to depart from Mexico in late March or early April when the ITCZ is typically narrower and less active. This timing allows them to take advantage of the strong northeast trade winds while minimizing the time spent in the ITCZ.

Once underway, many captains choose to stay north of the ITCZ for as long as possible, often following a course that takes them west to around 125W before heading south. This strategy allows them to avoid the most active part of the ITCZ, which tends to be located closer to the equator. However, once they reach around 132W, it's usually time to head south and make the crossing.

During the crossing, captains must be prepared for a wide range of conditions, from calm seas and light winds to squalls and thunderstorms. Keeping a close eye on the weather and being ready to adjust the sails and course as needed is essential for a safe and successful passage.

Case Study 2: Atlantic Crossing from Africa to the Caribbean

The Atlantic ITCZ, located between Africa and the Caribbean, is another region where careful planning and navigation are crucial. Many sailors crossing the Atlantic from the Canary Islands or Cape Verde to the Caribbean must contend with the ITCZ, particularly during the winter months when the zone is closer to the equator.

In this region, the ITCZ is often more active, with a higher likelihood of encountering squalls and thunderstorms. To minimize the time spent in the ITCZ, many captains choose to follow a more southerly route, staying below 10N for as long as possible before making the crossing. This approach allows them to take advantage of the stronger trade winds and avoid the most intense weather.

However, even with careful planning, the Atlantic ITCZ can be unpredictable, and captains must be prepared for a range of conditions. Regular communication with other vessels, monitoring of weather reports, and the use of satellite data can help in making informed decisions and ensuring a safe passage.

Dealing with Squalls and Thunderstorms: Best Practices

One of the most challenging aspects of navigating the ITCZ is dealing with squalls and thunderstorms. These intense and often short-lived weather events can pose a significant threat to the safety of the vessel and crew. Understanding how to recognize, prepare for, and respond to squalls is essential for any captain crossing the ITCZ.

Recognizing Squalls

Squalls are typically associated with the towering cumulonimbus clouds that form in the ITCZ. These clouds can be identified by their distinctive shape, with a flat base and a billowing top that often extends well into the upper atmosphere. As the squall approaches, you may notice a sudden drop in pressure, a darkening of the sky, and an increase in wind speed.

In some cases, squalls may be accompanied by thunder and lightning, which can add to the danger. It's important to keep a close eye on the sky and be alert for any signs of an approaching squall, particularly during the afternoon and evening when these events are most common.

Preparing for a Squall

When you identify an approaching squall, it's important to take immediate action to prepare the vessel and crew. This may include reducing sail, securing loose gear, and ensuring that all crew members are aware of the situation and ready to respond. Reefing the sails early is often the best course of action, as it allows you to maintain control of the vessel without being overpowered by the wind.

It's also important to secure any loose items on deck and below, as the sudden increase in wind and the rough seas that often accompany squalls can cause significant damage. Make sure that all hatches and ports are securely closed to prevent water from entering the vessel.

Navigating Through a Squall

As the squall hits, you'll need to maintain control of the vessel while minimizing the impact of the wind and waves. In many cases, the best approach is to head up into the wind, reducing your speed and allowing the squall to pass over you. This can help to minimize the strain on the sails and rigging and reduce the risk of damage.

If the squall is particularly intense, you may need to heave to, allowing the vessel to ride out the storm with minimal sail. This can be an effective way to maintain control and stability while waiting for the squall to pass.

Responding to Thunderstorms

Thunderstorms in the ITCZ can be particularly dangerous, as they are often accompanied by lightning, heavy rain, and strong winds. If you find yourself in the path of a thunderstorm, it's important to take immediate action to protect the vessel and crew.

First, reduce sail and secure the vessel as you would for a squall. Lightning poses a significant risk, particularly to vessels with tall masts, so it's important to minimize the risk of a direct strike. Ensure that all crew members are below deck, and avoid touching metal objects or electrical equipment.

If lightning strikes the vessel, it can cause significant damage to the electrical systems, so it's important to be prepared for the possibility of losing power or navigation equipment. Having a backup navigation plan, such as paper charts and a handheld GPS, can be invaluable in this situation.

