How Risky ?

We had to do a lot of research to understand the risk associated to Tropical Storms in French Polynesia, and thought of sharing our findings here. Albeit specific to the 2017 season – consisting of a weak LaNiña/Neutral one, pursuant to a strong ElNiño year – the sources we used in it could be of use to anyone interested on these waters.

<This is the Third in a series of Four posts covering our Learnings and Experiences of one season of cruising French Polynesia. Click here for the previous post in the series, covering learnings from the Passage between Mexico and French Polynesia, and here for the last post, containing information on the cost of provisioning there>

<Post Originally Written Oct 12th, 2016>

When we made the decision to cruise the South Pacific, and made a high-level design of our cruising plan, we knew we would most probably have to spend one season within the Cyclone Belt.

Source: Wikipedia

 

The options to avoid it altogether simply didn’t work for us from a logistics perspective. The Society Islands, in French Polynesia were defined as the mid-point base of our two-year plan and thus the place we would likely be during the Southern Hemisphere’s summer.

The Societies lie roughly at the eastern fringe of the South Pacific Cyclone belt and, even though away from the core activity every season, it does get visited by Tropical Storms occasionally.

The question then boils down to “How Risky is It”?

Well, it so happens that it isn’t that straight forward to find an answer to this question. Most locals – or at least the ones we spoke to – will say that “it doesn’t happen too often”, while some cruising guides assure it IS risky. Add the intrinsically chaotic nature of weather and throw in the fact that last year was a strong El Nino to stir the matter further.

So, I did some research.

The most available historical data of Cyclones in the South Pacific Basin dates back to the 1969-70 Season. Since then a disturbing 600 Tropical Systems formed and just over half of them developed into Cyclones, with sustained winds above 35kts:

www.familygonesailing.com
Sources: Wikipedia, based on reports from NADI

 

Focusing on the systems that matured into Cyclones – normal or severe – we see that every single season of the South Pacific Basin since 1969 has seen the development of at least 2 Cyclones, with an average of 7.7 per season:

www.familygonesailing.com
Sources: Wikipedia, based on reports from NADI

 

Now the question is: how many of these systems came close to Tahiti?

Using a mix of sources (see chart below), I selected all Tropical Cyclones that crossed anywhere within a rectangle which I arbitrarily defined as the “Close Enough” zone – an area within 144W-156W and 12S-22S, roughly 300 miles from Tahiti.

www.familygonesailing.com
Source: Cyclone tracks from Australia Government’s Bureau of Meteorology, validated/complemented by season reports from Joint Typhoon Warning Center. ElNino/LaNina monthly data from NOAA. Analysis from Familygonesailing.

 

There was a total of 34 “Close Enough” events during the 47 seasons analyzed above, all but one falling within the official Cyclone Season, from November to April.

  • Only 43% of the seasons had Cyclones passing “Close Enough” to Tahiti
  • 9% of “Close Enough” Cyclones happened in November, 18% in December, 12% in January, 27% in February, 24% in March and 9% in April

Next, I looked into the El Nino/ La Nina effect. These are cyclical oscillations on the average temperature of the ocean’s surface. The El Nino denomination is given to years when the surface temperatures are higher than average, while La Nina denotes years with lower-than-average surface temperatures. Historically, El Nino years tend to have stronger Cyclonic activity in the South Pacific, whereas seasons falling in La Nina year tend to be calmer. But how does this affect Cyclonic activity within the vicinity of Tahiti?

www.familygonesailing.com
Sources: see below
www.familygonesailing.com
Sources: Cyclone tracks from Australia Government’s Bureau of Meteorology, validated/complemented by season reports from Joint Typhoon Warning Center. ElNino/LaNina monthly data from NOAA. Analysis from Familygonesailing.

 

Of the 20 seasons that had one or more Cyclones passing “Close Enough” to Tahiti, 12 (60%) were during an El Nino year, the remaining 40% falling on Non-El Nino ones – either Neutral or La Nina years. Moreover, Cyclones tend to get “Close Enough” to Tahiti on the majority of seasons during El Nino years (75%), the opposite happening on Non-El Nino seasons (“Close Enough” events happening in only 26% of them).

