South Pacific Weather

Before I begin, please note that I'm an Electrical Engineer, not a weather professional, so please don't take this as gospel.  These are only my observations, but I've been watching the weather daily for several years, and I'm extremely motivated to learn.

The tradewinds in the Tropical South Pacific typically blow from the southeast or ESE, but they often don't start until a few hundred miles or so west of the Galapagos.  Closer to the South American mainland the winds are less well defined.  Sometimes they blow from the south, up the coast, fanning out somewhere off the coast of Peru, flowing NE into the Gulf of Panama and NW into the Pacific.  But these winds run into their counterparts coming down from North America.  Where they meet is called the Inter-Tropical Convergence Zone, or ITCZ.  The ITCZ usually manifests itself as a band of cloudy, squally weather with fluky winds.  When we sailed south from Panama, we were flying the chute all day in light and dying conditions.  As the grey cloud of the ITCZ appeared, we doused the chute, motored through about an hour of squalls, and came out the other side with both wind and current(!) in our faces (welcome to South America!).  Further west we ran into another convergence zone between the Marquesas and the Tuamotus, and that wasn't nearly so nice.

But the interesting thing about the Tropical South Pacific tradewinds is that they're driven by an alternating series of high and low pressure systems that move from west to east down near the roaring 40's.  This is very different from weather in the Caribbean, where virtually all weather (winds, tropical waves, hurricanes, and other weather systems) comes from the east and moves west and eventually north.

Typically, South Pacific lows (which rotate CW) and highs (CCW) alternate, one after the other.  These systems typically enter the South Pacific southeast of Australia and then move east, between about 30-40°S.  Although that's further south than most cruisers typically go, winds from these systems will either reinforce the tradewinds (in the case of a high) or diminish the trades (in the case of a low).  Interestingly, the magnitude of the highs will often tell how strong the tradewinds will be: a 1035mb high will usually generate winds of about 35 knots, which is a handy relationship to remember.

South Pacific Cyclones:

South Pacific cyclones usually form in the summer, from about November through April (remember, seasons in the southern hemisphere are opposite from northern hemisphere seasons).  When the ocean gets warm and if the air above it stops blowing and if a low pressure system forms, then the air warms and starts to rise like a giant hot-air balloon.  Coriolis forces start this giant bubble of rising warm air rotating (clockwise in the southern hemisphere, counter-clockwise in the north) and more warm air is sucked in at the bottom.  Surprising as it sounds, this cycle can build on itself until the rotational winds build to over 200 knots!

Luckily, there's a current of cold Antarctic water that comes north up the west coast of South America and then flows west into the Pacific at the equator.  When we sailed to the Galapagos, we had to dress warmly, even though we were within 50 miles of the equator!  This flow of cold water, called the Humboldt Current, cools the waters of the Pacific Ocean, especially the eastern Pacific, and therefore pushes cyclone formation further west.  If this flow of cold water slows down for any reason, then climatologists call it an El-Niño year.  For South Pacific cruisers, this means more cyclones will form, they'll form further east, and the cyclone season will start earlier and last longer - definitely a bad situation.

Because of the Humboldt Current, the islands of French Polynesia are usually cyclone free if it's not an El-Niño year.  Also, South Pacific cyclones don't usually hit within about 5 degrees of the equator, nor do they venture south of about 30°S.  So this defines a "cyclone-belt" from the east cost of Australia east through the Cook Islands, and from about 5-30°S.  This is the area that is most often hit by cyclones, usually from November through about April.  This is the area and season to avoid.

For our first cyclone season, we were east of the cyclones and relatively safe in Tahiti and the Society Islands of French Polynesia.  The following year we were in Fiji, watching the weather closely and always within a day's sail of 4 good cyclone-holes.  The year after that we decided we'd played with fire long enough and we sailed to Brisbane, Australia which is usually south of the cyclone belt.

South Pacific Weather Products:

Interestingly, our weather information generally arrives by email.  When we came though the South Pacific we depended on:

• GRIB files from SailDocs.com
• Weekly Weathergram emails from Bob McDavitt (pangolin.co.nz), and
• Text forecasts in French from Meteo-France

GRIB files come from big weather-modeling computers that take information from weather stations around the world and then process that data to come up with forecasts.  As such, they must be correctly interpreted and taken with a grain of salt as they arrive with no human interpretation!  This is the information that weather professionals start with (and modify) to produce their forecasts.  Typical information available via GRIB files include wind strength and direction, atmospheric pressure, rainfall (or cloud cover), sea temperature, wave height, as well as more esoteric weather information that we don't know how to use, like 500mb height.  As of this writing (2007) there are 5 different computer models available, but not all of them cover the entire world.  We typically use either the WW3 or GFS models.  We get our GRIB files from SailDocs as they allow us to define exactly what area and parameters we're interested in, but there are now about 20 different companies that will send GRIB files, usually for free.  GRIBs are relatively compact binary files and require a viewing program to see the information.  MaxSea can display isobars and wind-barbs in GRIB files, but better programs can be had for free on the Internet.  We use a program called ViewFax that displays more parameters and comes with AirMail, but it can also be used stand-alone and it's free.