The Psychological Impact of Navigating the ITCZ

Navigating the ITCZ can be a physically and mentally demanding experience for both the captain and crew. The uncertainty of the weather, the potential for long periods of calm, and the sudden onset of violent squalls can take a toll on morale and well-being.

As a captain, it's important to be aware of the psychological impact that the ITCZ can have on the crew and to take steps to maintain morale and mental health. This may include ensuring that the crew gets enough rest, providing regular updates on the weather and progress, and encouraging open communication.

It's also important to be patient and adaptable, recognizing that the ITCZ can be unpredictable and that plans may need to change as conditions evolve. By maintaining a positive and proactive attitude, you can help to keep the crew focused and motivated, even in the face of challenging conditions.

The Optimum Intermediate Waypoint
Obviously, based on the above illustrations, there is no precise optimum intermediate waypoint. All one can say is that at this time of year it is most likely to be somewhere between 03N and 07N at about 130W. Generally one needs to remain north of the ITCZ for as long as possible. This means one makes most of one’s westing in the northeast trades. Another reason to elect this strategy is that at this time of year the northeast trades tend to be stronger than the southeast
trades. A third reason is that one is more likely to find spaces between the convection cells the further one is to the west. However, it usually is not worth sailing further than about 132W before diving south; the extra distance isn’t worth it. There is a fourth reason that has to do with the doldrums. During the recommended season, there is a good probability that the further west one goes, the more likely one will find a sharp transition between the northeast and southeast trades. Cutting the corner before the recommended waypoint, i.e. diving south, increases the probability of encountering large areas of calm. If you leave late in the season, say in June, this becomes even more probable. The reason is that the low-pressure systems forming a few hundred miles off Southern Mexico during hurricane season have the effect of markedly reducing the strength of the winds in the far southeastern corner of the tradewind belt.

A fifth reason for making most of your westing in the northeast trades as well as not leaving it too late to depart, is that as hurricane season approaches, the southeast trades in the far Eastern Pacific steadily veer, becoming southwest winds. Not only would these winds be head winds, the severity of the ITCZ weather increases as the winds shift from a convergence situation to one of shear. On reaching the ITCZ most vessels power or motor sail due south so as to cut the ITCZ at right angles in order to transit in the shortest possible time. Once one is finally in steady southeast trades one sets course directly for the Marquesas. Sometimes the southeast trades are actually east winds.

Sailing this dogleg course is about 100 miles longer than the direct great circle or rhumb line route. That’s less than three percent more than the shortest route.
While this strategy minimizes the amount of time one may be exposed to the ITCZ, one should still expect squalls at any time along the route, especially after clearing the Revillagigedos Archipelago. After all, this entire 3000-mile passage is made in the tropics.

 

 

Navigating the ITCZ When Sailing from Panama to the Marquesas via the Galapagos

The journey from Panama to the Marquesas, with a stopover in the Galapagos Islands, is one of the most iconic passages for long-distance cruisers. This route takes you through some of the most challenging waters, where understanding the ITCZ and the broader meteorological patterns is essential for a successful and safe passage. The following considerations are crucial when planning and executing this voyage.

1. Timing the Departure from Panama

The timing of your departure from Panama is critical in determining how favorable the conditions will be as you approach and navigate through the ITCZ. Most captains aim to leave Panama between late February and early April, when the ITCZ is generally narrower and less active. This period also coincides with the transition from the dry season to the wet season in the eastern Pacific, which can help in avoiding the worst of the weather conditions.

Departing during this window also allows you to avoid the hurricane season in the eastern Pacific, which officially runs from May to November, with a peak in late August and September. By setting sail earlier in the year, you minimize the risk of encountering cyclonic activity, which can complicate the crossing of the ITCZ.

2. Navigating the Gulf of Panama

Before you even reach the ITCZ, you must contend with the conditions in the Gulf of Panama. This area is known for its variable winds, with the potential for both calm conditions and strong winds depending on the time of year and local weather patterns. The prevailing winds here are generally light and variable, especially during the dry season (December to April), but can pick up to 15-20 knots when influenced by the Papagayo winds from Central America.

