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South Pacific Winds, pt. 2

Last month, author Jim Corenman (Heart of Gold) introduced us to the curiosities of South Pacific weather. This month, he shows us how to apply this knowledge.

Weather Information Sources

In terms of getting weather information, looking out the window is of limited value. But like proper navigation or safe sex, it's something that everyone ought to do.

The problem is that the atmosphere in the tropics is never overly stable, and ugly towering clouds are usually just a sign of ugly towering clouds. Remember that the air on the north or east side of a disturbance will be warmer, and considerably more stable, than the approaching air mass, so the best indicator of bad weather to come is the presence of particularly nice weather, especially in the presence of a breeze that is backing (shifting north) and getting lighter. "The calm before the storm," as Mom used to say, but nice weather can also be a sign of nice weather, so don't be overly paranoid.

A barometer can be a help for certain kinds of disturbances. An approaching low pressure trough, for example, will show up as a drop of a few millibars, along with a wind shift backing towards the north. The diurnal variation of the barometer must be accounted for, however, and the pressure will vary about three millibars twice daily, peaking at about 10 a.m. and 10 p.m., local time. A barometer is not helpful with respect to the Convergence Zone (CZ), however, as the CZ is not a pressure-driven phenomenon. A barometer will show no change (other than diurnal) while sailing across even a strong convergence.

A big problem that meteorologists have in the South Pacific is the scarcity of reliable data. There are islands everywhere, but relatively few have any sort of weather station, and few of the ships at transit in the area are capable of observing and recording the weather. Satellite photos can help, as they indicate cloud height by temperature, which can be measured from the IR photos, and can help locate fronts and convergence zones.

Reliable weather forecasts are available for the South Pacific, but they are a little hard to find, and information on the activity and location of the SPCZ is a problem. The four offices that put together forecasts for the Pacific are the U.S. National Weather Service (NWS) Hurricane Center in Florida, the NWS office in Honolulu, and Fiji Meteorological Service in Nadi (pronounced "Nandi"), Fiji, and the New Zealand Meteorological Service in Wellington. Of these, the only office located in the Tropics is the one in Fiji. It's probably not a coincidence that they seem to do the best job; however, their maps and forecasts are also the hardest to get.

The French also do forecasts for the Societies, but they are useless for most of us, because the information is only broadcast in French. The beauty of fax charts are that they are independent of language, and even a text broadcast sent in Morse code can be translated with a little patience. Not so with a broadcast.

In terms of availability, you can get weather information and forecasts either in the form of charts or as written text. Charts are usually sent by weatherfax over the radio, but are sometimes coded as a sequence of numbers and sent by Morse code (CW). Written forecasts are either broadcast over SSB or sent by Morse code (CW) or teletype (SITOR).

The NWS fax charts from Washington and Florida are transmitted by the Coast Guard station NMC at Point Reyes, while the Honolulu fax charts are sent from KVM70 In Honolulu. The Tropical Analysis charts, from NMC Point Reyes, broadcast twice daily on fax, are the best source of information regarding the ITCZ and conditions between North America and the Marquesas.

KVM70 Honolulu covers a much broader area of the South Pacific, but their charts are of limited utility because they don't usually show fronts or convergence zones, and seem more designed to show the 'Big Picture', not very helpful when you are getting mauled by the details. Their Pacific Surface Analysis chart is hand drawn, and does show some features, while the Tropical Surface Analysis is computer-generated and harder to make sense of. Their 24 and 48-hour Surface Forecast charts are also computer-generated, and are useful for determining general surface winds, but again, the absence of features (fronts and CZs) is a real limitation.

The New Zealand Met Office Surface Analysis and Forecast charts are broadcast by ZKLF in Auckland, and cover the South Pacific to 140 degrees W, but don't provide much detail in the tropical areas. The isobar spacing is five millibars, pretty wide for the tropics where there aren't many isobars, and they don't reliably detail fronts and convergence zones in tropical waters. They sometimes show tropical disturbances, and sometimes not, so you never can tell what the absence of features on the charts means, and they don't show the position of the SPCZ on a regular basis. Interestingly, they share data and maps with the Fiji Met Service, so they've got the data. The New Zealand maps do give a good representation of fronts and disturbances to the south, however, a big help for do-it-yourself forecasters.

