Cyclone season in French Polynesia: Where to keep your boat safe ?
For sailors crossing the South Pacific, one of the central questions is “where to keep your boat safe during the cyclone season”, which lasts from November to April.
Figure 1: Approximately one natural disaster decree per year for each commune on the French coast.
In October 2015, Météo France forecast a 90% cyclone risk over French Polynesia over the coming months, triggering, if not an actual storm, at least a wave of panic among the population and visiting sailors. In the end, while Fiji and the Cook Islands were hit hard, the French Polynesian archipelagos only experienced moderate depressions.
So what is the real risk in Polynesia?
With French Polynesia covering an area equal to that of Western Europe, Météo France’s forecast was equivalent to predicting a 90% chance of a major storm over Western Europe during the coming winter. A statistical assertion that wouldn’t have surprised many people, but whose limited impact is immediately obvious. In reality, what interests each and every one of us is the probability of a major storm happening precisely where we live.
With a statistic of around one ‘arrêté de catastrophe naturelle’ (natural disaster decree) per year for each commune on the French mainland coast, the risk of storms there seems even higher than in Polynesia… However, the violence and size of the most significant cyclonic phenomena must be taken into account for a proper comparison of risks.
Analysis of cyclone risk in the South Pacific
Definition of the cyclone risk period
The cyclonic period corresponds to the Southern Hemisphere’s warm season, which lasts from October to April. However, the riskiest months are January to March, with much lower probabilities of major events at the beginning and end of the season.
In El Nino years, the period conducive to cyclone formation covers the entire warm season, with phenomena possible from October through to April.
So, whatever the year, we need to look at the precise conditions of cyclogenesis to measure the risk.
Cyclone incubators
In the Southern Hemisphere, cyclones originate in two distinct zones:
- the ITCZ, the Intertropical Convergence Zone or “Doldrums”, located slightly north of the equator
- the SPCZ, South Pacific Convergence Zone, which extends at an angle from Papua to south-west Polynesia.
Figure 2. French Polynesia covers an area the size of Western Europe.
Figure 3: Cyclonic risk is high in the warm season and nil in the cool season.
For a cyclone to occur, it is imperative that :
- a highly unstable and humid atmosphere, favorable to the development of convective activity (typically ITCZ and SPCZ conditions)
- warm seas (over 26.5°C) at least 50m deep
- an initial depression
During El Nino years, warm surface and subsurface water in the western Pacific moves eastwards, increasing the surface area over which cyclones can occur. This results in a sharp increase in the number of events affecting Polynesia, whereas in normal years Polynesia is almost never affected by these phenomena, which occur much further west.
Finally, the risk of cyclones increases significantly with the passage of a Madden Julian wave, a vast low-pressure area that crosses the globe from west to east. Such a wave takes between one and two months to pass over Polynesia.
As a result, the cyclonic risk in French Polynesia is virtually nil in normal years, higher in El Nino years, and generally only materializes when a Madden Julian wave passes over, which happens once or twice during the warm season.
Figure 4: In a normal year, cyclones are confined to the western South Pacific and the Indian Ocean.
Zone concerned by cylones
In a normal year, cyclones almost never reach French Polynesia, but remain confined to the western South Pacific and the Indian Ocean. Northern Australia is particularly exposed, while southern Australia and New Zealand are spared.
This makes French Polynesia a relatively safe place to leave your boat in normal years.
During El Nino years, however, Polynesia is more at risk, and certain areas should be avoided, while others remain safe due to their position or the quality of their shelters.
Cyclone risk analysis in French Polynesia
Analysis by zone
Most insurance companies consider that the risk zone in the South Pacific begins at 150° West, which includes the Leeward Islands, but leaves Tahiti, the Tuamotus and the Marquesas as a non-risk zone. What’s the reality?
- The Marquesas: located at around 139° West and 10° South, the Marquesas are to the south of the ITCZ and to the north-east of the SPCZ. As a result, the risk is generally considered to be nil. However, during the El Nino episode of 1982-83, tropical depressions formed just north of the Marquesas, causing some rain damage to the archipelago.
- The Tuamotus: located between the Society and Marquesas archipelagos, the Tuamotus are generally spared from cyclones, but are often subject to tropical depressions during the warm season. Given the lack of natural protection against wind and tides, extreme caution is called for in this area.
- The Australs: located in the south of Polynesia, the Australs are in the path of cyclones at the end of their life cycle, and are regularly affected by storms.
