Textbooks explain that the trade winds rush steadily eastwards around the globe, caused by the air-pressure difference between a high-pressure area (or ridge) in the subtropics and the low-pressure area along the equator. Story by Christian Feldbauer.

This weather model applies nicely for the North Atlantic, where the Azores High (or Bermuda-Azores High) behaves really well, ie; it is big and fairly permanent. Therefore, the trade winds in the Atlantic are generally stable and reliable. For the South Pacific, the model does not quite apply so nicely – the Pacific Ocean is simply too big.

Instead of a single ridge in the subtropics, there are (at least) two highs: in the east lies the South-Pacific High with its centre typically close to Easter Island and the other is the Kermadec High further west, closer to New Zealand.
Like the Azores High, the South-Pacific High is quite permanent. Consequently, steady trade winds prevail in the eastern South Pacific, which makes a passage from the Galapagos Islands to French Polynesia relatively easy. The Kermadec High, on the other hand, is neither permanent nor stationary. It arises from the Tasman Sea or somewhere north of New Zealand and travels eastwards via the Kermadec Islands, passes south of the Austral Islands of French Polynesia where it may fade away to be replaced by the next one. More persistent highs travel further southeast and join the eastward procession of lows in the roaring forties.
The transitory and wandering nature of the Kermadec High is the main reason for the frequently interrupted trade winds in the western and central tropical South Pacific. In other words, there is quite some weather going on.

The South Pacific Convergence Zone (SPCZ)
Two separate highs in the subtropics cause a different air pressure gradient in the tropics rather than a single ridge. Between the highs lies an area with lower pressure, and isobars encircle both centres. Where those circles come close to each other, air masses from different wind directions collide, causing a zone of convergence – the South Pacific Convergence Zone (SPCZ). This zone typically stretches from the Solomon Islands to Fiji and Tonga or Samoa, and often continues further to the Southern Cooks or the Austral Islands. As the Kermadec High moves eastward, the SPCZ – or a part of it – tags along with it and often there are several disconnected branches of convergence zones.
To the south (or west) of the SPCZ, dry, cool southerly or southeasterly winds are found; to the north (or east) the wind is from the northeast, north, or northwest, bringing warm, humid air. The consequence is cloud formation – squalls and thunderstorms as well as fronts develop. Due to lingering upward movement of air, air pressure falls and parts of the zone may evolve into a trough or low pressure system(s).

Around every 8-10 days a new transitory high starts its journey eastward, dragging the SPCZ and accompanying troughs along. An approaching trough is foreshadowed by the trade winds shifting first northeast and then north. When a trough passes, the wind turns further to the west and then south. The southerly wind is typically accompanied by a front with strong squalls. Deepened troughs and lows may also have a front on their northeastern side where the wind turns north.
When we compare weather charts from different met offices, we can see that even experienced forecasters seem divided over how to classify weather phenomena in the SPCZ. For instance, French Meteo may see a convergence zone with an associated front or just a stationary front whereas NOAA simply sees a (weak) trough.
A pattern we have observed over the last eight years is that strong and weak events usually alternate. Another observation is that grib files based on the GFS model have so far been very bad at showing those fronts.
Squash zones
When an eastwards-travelling high is very strong and/or moves very close to the tropics, the isobars on top of it are squeezed together and the resulting trade winds are strong. Such squash zones of enhanced trade winds occur frequently during the southern winter and are called mara’amu in Tahitian. The southeasterlies arrive almost always in conjunction with a nasty cold front after the convergence zone or trough has passed. The strong winds may last for several days and carry numerous squalls resulting from increased vaporisation and more cumulus cloud formation due to the agitated sea state.

Seasonal variations
During the cyclone season (southern summer) the subtropical highs are further away from the tropics than during the colder season. Therefore, trade winds are weaker, squash zones are rare and calm periods are more likely. The SPCZ moves more slowly or tends to become stationary.
For example, a branch of the SPCZ likes to linger over the Southern Cook and Austral Islands, bringing rainy weather to those areas and also to the Society Islands. This branch also causes plenty of warm and humid northeasterly and northerly winds over French Polynesia, particularly in December and January.

The SPCZ is the birthplace of most cyclones in the South Pacific. With the warm ocean surface in summer and heightened activity in the SPCZ, the risk of cyclone formation is great, particularly when an additional, extensive area of cloud formation and rainfall (like the 30- to 60-day re-occurring Madden–Julian oscillation) coincides with the SPCZ.
In the southern winter, the so-called sailing season, the trade wind belt paradoxically does not cover the entire tropics. As the highs are so close to the Tropic of Capricorn, the southern fringe lies already in the belt of variable winds, particularly in the central and eastern Pacific. In Tonga it is not unusual to get an extended period (5-6 days) of westerly winds around August.

Where is there less weather going on?
The eastern South Pacific has relatively undisturbed trade winds caused by the big and stable high around Easter Island. Also, a wide equatorial region has steady trade winds and fine (but hot) weather. When moving closer to the equator and further away from the subtropical highs, the isobars become more or less straight lines and resemble those caused by a single high-pressure ridge, and so the area does not see a convergence zone.
Also, the Intertropical Convergence Zone (ITCZ) is typically located well north of the equator at around 5-10°N. Apart from occasional warm fronts and squalls when the trade winds blow strong, the Marquesas Islands, Line Islands and Penrhyn in the Northern Cook Islands are blessed with stable weather all year round.
Further west or south, the SPCZ already starts to influence the weather. On the other hand, New Caledonia lies far enough west to also experience less convergence-zone typical weather.

In a nutshell…
Before setting sail, it pays off to take some time and do proper research, including pilot and climate charts – the Pacific is a huge and diverse area. Simplified text-book principles do not apply always or everywhere. For instance, it is not true that all archipelagos have their rainy season during the summer months.
Avoid extra tight itineraries as they leave no time to wait for favourable passage weather. Where possible avoid bold plans that include picking up and dropping off crew or visitors at places ambitiously far-apart.
It is not advisable to solely rely on grib files to find good passage weather – an additional look at surface-analysis weather charts shows where the convergence zone and nasty fronts are, particularly when sailing between French Polynesia and Fiji or places further west. The SPCZ that causes so much weather in the Pacific also has its good side: the numerous disturbances with their shifting winds provide good weather windows to sail eastward.
With some clever planning and plenty of flexibility, cruising the South Pacific is an undoubtedly an extraordinary experience.

Birgit and Christian have been cruising the Eastern South Pacific for eight years on their SY Pitufa. Check out their blog www.pitufa.at for cruising guides, advice about navigating and anchoring in coral-strewn areas. http://www.pitufa.at/