Translated from the original french - Credit,  Sail Tahiti - David Allouch and Nikki Puttergill sailtahiti.com

Where to put your boat safely during the Cyclone season - French Polynesia

One of the central questions for sailors crossing the South Pacific is "where to put your boat safely during the Cyclone season" which lasts from November to April.

 The publication in October 2015 of information from Météo France predicting a risk of a 90% cyclone over French Polynesia for the next few months had triggered, if not a real storm, at least a wind of panic among the population and passing sailors. Eventually, while Fiji and the Cooks were hit hard, the archipelagos of French Polynesia 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 that there was a 90% chance of a major storm over Western Europe during winter to come. Statistical statement that would not have surprised many people and whose limited scope is immediately visible. In fact, what interests each of us is the likelihood that there will be a major storm precisely where we live.

 With a statistic of around one natural disaster order per year for each municipality on the French mainland coast, the risk of storms seems even higher there than in Polynesia ... However, the violence and the size of the most important cyclonic phenomena must be taken into account to properly compare the risks.

Cyclone risk analysis in the South Pacific

Definition of the risk period

The cyclonic period is the hot season in the southern hemisphere, which lasts from October to April. However, the riskiest months are January to March with significantly lower probabilities of a major event at the start and end of the season. During the El Nino years, the period favorable to the formation of cyclones covers the whole of the hot season, with possible phenomena from October to April.

Within this period, therefore, regardless of the year, it is necessary to look at the precise conditions of cyclogenesis to measure the risk.

Conditions for the appearance of cyclones

In the southern hemisphere, cyclones originate in two distinct areas:

The ITCZ, Zone de Convergence Intertropicale or "Doldrums", located slightly north of the equator

The SPCZ, South Pacific Convergence Zone that stretches across Papua to southwestern Polynesia.

For a cyclone to appear it is imperative:

A highly unstable and humid atmosphere, favorable to the development of convective activity (typically the conditions of the ITCZ ​​and the ZCPS)

A warm sea (more than 26.5 ° C) at least 50m deep initial depression

During the El Nino years, warm surface and subsurface water in the western Pacific moves eastward, increasing the area over which cyclones can occur. This results in a very strong increase in the number of phenomena in Polynesia, whereas in normal years Polynesia is hardly ever affected by these phenomena which occur significantly more in the West.

Finally, the risk of cyclones increases significantly when a Madden Julian wave passes, a large low pressure area that crosses the globe from west to east. Such a wave takes between one and two months to pass over Polynesia.

Thus, the cyclonic risk in French Polynesia is almost zero in normal years, greater during the El Nino years, and generally only materializes during the passage of a Madden Julian wave, which occurs once or twice during the hot season.

Affected area

In a normal year, cyclones hardly ever reach French Polynesia but remain confined to the western South Pacific and the Indian Ocean. North Australia is particularly exposed, South Australia and New Zealand are spared.

French Polynesia therefore appears to be a relatively safe place to leave your boat there in normal years.

During the El Nino years, 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.

The Marquesas: located around 139 ° West and 10 ° South, the Marquesas are to the south of the ZCIT and to the North East of the ZCPS. Consequently, the risk is generally considered to be zero. However, during the El Nino episode of 1982-83 tropical depressions originated just north of the Marquesas Islands, causing some damage to the archipelago.

The Tuamotus: located between the Society archipelago and the Marquesas archipelago, the Tuamotus are generally spared by cyclones but often subject to tropical depressions during the hot season. Given the lack of natural protection against wind and tides, extreme care must be taken in this area.

The Austral Islands: located in the south of Polynesia, the Austral Islands are found in the path of end-of-life cyclones and are regularly affected by storms.

Tahiti and Moorea: are located on the edge of the risk zone but benefit from very good shelter depending on the direction of the wind and the sea.

Bora Bora, Raiatea, Tahaa, Huahine: are considered a risk zone by insurers, in particular because of the trajectories that we will study more precisely below. However, Raiatea has relatively safe shelters and a small dry port which allows it to be safe as long as the phenomenon remains at the level of a tropical depression.

Source: The islands of French Polynesia facing the cyclonic hazard, Sébastien Larrue and Thomas Chiron

Trajectory analysis

The depressions giving rise to cyclones generally form in the ZCPS, in the north-west of Polynesia. They then follow a path towards the South-East, as shown by the trajectory of Cyclone OLI (2010).