The Gulf of Panama is also home to strong currents, particularly the Humboldt Current, which flows from south to north along the west coast of South America. Understanding these currents is essential for planning your route out of the gulf and towards the Galapagos Islands.

3. The Approach to the Galapagos Islands

Once you clear the Gulf of Panama, your next major milestone is the Galapagos Islands, located around 900 nautical miles to the southwest. The passage from Panama to the Galapagos typically involves crossing the equator and navigating through regions of light and variable winds, especially as you near the equator.
Look for elebated northerlies to blow you south West - they are strongest between Mid December - early April

The equatorial countercurrent, which flows eastward near the equator, can also impact your progress, requiring careful course adjustments. It's important to monitor the current patterns closely and adjust your route to take advantage of favorable currents while avoiding any areas of significant adverse flow.

The approach to the Galapagos can be complicated by the convergence of several currents, including the Humboldt Current, the South Equatorial Current, and the Cromwell Current (an upwelling current that brings cold, nutrient-rich water to the surface). These currents, combined with the frequent occurrence of light winds, can make for a slow and sometimes frustrating approach to the islands.

4. The Stopover in the Galapagos Islands

A stopover in the Galapagos is not only a unique opportunity to explore one of the most ecologically rich and biologically diverse regions in the world but also a chance to rest and prepare for the long passage ahead to the Marquesas. During your stay, it’s essential to check the latest weather updates and forecasts for the ITCZ, as its position and intensity can change during the time you are in the islands.

Ensure that your vessel is well-prepared for the next leg of the journey, including performing any necessary maintenance, provisioning for the long passage ahead, and ensuring that all navigation and communication systems are fully operational.

The Galapagos Islands also have strict regulations regarding biosecurity and the protection of the local environment, so it’s important to comply with all entry and exit requirements, including obtaining the necessary permits and following guidelines for waste disposal and anchoring.

5. Crossing the ITCZ from the Galapagos to the Marquesas

After departing from the Galapagos, the next significant challenge is crossing the ITCZ en route to the Marquesas. This leg of the journey covers approximately 3,000 nautical miles, and the ITCZ typically lies between 02°N and 07°N in this region, depending on the time of year.

Position and Width of the ITCZ:

• The position of the ITCZ in this area can vary significantly, sometimes shifting north or south depending on the broader atmospheric conditions, such as the El Niño-Southern Oscillation (ENSO). During an El Niño event, the ITCZ may shift further south, bringing wetter conditions to the region. Conversely, during La Niña, the ITCZ may shift north, which can lead to drier and more stable conditions in the equatorial Pacific.

Tactics for Minimizing Time in the ITCZ:

• To minimize your time in the ITCZ, it’s advisable to sail west as far as possible before turning south to cross the ITCZ. A common strategy is to stay near or slightly north of the equator, taking advantage of any favorable currents and winds, before making a southwesterly turn to cross the ITCZ at its narrowest point.

Navigating Squalls and Doldrums:

• The ITCZ is notorious for its unpredictable weather, including sudden squalls, heavy rains, and long periods of calm, known as the doldrums. When preparing to cross the ITCZ, it’s crucial to be ready for both extremes. Ensure that your crew is prepared for rapid changes in conditions, including reefing sails quickly during a squall and maintaining morale during extended periods of calm.
• Regularly monitoring the weather, using satellite imagery and onboard radar, can help in identifying areas of convective activity and planning your route to avoid the worst of the weather. Staying north of the ITCZ for as long as possible is often recommended, as this allows you to take advantage of the more consistent northeast trade winds before making the crossing.

Crossing Strategies:

• Once you decide to cross the ITCZ, maintaining a flexible approach is key. Be prepared to adjust your course to avoid the most active areas of convection, and consider motoring through periods of calm if you have sufficient fuel reserves. Many captains find that crossing at a slight angle to the ITCZ, rather than perpendicular to it, can help in reducing the time spent in the zone and avoiding the most intense weather.