The upcoming Cyclone Season – 2016 to 2017 – is shaping up to be either a Neutral or Weak La Nina one, thus let’s take a closer look at what happened during such seasons historically:

www.familygonesailing.com
Sources: Cyclone tracks from Australia Government’s Bureau of Meteorology, validated/complemented by season reports from Joint Typhoon Warning Center. ElNino/LaNina monthly data from NOAA. Analysis from Familygonesailing.

 

So here we see that since 1969, only 10 Cyclones passed closer than ~300 nautical miles of Tahiti during non-El Nino seasons. Additionally:

  • All of these storms had maximum sustained winds of less than 65 knots (near their center)
  • All but one passed farther than 100 miles away from the island
  • Leo, the closest event, actually formed almost on top of Tahiti, with winds of under 35 knots at that stage
  • February and March were the months of most incidence
  • Most of them passed to the South or Southwest of Tahiti

Let’s explore the last point further. Papeete and most of the local mooring areas – including where we are – are located in Tahiti’s Western coast. Now, Cyclones spin clockwise below the equator. As such, when a Cyclone passes to the SW of the island, its West coast is directly exposed to the Cyclone’s prevailing wind and waves, which is a negative aspect. On the other hand, however, winds tend to be less strong on the NE quadrant of the Cyclone – kind of a mix-and-match situation.

www.familygonesailing.com

 

So, back to the original question of “How Risky is it?”, in a nutshell:

  • The upcoming Cyclone Season (2016-17) is shaping up to be a Neutral or Weak La Nina one
  • Historically, Cyclones came to within ~300 nautical miles of Tahiti in 26% of the Neutral or La Nina Seasons, to a total of 10 Cyclones in 47 years
  • Only one Cyclone passed less than 100 miles of the Island during such seasons
  • All of these Cyclones had winds of 65 knots or less (near their center) at the times they were the closest to Tahiti
  • Most of them passed to the SW of Tahiti, causing winds/waves from a West-to-North direction to hit the west coast of Tahiti

That’s it. Should the 2016-17 season NOT redefine the statistics – and I truly hope it doesn’t – we have a 1-in-4 chance of having one Cyclone passing close to Tahiti and, if it does, it should be a Category 2 or below, with winds of not much more than 50 knots hitting the west coast here.

Finally – and now I will indulge on a brief moment of wishful thinking – I was taking a look at the most recent Sea Temperature Index for this Season. Without getting into much technicality, a positive index means warmer seas and negative values indicate cooler ones. A consistent series of monthly indexes above 0.5 constitute an El Nino Year, whereas a consistent series of indexes below -0.5 indicate the la Nina phenomenon. Well, the last two indexes issued by NOAA for the 2016-17 have been -0.3 (average for June, July, August) and -0.5 (average for July, August and September). I looked across the entire series of Seasons, selecting the ones which had indexes between 0 and -0.5 for each of these two periods. Thirteen seasons match such criteria, and only in ONE of them has there been a “Close Enough” Cyclone (Ima, February ’86). So, who knows, maybe this factoid reduces our 1-in-4 chance to even better odds …

 

POST SCRIPTUM <Written Oct23rd, 2016>:

After I wrote this post, I came across an excellent analysis made by the National Institute of Water and Atmospheric Research in NZ (click here to access their database of cyclone-related studies). Using a definitely more scientific approach, and considering factors that I didn’t above, NIWA identified 4 historical seasons which had weather and oceanic patterns closest to the ones of this upcoming season. Based on this, they issued the below Cyclonic Activity Outlook for the 2016/17 Season:

Cyclonic Activity Outlook for 2017. Issued in Oct 2016 by NIWA.

This assessment shows that :

  • The Society Islands are indeed within the least risky areas of the South Pacific Basin for a Non-El Nino Season
  • Their forecast activity for this region is of 0 to 1 Cyclone in the 2016-17 Season

For the detail-oriented, I am including here NIWA’s images of all cyclone tracks during these “similar” historic seasons:

Source: NIWA
Source: NIWA
Source: NIWA
Source: NIWA

 

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