- Tahiti and Moorea: are located at the edge of the risk zone, but benefit from good shelter depending on wind and sea direction.
- Bora Bora, Raiatea, Tahaa, Huahine: are considered at-risk zones by insurers, in particular because of their trajectories, which we will examine in greater detail below. However, Raiatea has relatively safe shelters and a small dry docks, so you’re safe as long as the phenomenon remains at tropical depression level.
Figure 5: Tropical storm and cyclone frequencies (by degree of latitude and longitude) observed between 1970 and 2009 in French Polynesia.
Source: Les îles de Polynésie française face à l’aléa cyclonique, Sébastien Larrue and Thomas Chiron
Figure 6. Lows generally form in the SPCZ, then follow a south-easterly path (cyclone OLI – 2010)
Cyclone Track Analysis
Cyclone-generating lows generally form in the SPCZ, northwest of Polynesia. They then follow a south-easterly path, as shown by the trajectory of cyclone OLI (2010).
In El Nino years, these paths originate further east than in normal years, and travel between the Society Archipelago and the Cook Islands. Western Polynesia, and the Leeward Islands in particular, are therefore potentially at risk in El Nino years.
In 1983, several lows were born in the ITCZ to the north of the Marquesas, before moving westwards and then veering south-eastwards. These phenomena affected the whole of Polynesia, but only in 1983.
Cyclone Shelters in French Polynesia
In the Tuamotus
Figure 7. Apataki Carrenage, a serious dry dock opein since 2010.
Unlike the rest of Polynesia, the Tuamotus offer no protection against severe weather. In the event of a cyclonic tide, the swell overtops the coral reef and you find yourself not in the middle of a lagoon but in the open sea, with reefs all around you. Clearly not a good solution! In Apataki, however, there’s a very serious dry dock and yard with a few places where you can leave your boat. The boat is then semi-buried and firmly moored, providing adequate protection in the event of a strong low-pressure system.
Tahiti and Moorea
During the trade wind season, there is a strong acceleration of easterly winds along the north and south coasts of Tahiti and Moorea.
Conversely, as we have seen in the study of trajectories, tropical depressions and cyclones generally circulate to the west of Tahiti, in the corridor between the Cook Islands and the Society archipelago. Winds are initially north-north-westerly, then veer south once the phenomenon has descended south of the archipelago. In this case, the north and south coasts offer excellent shelter thanks to the buffering effect of the mountains.
The following shelters can be found:
- Tahiti north coast: downtown marina and Tahiti yacht club in Arue
- Tahiti south coast: Taravao bay
- North coast of Moorea: Opunohoo and der Cook bays.
The Taina marina, located on a superb spot on Tahiti’s west coast, is well protected from the trade winds, but can be dangerous in the event of a cyclone.
Leeward Islands
Like Tahiti and Moorea, the Leeward Islands are protected both by the coral reef and by their relief. However, these islands are much more at risk than the rest of French Polynesia, and their lower relief than Tahiti’s offers less protection. Raiatea also offers 2 marinas and a dry dock. Before leaving your boat there, check with your insurance company that it will cover you, as most consider the Leeward Islands to be a high-risk zone and therefore uninsured…
Marquesas Islands
Figure 8. Marquise Service Carrenage in Hiva Oa – exit from a Catana 47
As we saw earlier, the Marquesas are outside the cyclone zone. However, the archipelago was hit by tropical depressions in 1983. Thanks to their very steep relief, the Marquesas offer a large number of protected anchorages, particularly on the north coast (cliff effect blocking the wind). As the seabed is very steep, and there is no reef barrier, the main risk is swell and poor anchoring.
A dry dock and yard has recently been opened at Hiva Oa. The Marquesas Islands are thus becoming not only an excellent place to stay during the warm season, but also a place where you can take your boat out of the water for care or storage if you return home for a few months.
Figure 9. During the trade wind season, strong coastal acceleration of the easterly wind can be seen along the north and south coasts of Tahiti and Moorea.
Conclusion
French Polynesia as a whole is generally not subject to cyclonic risk. This risk increases sharply in El Nino years.
Within Polynesia, the Marquesas Islands are not at risk. The Tuamotus and Australs offer little protection and can be in the path of depressions or even cyclones.
Tahiti and Moorea offer shelters and technical infrastructure.
The Leeward Islands are in a riskier zone, with less natural protection and infrastructure than Tahiti. Nevertheless, there is a dry dock, two marinas and some good anchorages in the deep bays.