As mentioned, the best source of weather in the western South Pacific is the charts and forecasts from the Fiji Met office in Nadi. They report the position and activity of the fronts and CZs, and draw their charts with an isobar spacing of two millibars. The catch is that their charts are not widely distributed, at least aside from the Fiji Times, but there are a lot of us that believe that the day-old Nadi chart in the newspaper is still better than any of the available fax charts. Getting the paper can be a problem when you're offshore or in the outer islands, however.

Arnold (ZKlDB) in Rarotonga (Cook Islands) and others get the Nadi charts by phone fax, but for us yachties the only way to get the data directly is to copy the coded map transmitted by ZKLF (New Zealand) in Morse Code at 0500 and 180OZ. It comes at 18 words a minute, but isn't as hard as it sounds as it is all numbers (only 10 codes to learn). You can also copy Morse with a computer if you have a laptop and the right sort of decoder.

You will also need a decoder sheet for the 'IAC Fleet Code'. These are included in the Admiralty List of Radio Signals, an otherwise bulky and mostly useless set of volumes, and are available from some Weather Service or Met offices. We used to copy and draw coded maps in the 'old days' before we had a fax (we're talking '82 here), and thought that we had seen the last of them. Imagine our surprise when we got here and discovered that the best weather source in the South Pacific was a coded map, and on Morse, no less! Don't forget your secret decoder sheets!

The other reliable source for information on the SPCZ is Arnold's Weather Net, if you can get it. The problems are that voice is often hard to copy in general, and Arnold speaks Kiwi (which is certainly easier than French but still hard for a lot of Norte Americanos), and his signal is not very strong in most areas of the Pacific. Another unfortunate happenstance is that he comes on at the same time as the warm-up session of the Pacific Maritime Net, a popular ham net only 5 kHz away. It's quite common for someone in a busy anchorage to check into the net, wiping out Arnold's forecast for the immediate vicinity. Unfortunate, but that's life.

This past season the coded map was copied and deciphered every day by Peter and "the lovely Christina" on Wild Spirit, who pass along the weather each morning on the 'Comedy Net', an informal 40 meter Ham Net that got started in French Polynesia. This was a great service, and hopefully will continue next year.

Most South Pacific weather comes across Australia, and copying the surface charts from Melbourne (AXM) can be a help, especially for planning a crossing to New Zealand. They transmit surface analysis charts four times daily, plus prognosis (prog) charts, and an especially nice prog chart at OOZ that overlays the highlevel contours to provide a sense of relative motion.

Voice or text broadcasts can also be useful, and often contain interpretations not contained on the fax charts, but they are harder to find. The Nadi Met office is the source for a text forecast broadcast on Morse code by ZKLF from Auckland at 0920Z and 212OZ (as well as the coded maps mentioned above). NMO Honolulu broadcasts voice and SITOR (telex) forecasts, which give the position of the ITCZ, but little else of value. There are no other SITOR broadcasts in the South Pacific, but Fiji and New Zealand do broadcast weather on SSB (voice).

Some computer-type fax decoders can also 'read' Morse code and SITOR, but some have trouble with Morse. As an aside, KMI transmits their traffic list continuously on SITOR on 8429.3 and 12627.8 kHz (as well as others copy long enough and you'll get all the frequencies). It's an easy way to check for messages.

What we would recommend is to use NMC's Tropical Analysis and KVM70's Pacific Surface Analysis charts to get you to the Marquesas, continue to watch the KVM70 charts as far as, say, the Cooks or maybe Tonga, and start watching the ZKLF charts for the Southwest Pacific when you can copy them (maybe in the Societies). Decoding at least the part of the Nadi chart that deals with fronts and the SPCZ will also be important if you want to avoid tangling with that little hummer.

Sailing Strategies

So the weather reports will tell us generally where the SPCZ Is, and roughly how active it is. We also know that troughs and their associated fronts don't dissipate quickly in the tropics, and we need to remember that they are there, even after the weather service stops drawing them on the charts. Can we avoid the bid weather? Maybe, If we're willing to be a little flexible.

