Planning a tropical passage

Despite its importance, we like to think of route planning as a game. A chart of your voyaging ground is the playing field, while pilots, pilot charts, voyaging guides and other information determine what you’ll find there and how the weather will be on the way. The goal is for your boat to wend its way across the ocean, deftly avoiding foul weather and reefs, stopping for a time in beautiful anchorages, and once more setting sail. If the pilot charts say you’ll meet excessive headwinds, storms or big seas, a voyaging guide has some discouraging remarks about piracy, or a pilot says you’ve no hope of anchoring off that deserted island, then simply turn back the clock and revise your schedule or choose a different route.

Any clever tricks you can think of to beat the odds are fair game. You can log on the Net and have a look at the El Niño/southern oscillation (ENSO) predictions to get a handle on how trade-wind strength and the chance for tropical cyclones might vary from the norm. Or perhaps dig out a copy of an old pilot book to see if there are some remarks about small-boat anchorages that have been deleted from the recent editions. And it never hurts to have a chat with some sailors who’ve recently been where you’re going to get their take on the risks of sailing there. If you win, your real passage will be a good one, and you’ll have a great time; if you lose, you pay, be it in bad weather, islands, people that aren’t what you thought they’d be or a host of other problems or disappointments.

Here we’ll focus in detail on those aspects of route planning that have long been of central concern for sailors: weather, winds and currents. Essential information and resources that you should have include:

1. An understanding of the seasonal patterns characteristic of tropical weather and how to use trade winds, depressions and currents to your advantage. It’s also important to know where and how cyclones occur frequently and develop strategies for avoiding or weathering them.

2. Additional details regarding the winds, weather and currents in the region where you’ll be voyaging. Important reference works include pilot charts, pilots and voyaging guides. You should supplement such information (which presents average conditions) with specific updates of how conditions in the season or even month in which you’re sailing may differ.Seasonal patterns in the tropics

Winter is the prime time for sailing in the tropics. The tropical voyaging season stretches from mid-fall to mid-spring: generally December to May in the Northern Hemisphere, and May to November in the Southern Hemisphere. In most tropical areas, the predominantly easterly trade winds blow most consistently during the winter months. The winter season also brings cooler, drier weather and a much lower risk of encountering tropical cyclones, which occur primarily during the summer. Although it is possible to safely spend the summer months in the tropics, most voyagers choose to stay out of the area during this time. As a result, the timing of a tropical voyage — whether it is limited to a few island groups or takes you across an ocean — will be determined largely by the seasons.

There are many variations to thesey´broad patterns across the earth, and many options that can be exercised. Often it’s not until one is sailing a classic downwind route (but experiencing winds that don’t conform to expectations) that considerable variations in the trade winds become apparent. These variations can often be predicted to a certain degree; and by sailing when winds are more favorable, it’s possible to speed or ease one’s passage considerably. This tactic is also valuable for sailors searching for alternatives to the tested (but sometimes crowded) downwind routes.Variations in the trades

Few sailors care to make lengthy passages to windward against the trades, especially if these are strong, and it’s important to plan your route carefully to avoid a bash to windward if possible. This can be done in part by anticipating shifts in the trades that typically occur during the course of a season. For example, in the Caribbean’s Lesser Antilles, trade winds tend to be northeasterly in the winter months, turning southeasterly late in the season and during the summer. While such shifts are not large, they may be sufficient to mean the difference between a passage to windward and a close or beam reach.

In addition to seasonal shifts, trade winds in many areas (especially those influenced by non-stationary highs) will shift with the passing of each high-pressure cell. In the southwest Pacific, the trades are caused by a series of high-pressure cells that move eastward from the Australian continent into the Tasman Sea and then east past New Zealand. Winds in the Coral Sea — in the vicinity of New Caledonia, Vanuatu and the Solomon Islands — will be southerly when a high is to the west, still in the Australian Bight. These winds will shift to the southeast as the high moves eastward across the Tasman, becoming easterly, and at times even northeasterly, as the high moves past New Zealand.

Wind strength can be as important as direction. Sailors embarking on a downwind passage will welcome much stronger winds than they would if their course placed the wind on the beam or the nose. Trade winds are generally at their strongest during the winter months, but will invariably gain strength as high-pressure systems build and lose strength as the pressure decreases or a high moves away. As a result, although the consistency of the trades would seemingly suggest that a passage can be undertaken at almost any time during the favorable season, it is in fact important to time your departure so that the winds are more likely to be the strength you want for the passage you’re making.