6. Reaching the Marquesas

As you emerge from the ITCZ and enter the southern hemisphere, you’ll encounter the southeast trade winds, which will carry you toward the Marquesas. These winds are generally more stable and reliable than those in the northern hemisphere, making for a relatively straightforward passage to your destination.

The approach to the Marquesas is often a welcome relief after the challenges of the ITCZ. The islands themselves are a dramatic sight, with towering volcanic peaks rising out of the ocean, and they offer a safe and sheltered anchorage after your long passage.

Upon arrival, it’s important to remember that the Marquesas are part of French Polynesia, and you’ll need to comply with the local customs and immigration requirements. This may include obtaining a visa and completing entry formalities, so be sure to have all necessary documentation ready.

In addition to the formalities, take the time to enjoy the stunning natural beauty of the Marquesas, from the lush green valleys to the rich cultural heritage of the local communities. The islands are a true reward after the long and challenging passage from Panama, and they offer a chance to rest, resupply, and prepare for the next leg of your journey.

Conclusion: Navigating the ITCZ from Panama to the Marquesas via the Galapagos

The passage from Panama to the Marquesas, with a stopover in the Galapagos, is one of the most rewarding and challenging routes in long-distance sailing. By carefully planning your departure, understanding the unique challenges of each leg of the journey, and being prepared for the unpredictable nature of the ITCZ, you can navigate this iconic route with confidence.

As you sail through the ITCZ, remember that patience, preparation, and adaptability are your greatest allies. By staying informed, monitoring the weather closely, and being ready to adjust your plans as conditions change, you can ensure a safe and successful crossing, leading to the unforgettable experience of making landfall in the remote and beautiful Marquesas Islands.

Dr. Donald J. Anderson;   Feb '07

The Intertropical Convergence Zone, usually referred to as the ITCZ, is a band of disturbed unsettled weather surrounding the earth a few degrees north of the equator where the northeast and southeast tradewinds converge and where the sea surface temperatures reach their maximum values. It is characterized by cells of moderate to strong convection interspersed with areas of flat calm (the doldrums). The convection cells appear as tall cumulonimbus clouds reaching heights well above 20,000 feet. At the surface beneath these cells there is frequent torrential rain with thunder and lightning.

Conditions are often squally with winds from any direction. The squalls rarely exceed 35 knots.
Unlike the bad weather associated with frontal systems at higher latitudes where they travel from west to east at about 20 knots or more, those in the ITCZ seem to form and dissipate in place, in other words they are stationary systems. Furthermore they can form and disappear over very short periods of time, sometimes less than 24 hours. Seas usually are moderate to small but confused since they can come from any direction or several directions at once. The ITCZ migrates north and south following the sun, lagging it by about two months. The north-south width of the disturbed area varies from none at all to as much as 300
miles wide. The north-south seasonal boundary migration is usually between about 01N and 14N with the northern extreme occurring during August and the southern extreme during February.

During the winter and spring months, when the ITCZ is closest to the equator, cyclonic storms can not form in the ITCZ because there is negligible coriolis force close to the equator. However, small area low level cyclonic circulations (LLCC’s) up to about 120 miles in diameter with winds to 35 knots do occasionally form with very heavy rain and of course confused seas. These have the characteristics of miniature tropical disturbances. They travel east to west at 10 to 20 knots embedded in the southern boundary of the northeast trades. The strongest winds will be found in their northwest quadrant where they enhance the trades.
During the late summer months, when the ITCZ is at its maximum north migration, it experiences a larger coriolis force (actually a conservation of momentum effect rather than a force). This physical situation is an essential component of cyclogenesis leading to tropical storms and hurricanes. It is the principal reason why the highest frequency and the greatest intensities for hurricanes occur during the months of August and September.

Another tropical weather phenomenon affecting conditions along the ITCZ is the tropical wave. This is a whole subject unto itself so suffice it to say here that if one is prudent and makes passage from Mexico to the Marquesas between mid-March through the end of April, one does not have to worry about them. However, later in the season they could really spoil you day.