To subscribe to the automated SailDocs system, send an email that looks like the following example:

 Parameters explained:

• send/sub - send once, or subscribe (default subscription length is 14 days)
• GFS/GRIB - GRIB model, not sure of the differences
• 10N,24S,034E,090E: boundaries of GRIB request, latitude first, then longitude
• 2,2: spacing between data points in degrees - latitude, longitude
• 24,48,72: Hours ahead for forecast (in this case, 3 days)
• Parameters to send at each data point (only select those you want):
  • PRMSL: Pressure isobars
  • WIND: Wind barbs (speed and direction)
  • HGT500: 500mb height? (we don't use this)
  • SFCTMP: Surface temperature of the ocean
  • TMP500: ?? air Temp at 500m?
  • WAVES: Wave heights
  • RAIN: Rain clouds/storms
• Optional additional parameters:
  • 5.0 - Forward speed in knots for moving center GRIB (we don't usually use this)
  • 270 - Course of progress for moving center GRIB (we don't usually use this)
  • Days=30 - days for subscription to run (default is 14 days)
  • time=06:00 - UTC time you want GRIB to be generated (06:00 gives the best forecast)

Bob McDavitt is a weatherman in New Zealand who has a strong interest in the cruising community.  If you send a subject-less email to yotreps@pangolin.co.nz with the words JOIN WEATHERGRAM in the message body, his system will send you his weekly Weathergram of the South Pacific.  A similar email with the word LEAVE will take you off the list.  Bob's Weathergram is obviously fairly general (it covers the whole South Pacific) but he provides a lot of insight and valuable information.  We subscribed to his newsletter for the entire 3 years that we were in the Pacific.  Highly recommended.

Text weather forecasts can come from a variety of sources, including Pangolin.  We used weather products available through the Winlink catalog system.  While we usually received the cyclone forecast from Pearl Harbor, Hawaii, the best local forecasts came from Meteo-France and were in French (FRENCH_POLY and F_POLY_MAR).  One of the net controllers of the 8 MHz French Polynesia Breakfast Net would usually translate these, but translating them yourself is not difficult, especially if you use the handy French-to-English weather translation table at the bottom of this page.

A note for Winlink users on requesting text weather products from the Winlink catalog: Have the catalog generate some requests for you and then go look at them so you can see what the format is, but I recommend building your own requests.  That way you can request several products at the same time and make sure they're always the first email to go out, which means you'll usually get responses back without having to reconnect at a later time.  The format of the Inquiry/Requests is fairly straightforward - all you need is the code from the catalog.  You can request multiple documents by simply putting the codes on their own lines.  One thing that is not documented is that a semi-colon (;) is a "comment character" that tells the system to ignore the rest of the line, allowing you to document your request files with comments.  Once you've built the Inquiry/Request file that you want, just send the same file over and over.  Go to your OutBox, open the old, sent request, click the Send-Via button, delete whatever was in the box and replace it with HAM or WL2K, change any of your requests that need it, and post the (new) message.  Simple and much faster than wading through the Catalog.

Some people liked the "Fleet-code" products.  These are similar to GRIBs in that they're relatively small binary files requiring a special viewing program, but we always found that they were at least 24 hours old by the time we received them.  After getting several and comparing them to other available weather products, I stopped requesting Fleet-code products.

French to English Weather Term Translations:

The following table lists common French weather terms and their English translations.  This table can be quite helpful when translating the (excellent) French weather reports in French Polynesia and New Caledonia.  Some examples are listed at the bottom.

Some common examples:

1. "NORD-NORD-EST 08/12KT, MER PEU AGITEE, AVERSES EPARSES"
   = Winds NNE 8-12 knots, seas with a light chop, scattered showers
2. "SECTEUR EST-SUD-EST 04/07KT SE RENFORCANT 13/17KT SUR SUD ZONE POINTES 30KT SOUS GRAINS, MER BELLE A PEU AGITEE AU NORD ET AGITEE AU SUD, AVERSES OU PLUIES ET GRAINS."
   = Winds ESE 4-7 knots, increasing to 13-17 over the southern part, up to 30 knots in squalls, seas smooth to a light chop in the north part, and choppy in the south part; showers or rain and squalls.
3. "18/22KT FAIBLISSANT A 08/12KT" = 18-22 knots weakening to 8-12 knots.
4. "ANTICYCLONE 1031HPA PAR 35SUD/145OUEST DEPLACEMENT EST 05KT"
   = A 1031mb high near 35° South 145° West, moving East at 5 knots.
5. "VENT DE SUD-EST A EST 25/30 KT (6/7B) RAFALES 35/40KT. MER FORTE."
   = S to SE winds at 23-30 knots (Beaufort Force 6-7) with gusts of 35-40 knots.  Rough seas.
6. "HOULE: SUR LA SOCIETE ET LE NORD TUAMOTU, HOULE DE SUD-EST DE 2M A 2M50. AILLEURS HOULE DE SECTEUR SUD DE 2M A 2.50M"
   = Swell: Over the Societies and Northern Tuamotus, SE swell at 2-2.5 meters. Elsewhere, South swell at 2-2.5 meters.
7. "POUSSEE D'ALIZE SUR LES FACES NORD ET EST D'UN ANTICYCLONE 1029HPA CENTRE LE 24 A 18 UTC PAR 35 SUD 145 OUEST SE DECALANT VERS L'EST-SUD-EST"
   = Reinforced tradewinds North and East of a 1029mb High which, at 1800z on the 24th, was centered near 35° South, 145° West, moving towards the ESE.

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