Sailors moving from one hemisphere to another should be mindful of the need to avoid (or otherwise be prepared for) cyclone season in both hemispheres. For example, a sailor heading south from Hawaii will want to leave port and cross to the “safe zone” south of about 8° S before the onset of the North Pacific cyclone season (in June). This will also allow maximum voyaging time in the Southern Hemisphere before the onset of the cyclone season there (in late November). Voyagers who depart Hawaii late — in June or July — will still have a number of months in which to sail safely in the South Pacific, but they run an increasing risk of encountering hurricanes as they cross the zone between 18° N and 10° N, especially in an active cyclone year.Using lows

Although trade winds dominate the weather picture in the tropical winter, other winds also occur. Most are associated with disturbances, or lows, of various sorts. You can take advantage of these weather features to make passages that would be difficult in typical trade-wind conditions. Subtropical lows bring westerly quadrant winds in both the Northern and Southern hemispheres. These are a primary means by which to make a short passage against the trades. They are not helpful for long passages, as these systems are generally fast-moving and will quickly leave your boat behind. The strategy is simple: Wait for such a system to come through and then use those winds to sail east. The winds associated with such a low rarely exceed about 35 knots within the tropics and will more often be lower. Be sure to obtain a good forecast before committing to a passage with a low in the vicinity.

Tropical low-pressure zones (though not tropical cyclones) provide another alternative to trade-wind sailing. Although tropical lows generally move with the trade-wind flow, they will alter the wind patterns in their vicinity, at times enough to make an otherwise difficult passage possible. Be sure to do this only during the winter months, when the potential for cyclone development is at a minimum.Coping with currents

Ocean currents can serve to speed or slow a passage, and it makes sense to plan your route with currents in mind for that reason alone. Fortunately the currents along the major (westbound) trade-wind sailing routes are generally favorable. The boundaries of the major tropical currents (the North and South Equatorial Current and the Equatorial Counter Current, present in both the Atlantic and Pacific oceans) are not fixed. These currents shift north and south seasonally, and a small change in your route (north or south) may make a considerable difference in the current that you experience. Currents in areas subject to monsoon wind systems alter with the monsoons.

Currents can turn what would otherwise be an average passage into a potentially treacherous one if fast-flowing currents encounter strong winds blowing in opposition. Large, steep waves develop very quickly under such circumstances. This situation arises at times in many parts of the world; well-known examples of this phenomenon are associated with the Gulf Stream, which flows along the U.S. East Coast, and the Agulhas Current, flowing along the East Coast of South Africa. Although both of these currents lie outside the tropics, they routinely affect passages to and from the tropics.Moving beyond the averages

Classic information sources, such as pilot charts, will help flesh out the details of what you can expect in the way of winds in any tropical region and serve as the foundation of route planning. It’s possible (and advisable) to supplement these, however. Modern information-gathering tools (such as satellites) and almost instant communications make it possible to assess conditions as they develop. Of course the updates you get may vary dramatically in accuracy, and you should always be wary of assuming that a single incident signifies a trend. (That’s the beauty of the classic references; they’re compiled from data collected over many years, and so they give a reasonable picture of the probability of an event.) But some predictions, and especially those relating to climatic phenomena, are worth heeding.

Although long-range weather forecasting is still difficult, forecasters and climatologists are able to predict the likelihood of some conditions or features months in advance. So while they may not be able to tell you what the weather will be doing next week, they can provide a guide to an upcoming season in the way of relative trade-wind strength or cyclone probability. That’s in part because these are closely linked to the El Niño and La Niña phenomena, which affect weather across much of the globe.El Niño/southern oscillation

El Niño has long been used describe a warm current that replaces the normally cold waters off the west coast of South America. Written records relating to the periodic event go back almost 500 years, and there is geologic evidence of past El Niños going back thousands of years. Today we know that the El Niño current is just one part of a much larger weather cycle that’s linked to weather changes across the globe.

During “normal” years the trade winds in the Pacific blow with sufficient force to push the surface water layer westward, causing a buildup of warm water in the western Pacific around Indonesia (where sea level is typically some 18 inches higher than in the eastern Pacific). Cold, deep-ocean water wells up along the South American coast to replace the water that is pushed westward, bringing with it nutrients and thus a prolific fishery.

During El Niño years, the Pacific Ocean’s easterly trade winds are lighter than normal. There is insufficient wind to power the normal west-setting currents properly, which decline in strength, and water in the eastern Pacific becomes hotter than usual. This causes a reduction in the cold, upwelling currents and abnormally high rainfall along the west coast of the Americas. Countries in the western Pacific and Indian Ocean tend to experience droughts.

La Niña describes a pattern that is the reverse of El Niño, in which normal patterns are reinforced: trade winds are stronger than normal, moving warm water even farther west than normal. The result is drought and cool temperatures in much of the Americas and heavy rainfall in northern Australia and Southeast Asia.