In El Nino years these trajectories originate further east than in normal years and circulate between the Society Archipelago and the Cook Islands. Western Polynesia and in particular the Leeward Islands are therefore potentially threatened in the El Nino years.

In 1983, several depressions formed in the ITCZ ​​north of the Marquesas Islands, before moving west and then veering south-east. These phenomena therefore affected the whole of Polynesia, but it did not happen until 1983.

Unlike the rest of Polynesia, the Tuamotus offer no protection against severe weather phenomena. In the event of a cyclonic tide, the swell passes the coral reef and you therefore no longer find yourself in the middle of a lagoon but in the open sea, with reefs all around you. This is clearly not a good solution! However, in Apataki there is a very serious refit yard which has a few places to leave your boat. This is then semi buried and firmly moored, which protects it suitably in the event of a strong depression.

In Tahiti and Moorea

During the trade winds season, strong coastal accelerations of the easterly wind can be observed along the north and south coasts of Tahiti and Moorea.

Conversely, as we saw in the study of trajectories, tropical depressions and cyclones generally circulate west of Tahiti, in the corridor between the Cook Islands and the Society archipelago. The winds are therefore first from the north-northwest and then shift to the south once the phenomenon has descended to the south of the archipelago. In this case, the north and south coasts offer very good shelter thanks to the buffering effect of the mountains.

Thus we find the following shelters:

North coast of Tahiti: downtown marina and tahiti yacht club in Arue

South coast of Tahiti: Taravao bay

North coast of Moorea: Opunohoo and Der Cook bays

The Taina marina, located on a superb spot on the west coast of Tahiti, is well protected from the trade winds but is dangerous in the event of a cyclone.

In the Leeward Islands

Like Tahiti and Moorea, the Leeward Islands are protected by both the coral reef and their relief. However, these islands are much more at risk than the rest of French Polynesia and their lower relief than that of Tahiti offers less protection. However, you can take shelter in the deep bays and Raiatea also offers 2 marinas and a fairing. Before leaving your boat there, you must confirm with your insurance that it will cover you, because most consider the Leeward Islands to be in a risk zone and therefore uninsured …

As we have seen previously the Marquesas Islands are outside the hurricane zone. However, the archipelago was affected by tropical depressions in 1983. Thanks to their deep relief, the Marquesas offer many protected anchorages, especially on the northern coasts (cliff effect that blocks the wind). The bottom falls very steeply and in the absence of a reef barrier the main risk is that of the swell and the poor behavior of its anchorage.

Recently a refit has been opened at Hiva Oa. The Marquesas Islands are therefore on the way to becoming not only an excellent shelter to stay in during the hot season, but also a place where you can take your boat out of the water to take care of it or put it away if you are goes home for a few months.

Conclusion

French Polynesia as a whole is generally very little subject to cyclonic risk. This risk increases sharply during the El Nino years.

Inside Polynesia, the Marquesas are not at risk. The Tuamotus and the Austral Islands offer little protection and can be found in the path of depressions or even cyclones.

Tahiti and Moorea offer safe shelters and technical infrastructure.

The Leeward Islands are in a riskier area and natural protections and infrastructure are weaker than in Tahiti. There is, however, a refit yard, two marinas and a few good anchorages in the deep bays.

All of the international climate models surveyed by the Bureau indicate central tropical Pacific sea surface temperatures in the NINO3.4 region will cool further in the coming months. Three of the eight surveyed models exceed the La Niña threshold during September, with two more models surpassing the threshold in October. For November and December six out of eight models indicate La Niña is most likely.

Recent cooling of the surface of the tropical Pacific Ocean, changes in tropical weather patterns, and continued ocean cooling forecast by climate models suggest La Niña could become established in spring 2020.

The Bureau's ENSO Outlook remains at La Niña ALERT. This means the chance of La Niña forming in 2020 is around 70%—roughly three times the average likelihood.

While most key indicators remain within ENSO-neutral range, there have been further signs of La Niña development in the past fortnight. The central tropical Pacific Ocean has continued to cool and trade winds remain stronger than average, while the Southern Oscillation Index (SOI) has exceeded La Niña thresholds in recent days. Equatorial cloudiness near the Date Line also remains below average.

All of the surveyed international climate models surveyed anticipate further cooling of the tropical Pacific Ocean. Five of the eight models reach or exceed La Niña thresholds during October, with six models indicating that if La Niña forms it is likely to persist into December.