Should you be unfortunate enough to be making this passage late in the season then here is the stuff to watch for. Each year on average, we experience about 60 tropical waves moving east to west. They begin their journey along the coast of West Africa and march west at 10 to 15 knots eventually dissipating near the date line. They are north-south troughs lying between about 05N and 18N, about 800 to 1200 miles long. Width is about 300 miles at the surface. They occur mid-May through the end of November so that means a wave passes about every three to four days. Most pass relatively unnoticed to the cruising sailor. Others cause a significant clockwise wind shift as they pass and some have torrential rain on the back side. A small number, about 16 in all, interact with disturbed areas in the ITCZ such that they become part of a significant tropical cyclone development, i.e. cyclogenesis.

In the Northeast Pacific, all tropical storms and hurricanes have at least one tropical wave as a component of their cyclogenesis. Only about half of the tropical cyclones on the Atlantic side have such a component, why I don’t know. Frequently in the transition zone between the stable northeast and southeast trades there is an area of southerly winds of 5 to 20 knots over a north-south distance of up to 100 miles. Northerly winds in the transition zone are very rare. Sometimes there is sufficient separation between the convection cells that one might find a smooth transition between the northeast and southeast trades with the winds never dropping below 15 knots. If one is fortunate enough to find this condition while transiting the ITCZ, it is more likely to be blind luck than astute strategic planning. The convection cells simply form, dissipate and reform in a seemingly random fashion over periods of less than 48 hours which is far too brief a period for a slow moving vessel to effectively steer a course that will assure her of a smooth transit of the ITCZ. The location of the actual convection is rarely right over the convergence zone. Strictly speaking, the convergence is a zone of wind convergence and the convection cells usually lie on the north side of the convergence zone with their southern boundaries close to the convergence line.

 

This annual migration is illustrated by the climates of those islands lying in the tropics versus those situated close to the equator. Isla del Cocos (5-30N 87-00W) and the Palmyra Atoll (5-55N 162-05W) are both situated at about 06ºN. Although they are 4500 miles apart, they both are within the ITCZ for most of the year which is the reason they have average annual rainfalls of about 280 and 180 inches a year respectively. This accounts for the large tropical rain forests on both islands. Isla del Cocos is the more humid of the two because it is situated in the far Eastern Pacific where winds are mostly light while the Palmyra Atoll is in the Central Pacific where the trade winds are well established and consistent. The Galapagos Archipelago (00-30N 90-40W) straddles the equator and Christmas Island (01-55N 157-25W) is only 115 miles north of the equator. Although they are 4000 miles apart, their annual rainfall is similar; about 20 and 35 inches respectively. Annual rainfall in the Galapagos is fairly consistent whereas on Christmas Island it goes through wide swings between moderate rainfall to periods of drought lasting 18 months or more. Part of the reason is the tendency of the ITCZ to thin out as it progresses west toward the Central Pacific. During a strong El Niño episode, when sea surface temperatures can be two to four degrees Celsius above normal, the trade winds are weaker and the annual rainfall in the Galapagos and Easter Island can exceed five times the average amount. As already mentioned, the recommended period for passages from Mexican ports to the Marquesas is about the middle of March through April. During this period, the ITCZ usually lies between about 03N and 07N.