The El Niño/La Niña phenomenon has been linked by meteorologists to an oscillation in the air-pressure relationship between the Pacific and Indian Oceans. This has been measured and termed the southern oscillation index (SOI) — a daunting term, but one that’s easy to interpret and serves as another clue regarding the status of El Niño and La Niña. During El Niño events, pressure is higher than normal in the Indian Ocean and lower than normal in the Pacific. This results in a negative value for the SOI. During La Niñas, the SOI is positive, because pressure in the Pacific is high, and that in the Indian Ocean low. You may see the phenomena referred to as ENSO, shorthand for the cyclic change in pressure, wind and weather patterns that these are associated with.ENSO predictions

Because ENSO cycles are long, lasting many months and sometimes years, you don’t need frequent updates. The simplest way to keep tabs on what’s happening is to log on the Internet every few months and check an ENSO forecast. A wealth of information is available, from simple predictions in plain English to highly technical reviews of the latest data. Sea-surface temperatures are also available, updated on a daily basis. These sources can be accessed from anywhere at no cost (other than any Internet service provider fees you may have). We provide a listing of Internet addresses below.

Both El Niño and La Niña seem to be occurring more frequently. The 20th century saw some 23 El Niños and 16 La Niñas. The four most powerful El Niños have occurred in the last 20 years. If this trend continues, then typical weather patterns will occur less often, and conditions will be more variable. This can be frustrating if you’re counting on having typical weather, but it can be a boon if you can take advantage of relatively unusual winds.

For example, El Niño years (when the SOI is negative) may allow one to make a passage against the prevailing trade winds in the South Pacific more easily, because trade winds are replaced by westerly winds more often than normal. Trade winds are also lighter than normal.

In the Caribbean, an El Niño is associated with a much lower incidence of hurricanes and may make a summer or early fall passage safer than normal. During La Niña (a positive SOI) the South Pacific convergence zone tends to move south and west, while the Intertropical convergence zone moves north. The doldrum belt moves west and north as well, and a strong South Pacific high results in stronger than normal trade winds.

Voyages in the Atlantic and Caribbean may be riskier, thanks to the higher incidence of cyclones.ENSO and tropical cyclones

One of ENSO’s most important impacts (from the perspective of tropical voyaging) is on cyclone frequency. Table 1 lists average cyclone seasons and frequency around the world. The neat, precise figures and seasons in this table may be statistically accurate, but the actual incidence often varies dramatically from one year to the next.

This fact has been borne out in part by cyclone tracks, which show considerable variation from year to year. It has also been borne out by research that compares cyclone frequency and tracks with ENSO conditions.Atlantic Ocean

North Atlantic: Contrary to what one might expect, the biggest impact on cyclone frequency is in the North Atlantic, where an El Niño reduces hurricane frequency by as much as 60 percent. The biggest drop is in low latitudes. Intensity is also reduced. During a La Niña, hurricane frequency and intensity both increase. The alteration in Atlantic hurricane frequency is due primarily to changes in upper-level westerly winds and wind sheer, not to temperature changes at the surface.

South Atlantic: Tropical cyclones are essentially unknown in the South Atlantic.Pacific Ocean

Eastern North Pacific: El Niño brings an increase in both hurricane frequency and intensity, while both decrease during La Niña.

Western North Pacific: This region has the greatest number of tropical cyclones worldwide. The impact of El Niño and La Niña are opposite in the eastern and western parts of the region. In the eastern part of the western North Pacific, the frequency and intensity of typhoons between 140° and 160° E (an area that includes the Caroline and Marianas Islands) increases markedly during an El Niño and decreases in a La Niña. In the western part (the South China Sea), typhoons are less likely during an El Niño and more likely during a La Niña.

South Pacific: The total number and intensity of tropical cyclones varies little overall during ENSO cycles, but the distribution of tropical cyclones does change. During El Niño events, there are more cyclones in the eastern Pacific than usual, impacting French Polynesia and the Cook Islands with greater-than-normal frequency.

El Niño means fewer cyclones in the Coral Sea and east coast of Australia. Following an El Niño, the South Pacific cyclone season may be delayed somewhat. A La Niña will generally result in fewer cyclones in the eastern South Pacific, and a greater number in the Coral Sea region. The impacts on South Pacific frequency appear to be caused by changes in surface temperature and pressure.Indian Ocean

The ENSO cycle does not seem to significantly alter the overall pattern of Indian Ocean tropical cyclones. All of the above points to the importance of voyage planning to avoid a slow passage or, even worse, getting clobbered by a powerful storm.

Mark Smaalders and Kim Des Rochers are sailors based in New Caledonia in the South Pacific who write regularly on voyaging topics.Their recent book Tropical Cruising Handbook, published by International Marine Publishing/McGraw Hill is available online at: tropicalcruising

By Ocean Navigator