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Grapefruit 2018 728x90

Panache and the South Pacific Cyclone Season

by Price and Gail (Benny) Powell 25 Jan 06:49 UTC
South Pacific Cyclone Season © Price and Gail (Benny) Powell

Last year, when we were in the Caribbean, everyone wanted to know what we were planning to do about hurricanes. Hurricanes are more prevalent in some areas of the Caribbean than they are in others. Our plan was to go where hurricanes seldom travel. Hurricane season runs from June through October, with 90% of hurricanes occurring in the three-month period August to October. We spent the first part of the season in Grenada, which is rarely hit by hurricanes, and then moved westward for peak season to Bonaire, which is almost never affected.

This year we are in the South Pacific, where hurricanes are called "cyclones". Cyclone season runs from November to April, essentially the opposite of the Caribbean hurricane season. As in the Caribbean, cyclones are more prevalent in some areas than others. Generally speaking, there are many more cyclones in the Western South Pacific (Tonga, Samoa, Fiji, Vanuatu and New Caledonia) than in the Eastern South Pacific (Cook Islands and French Polynesia). For example, Fiji has experienced more than three times as many cyclones as Tahiti over the last five decades.

Cyclone History in Tahiti

Panache will be based in Tahiti, French Polynesia for the 2018/19 Cyclone Season. French Polynesia is on the eastern edge of the South Pacific Cyclone Zone. It covers a large area, roughly the size of Northern Europe (think Poland to Istanbul to Spain). The most north-easterly archipelago of French Polynesia is the Marquesas, which has never been hit by a cyclone. The most south-easterly archipelago is the Gambiers, also rarely hit by cyclones. As you move westward, the risk of a cyclones slowly increases.

The Australian Bureau of Meteorology has a fantastic website that shows the tracks of all the South Pacific Cyclones since 1969. The chart below shows the tracks of the cyclones that have been within 300km (162NM) of Tahiti over the last 48 years.

There are 20 cyclones shown on the chart ("All Cyclones"). That is an average of one significant weather event every 2.4 years. However, only one cyclone appears to have hit Papeete, Tahiti directly. That said, the others are scattered randomly on either side of the Island, so any one of them could have veered slightly one way or another along its 1,000km to 5,000km path, bringing Tahiti into its sights. Over a 1,000km path, a heading change of only 10 degrees could bring a cyclone that would have passed within 300km of Tahiti to within 100km. That's in the danger-zone for a Major Cyclone. So, while it's true that cyclones have rarely hit Tahiti, with a few small course changes, that reality could have been quite different.

It is interesting that New Zealand, where most South Pacific cruisers go to "escape" cyclone season, has had 18 cyclones within 300km over the same period essentially the same number as Tahiti.

Cyclones vs Hurricanes

The Australians use a different wind speed scale to describe cyclones than the Saffir-Simpson Scale used for hurricanes in the Atlantic/Caribbean. The Australian Tropical Cyclone Scale starts at 34kts and the Saffir-Simpson Hurricane Scale starts at 64kts. Cyclones of Category 1 and 2 on the Australian scale would be classified as a "Gale" and a "Storm," respectively, in the Atlantic/Caribbean. These are still strong winds, but they don't cause the same amount of damage that is associated with hurricanes.

If we remove cyclones of Categories 1 and 2 from the mix of 20 cyclones that have been within 300km of Tahiti, the total number drops to 10 ("Major Cyclones"). The four strongest had maximum wind speeds of 100kts, which is equivalent to Category 3 hurricanes in the Atlantic/Caribbean. Below is a chart of All Cyclones near Tahiti. Those that are inside the blue shaded box are Major Cyclones (i.e. have wind speeds of 64+kts). Note that the wind speed for each cyclone is the maximum that the cyclone reached anywhere along its path, not necessarily the wind speed it had when it was adjacent to Tahiti. This same caveat applies to the 18 cyclones near New Zealand (13 of which were Major Cyclones).

El Nino Years

In the South Pacific generally, cyclone frequency, intensity and distribution are influenced by the temperature of the ocean in the west relative to the east. Years in which the eastern ocean is hotter than normal are called, "El Niño," when it is colder they are called, "La Niña," and years when it is normal they are called, "Neutral." El Niño and La Niña occurrences are further classified as Weak, Moderate, Strong or Very Strong.

The chart below tracks these occurrences using the El Niño Southern Oscillation index ("ENSO"). Areas above the base line ("0.00" line) are El Niño years and areas below are La Niña.