The key is to avoid being in the wrong place at the wrong time, specifically in the vicinity of the convergence zone when a new load of cool air comes rolling in. If the CZ is in the neighborhood, and looks like it will stay, then don't hang around waiting for something really bad to happen. Go elsewhere. Time your passages for a period of minimum activity, then put some miles between yourself and The Beast.

If you're going to hang out in CZ territory, then a protected anchorage is important, especially protection from the south. Remember that a northerly shift is common as a disturbance approaches, and don't be quick to abandon an anchorage protected from the south just because it is a little exposed to the north. Finding a good anchorage in the Tuamotus can be a problem, because of the shape of the atolls, and because the group lies near the usual haunt of the SPCZ.

An anchorage off a motu that offers good protection in an easterly will not be so attractive if the wind goes south, and most of the atolls are big enough to offer plenty of fetch to develop a nasty swell. You won't see the three or four meter ocean swells you would outside, to be sure, but a meter of steep chop is more than enough to wreck your ground tackle, especially when the chain is hooked up short under a coral head with a reef at your back.

If you think there is potential for a disturbance, try to choose an anchorage protected from the southwest through the southeast, even if it's a little uncomfortable in a light northeasterly. The potential for strong southerlies is a lot higher than for a strong north-anything. Remember that a disturbance will be heralded by a strong wind from a new direction, which will almost invariably wrap the anchor chain around a coral head before it pulls tight and off the bottom. This will leave you well secured, but with a very short scope. The best thing to do, rather than trying to unwrap it in a blow, is simply let out more chain and rig a nylon snubber to take the shock loads, or maybe one of those rubber dockline snubbers. If you wind up with the reef at your back, consider going elsewhere if there is enough daylight left to navigate the coral. Sometimes going out through the pass and waiting it out on the outside (in the lee of the atoll) is the best answer.

If you get caught by a trough or a strong convergence offshore, you've got to sail away from it, and not heave to and wait for it to move away from you. Remember that The Beast can be deceptively slow moving in spite of its general boisterousness, and if you choose to wait, you might wait a very long time. Running off is equally useless, because the converging nature of the winds will send you right down the axis of the convergence. Worse, by staying with the convergence you are almost guaranteed to get pasted by one of the truly nasty cells that get imbedded in the unstable tropical air.

The only sailing strategy that makes sense is to put as many miles as you can between yourself and the convergence, and that generally requires making good distance to the south. Sailing above a beam reach is usually required to make good any distance from the convergence, and the higher you can sail, the faster life will get better, although it will be a little sloppy in the meantime. A boat with too much windage and too little keel for her load will have a very hard time making much progress to windward, but there's no other way, so give it your best shot.

The Bottom Line

So, on the whole, just how goofy was the South Pacific Weather? That's a tough question to answer, because it depends a lot on individual expectations. Most of the time it was lovely, classic tropical weather. In the vicinity of the convergence zone, however, it was unsettled and periodically rained quite a bit, sometimes for days, which everyone got a little tired of. The occasions when it got downright boisterous were more rare, but everyone got caught out in it at least once. In only a few cases did anybody have problems, as most boats are well equipped to deal with heavy weather.

Was this a normal year weatherwise? Probably not, but what's normal anymore? The weather gurus say that we are still in an El Niño pattern, albeit a weak one, for something like the third year running. The occurrence of an El Niño doesn't alter the basic weather mechanisms of the South Pacific, but any change in sea temperature will have an effect, good or bad, on the general activity level of the tropics.

Would we sail the South Pacific again? Yes, absolutely. We had done a careful job of preparing out, without really knowing why, other than it is the right thing to do. We now know why, and wouldn't do anything different, other than plan to be a little more flexible with respect to places and schedules. It's been such an incredible experience, being among relatively Isolated places and cultures so different from our own, that a few hassles with the weather seem minor by comparison. Those who would venture unprepared, however, would likely find more than hassles.

- jim corenman

This story was reprinted from the the March 1994 issue of Latitude 38. To order a copy (complete with black & white photos), use the subscription order form, and specify the 3/94 issue, or just drop us a note with a check for $7 to Latitude 38, Attn: Back Issues, 15 Locust Ave., Mill Valley, CA 94941.

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