The north-south width of the convection cells tends to decrease towards the west, especially west of about 125W. Figures 1 and 2 are color-coded infrared satellite images. Note the thinning out of the ITCZ as one moves west. Of even greater significance is the change in the ITCZ over a very short period of time. These images were taken only thirteen days apart. With that in mind, note the significant shift towards the south and the essential disappearance of any convection west of 130W on March 9. This does not mean the ITCZ is moving south in a predictable manner. It simply illustrates dramatically the large variations that can take place over very short periods of time. The distribution of convection could easily return to something like what existed thirteen days earlier. Such is the meandering variability of the convergence zone. The location of the ITCZ is given every six hours in the NWS High Seas Forecasts. The format is a sequence of lat/lon coordinates with comments on where along the ITCZ there is moderate or strong convection. This is useful information but one should not expect it to be current. The reason is that the convection cells can develop as strong thunderstorms and then be completely dissipated over a period of less than 24 hours. Unlike thunderstorms at higher latitudes, where they move from west to east at about 20 knots, those in the ITCZ tend to remain stationary. On the other hand, the latitude or line of the ITCZ, wiggles or meanders like the Gulf Stream. For a given longitude, the north-south position of the ITCZ can vary as much as 120 miles in a 24-hour period. What all this means is that by the time a vessel receives the ITCZ data, conditions have probably changed significantly. A slow moving vessel can not expect to find a hole in the convection chain based on information received via INMARSAT-C or USCG HF-voice weather forecasts. One might be able to improve one’s chances if one has the capability to receive (real time) the low altitude polar orbiting NOAA weather satellite infrared encoded images directly as they pass over one’s location. Aboard “SUMMER PASSAGE” we use the OCENS software and receiver. Forecasting the day-to-day meanderings of the ITCZ is for me at any rate impossible. It’s rather like trying to predict which way the rabbit is going to go when I chase him across the meadow. I know he probably will stay somewhere ahead of me, but when and where will he zig or zag? The only charts I’m aware of in the public domain that come reasonably close to forecasting the ITCZ are the streamline charts generated by the USN’s NOGAPS model. But even then, one has to weight or bias them each day based on QuikSCAT scatterometer surface wind data. Computed wind vectors from scatterometer measurements are remarkably accurate. However, one has to be careful not to put too much faith in the data on the edges of the swaths or in the vicinity of very heavy precipitation. So how does a slow moving vessel make something useful out of ITCZ data? Well, if one plots the coordinates every 24 hours, beginning several days in advance, and if one uses the same piece of paper or electronic chart, then one will have a band that shows the most probable width and location of the ITCZ. To illustrate this exercise I have plotted the daily coordinates given for the ITCZ over an eight-day period. Figure 3 shows these plots. Note the wider swings at the eastern end versus the smaller swings at the western end. Also note that during this brief period of time, the north-south swings range from about 175 to 475 miles. This illustrates my point that a slow-moving vessel can not expect to exploit the ITCZ to advantage. The rate of changes in the ITCZ far exceed the rate of progress of a slow-moving vessel. Obviously, based on the above illustrations, there is no precise optimum intermediate waypoint. All one can say is that at this time of year it is most likely to be somewhere between 03N and 07N at about 130W. Generally one needs to remain north of the ITCZ for as long as possible. This means one makes most of one’s westing in the northeast trades. Another reason to elect this strategy is that at this time of year the northeast trades tend to be stronger than the southeast trades. A third reason is that one is more likely to find spaces between the convection cells the further one is to the west. However, it usually is not worth sailing further than about 132W before diving south; the extra distance isn’t worth it. There is a fourth reason that has to do with the doldrums. During the recommended season, there is a good probability that the further west one goes, the more likely one will find a sharp transition between the northeast and southeast trades. Cutting the corner before the recommended waypoint, i.e. diving south, increases the probability of encountering large areas of calm. If you leave late in the season, say in June, this becomes even more probable. The reason is that the low-pressure systems forming a few hundred miles off Southern Mexico during hurricane season have the effect of markedly reducing the strength of the winds in the far southeastern corner of the tradewind belt. A fifth reason for making most of your westing in the northeast trades as well as not leaving it too late to depart, is that as hurricane season approaches, the southeast trades in the far Eastern Pacific steadily veer, becoming southwest winds. Not only would these winds be head winds, the severity of the ITCZ weather increases as the winds shift from a convergence situation to one of shear. On reaching the ITCZ most vessels power or motor sail due south so as to cut the ITCZ at right angles in order to transit in the shortest possible time. Once one is finally in steady southeast trades one sets course directly for the Marquesas. Sometimes the southeast trades are actually east winds. Sailing this dogleg course is about 100 miles longer than the direct great circle or rhumb line route. That’s less than three percent more than the shortest route. While this strategy minimizes the amount of time one may be exposed to the ITCZ, one should still expect squalls at any time along the route, especially after clearing the Revillagigedos Archipelago. After all, this entire 3000-mile passage is made in the tropics.

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