As you can see, it fluctuates back and forth. In the last 68 years there have been 25 El Niño years, 21 Neutral years and 22 La Niña years, or roughly 1/3 each.

Impact of ENSO on Cyclones

Ten Major Cyclones over 48 years would be an average of about one every five years, if they were spread evenly across time. However, there is a significant correlation between cyclones and El Niño years. As you can see in the table below, 80% of All Cyclones and 70% of Major Cyclones have occurred in El Niño years. So, the ENSO state appears to be a key indicator of the occurrence of cyclones, although not a perfect one.

Distance from the Eye

Let's look at how your proximity to the center of a cyclone affects its impact on you (the Bureau of Meteorology of the Government of Australia has a very useful website on tropical cyclone intensity). The chart below shows that the peak wind speed of a "Typical Tropical Cyclone" is about 75kts. The wind speed drops in half if you are 75km from the center or the "eye" of the cyclone. www.bom.gov.au/cyclone/about/intensity.shtml

Extrapolating to the 100kts maximum wind speed exhibited by the four strongest Major Cyclones suggests they would have had to have passed within about 100km (54NM) for Tahiti to have experienced "hurricane force winds (64kts)". One Major Cyclone got that close: Cyclone Venna in 1983, a "Very Strong" El Niño year.

South Pacific Cyclone Forecast

The New Zealand Meteorological Service produces an annual outlook for cyclone activity in the South Pacific. The chart below shows their prediction for each region. The outlook for Tahiti (marked with a star) is for 0-1 cyclones in the 2018/19 season. The risk level is rated as "normal". While that looks like a pretty low risk, it only takes one cyclone to ruin your whole day!

Putting It All Together

The Risk Factors

  • There have been 20 cyclones within 300km (162NM) of Tahiti in the last 48 years. Any one of them could have hit Tahiti if it had veered somewhat left or right along its 1,000-5,000+km path.
  • 10 were Major Cyclones, equivalent in wind speed to named hurricanes in the Atlantic/Caribbean.
  • Even the lower level cyclones pack a punch.
The Mitigating Factors
  • 65%-75% of All Cyclones that came within 300km of Tahiti occurred in Moderate to Very Strong El Niño years.
  • This is currently a Neutral year on the ENSO scale, but is expected to turn to a Weak El Niño in early 2019. None of the Major Cyclones occurred in Weak El Niño years. That doesn't mean that it's not possible, as two Major Cyclones occurred in Neutral years (1981 and 1986).
  • A Major Cyclone, with wind speeds equal to the maximum that has been seen in the last 48 years, would have to pass within about 100km (54NM) of Tahiti to deliver "hurricane-force winds." That's pretty close. Only one of the Major Cyclones has ever passed that near to Tahiti.
  • The New Zealand Met Service has rated the risk level for the Society Islands (Tahiti is in the south-east of the Societies) to be "normal," with 0 to 1 cyclone expected.
The Bottom Line

Tahiti is clearly within the cyclone zone. That said, there have been relatively few Major Cyclones, and ¾ of those occurred when there were stronger El Niño conditions than is expected in the 2018/19 season. Only three Major Cyclones occurred outside those ENSO conditions (an average of 1 every 16 years).

Only one Major Cyclone came close enough to inflict hurricane force winds on Tahiti. While this is true (and certainly reassuring), it seems to understate the risk, as other Major Cyclones could have caused problems if circumstances were slightly different.

While we feel the likelihood of a Major Cyclone hitting us is low, Panache is fully insured for cyclone damage and is securely moored at Marina Taina in Tahiti for the season. Better safe than sorry!

The Backstory

We wanted to spend a year in French Polynesia as it is one of the world's best cruising areas. To do so meant having to spend the five months of Cyclone Season here. As mentioned, the Marquesas and the Gambiers are very low risk areas, but they are also quite remote. Tahiti is admittedly somewhat riskier but within acceptable limits. West of Tahiti are the Leeward Societies (think Bora Bora). They are riskier still and are no-go for insurance coverage during Cyclone Season. Papeete, the main city on Tahiti, is the capital of French Polynesia. Its population is 140,000, so it is big enough to offer most things you need, yet small enough to retain its friendly Polynesian appeal. To get around, we purchased a 50cc scooter, which delivers plenty of freedom and a ton of fun. And our stay here will give us ample time to deal with all the boat chores that have been accumulating over the last eight months of travel in more remote regions.

This article has been provided by the courtesy of Bluewater Cruising Association.

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