South Pacific Weather

We are knee deep in preparations for our trans Pacific journey beginning in May.

I depart for New Zealand April 30 and Steve Parsons will be joining me in Whangarei. Deb will remain in Pittsburgh and join us in French Polynesia.

Steve and I will be busy bringing Iron Lady back to life. Lots of new systems to test – our Yanmar get home drive, new four blade prop on the main, new communications gear, and new monitoring software and sensors for the Maretron system. We will be doing some local cruising (probably Great Barrier or the Bay of Islands) to fully exercise all the onboard systems – our big John Deere and drive line, all the electrical systems (12 volt and 24 volt DC systems), the 120 and 240 volt systems and inverters, potable water and watermaker, heads and holding tank systems, refrigeration and galley appliances, Kabola for boat heat and hot domestic water, air con, ground tackle, bow thruster, all of ur electronics including the all important autopilots, and, of course, our trusty dink, Beer Can. Lots more stuff including going over our spares, fueling the boat and provisioning, but you get the idea. A lot to do and a lot of water between NZ and French Polynesia with very few places to stop so preparations are all important.

Our plan is to depart NZ before May 30 – preferably by May 15. We will be joined by one of Steve’s sailing buddies for the transit so there will be three of us on board for the crossing. Travel time to French Polynesia should be on the order of 2 weeks.

I have been reading up on South Pacific weather and routes from NZ to French Polynesia. For those of you who have been around boating for awhile, this is NOT the preferred direction to go as it is mostly uphill against the prevailing southeast trades. In studying the pilot charts for May (and reading some recommended suggestions on a route), one of the strategies is to head east from New Zealand and remain in the Southern Ocean well south of 30 degrees. At this latitude, the procession of alternating lows and highs moving out of the Tasman, across New Zealand and then across the Southern Ocean generates westerlies that can put the wind behind you for portions of the trip. It also, of course, puts one smack on the storm track and encounters with gales are a likely consequence of this strategy. On this routing, a course south of the great circle route is maintained until one is well toward French Polynesia when a northerly turn is made to head to the Tahiti.

Steve, who has been making (and is currently on) a run to Pitcairn Island on a commercial trader is advocating the great circle route as his experience is that the weather will typically be fairly benign that time of year. Part of that obviously depends on the strength of the southeast trades.

The following graphic (courtesy of the Australian Met office) gives a pretty good picture of South Pacific weather patterns. I have also found that the book “Landfalls in Paradise” provides some excellent material on understanding South Pacific weather as well. Also a hat tip to the blog Soggy Paws (www.svsoggypaws.com) – they have put together some excellent compendiums on their journeys thru the South Pacific and reference some excellent material by others.

It is tempting to believe that the southeast trades would cause weather systems to flow from east to west. Tempting but wrong. Weather systems generated in the Tasman and the Southern Ocean progress from west to east in an alternating pattern of low and high pressure systems. As these systems run in to the warm and moist southeasterly trades, the result is convective activity in a zone called the South Pacific Convergence Zone (SPCZ). The SPCZ is not to be confused with the ITCZ north of the Equator but is similar in nature. Like the ITCZ, the SPCZ is a semipermanent feature. One of the sources I have been reading characterizes it as the grave yard for Southern Ocean weather features.

This prevailing weather pattern also has some interesting consequences for our time in French Polynesia. When a high pressure system presses into the islands, the winds will go north as it passes (highs are anticyclonic in the southern hemisphere). This serves as a warning that a low is probably not far behind with squally weather and a sudden blustery wind shift to the south. The telltale from what I have read is fair benign weather with light northerlies. The recommended advice is to forget the northerlies and seek safe shelter from strong southerlies.

This is all pretty basic and over simplified but this is not intended to be an exhaustive treatment on South Pacific weather – that is beyond the scope of this post (and the expertise of this writer).

Along with all of this, I have been trying to line up my sources for weather (beyond the look out the window, look at the wind direction, strength and barometer eyeballometric guestimation that we always to). More on that next time, but I will note two obvious problems. First – there’s an awful lot of water and very little land between NZ and FP – hence not many real time observations of what is really going on out there and, second – weathermen are frequently wrong even when they do have good information to forecast from.

Next time – the sources that we will try to use for those who have an interest in such things.

Cheers