Bob McDavitt’s ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the patterned world.
Compiled Sunday 28 April 2024
WESTWARD HO … for those planning to get from Tahiti to Tonga
An introduction to the South Pacific Convergence Zone (What and Why)
Now that the cyclone season finishes mid-week, many yachts are about to travel west from Tahiti to Tonga, and are thus about to sail thru or around the SPCZ, this obstacle has become a talking point…as if it guards the eastern entrance to the South Pacific like a protective dragon or demon, and some have asked what is it, why is it there, how does it differ from the ITCZ and what makes it tick.
Weather is a mix of pattern and chaos, and meteorologists concentrate on the pattern.
In tropical meteorology the first idea given is the Hadley cell.
Recipe for the ITCZ
1. Because the sun is most directly overhead near the equator, that’s where the warmest seas are, and this causes rising air.
2. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out.
3. The sinking air reaches the surface again around 30N or 30S (subtropical ridge) and then recirculates back to the equator as surface winds know as trade winds.
4. The trade winds from each hemisphere converge together in a zone, and this convergence narrows the zone of rising air into a feature called the Intertropical Convergence Zone or ITCZ
But in the Southern Hemisphere, the Andes of South America cause a split in the trade winds. They block a HIGH near 30S around 90 to 110W, or south of Easter Island. It is quasi stationary, just like the High between California and Hawaii, and has a gyre that collects a rubbish heap just as badly (see Henderson Island: blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-in-paradise/).
Recipe for the SPCZ
1. There are easterly winds on the north side of this “Andes” High: they are dry due to continental outflow from off South America. These easterly winds travel well to west of the dateline along around 10 to 15S.
2. And there are migratory Highs that travel east along the subtropical ridge from Australia to east of NZ, with a zone of south to southeast winds on their northern side. These South/SE winds come and go according to the migratory high and are usually found around 15 to 25S.
3. The convergence zone between these easterly and Southeasterly winds is called the South pacific Convergence Zone, or SPCZ.
It is typically located from the Solomon Islands southeastwards to the Southern Cooks, and is around 1 to 5 degrees latitude wide, but sometimes may have large gaps or be very quiet.
It is affected by many things:
· the Pacific Decadal Oscillation or PDO which takes many years to switch.
· the El Nino/La Nina which lasts a year or so.
· the strongest cycle is the ANNUAL cycle as earth orbits the sun, making the seasons.
· and the MJO which comes for a week or so every six weeks or so.
Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone
From www-gte.larc.nasa.gov/pem/pemt_flt.htm
I have found that the easiest way to determine the position and severity of the SPCZ is to use satellite imagery, and the easiest way to decide what it may do over next few days is to use the 5day rain accumulation parameter on windy.com.
The latest Satellite image of the SPCZ is available as a small attachment via an email request.
Send an email to query@saildocs.com with the message (no subject line needed)
SEND https://www.y2ksail.com/himawari/latest-SpacificLarge-small.jpg
In a future blog I’ll compare and contrast the three routes from Tahiti to Tonga: when to take or avoid the northern, direct or southern route and how to decide which is best over the week ahead.
TROPICS
The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu and Tropical Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html
It is a quiet time with no named cyclones.
WEATHER ZONES
Weather Zones Mid-week GFS model showing isobars, winds, waves (purple), rain (red), MT (Monsoonal trough), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) CZ (Convergence Zone)
Rain Accumulation next five days from windy.com
It’s a busy weather map this week with four lows: 2 in the tropics and 2 in the mid- latitudes.
The South Pacific Convergence zone is active from Northern Coral Sea to northern Vanuatu to south of Fiji, with another arm over Northern Cooks. There is a passing trough to southeast of Tahiti.
HIGHS and LOWS
HIGH H1 over Chathams tonight is expected to travel ESE from 43S to 50S leaving a ridge over central NZ.
Low L1 is deepening between northern NZ and Tonga on Monday and then travelling SE, peak central pressure below 995 when east and well offshore of Gisborne on Wednesday. From Thursday as L1 moves away a trough is expected to cross NZ followed by SW swells over 3m extending north and reaching New Caledonia by the weekend.
Low L2 I expected to form off the eastern seaboard of Australia around Friday with associated trough traveling east across Lord Howe Monday 6 May
Looking Ok this week for getting from NZ to the tropics, maybe going around L1.
If planning a Trans-Tasman trip going east, then work in with L2.
Looking Ok for getting from NZ to the tropics,
Panama to Marquesas:
Only light winds for starters this week.
Doldrums between Panama and 2N with erratic winds and squally showers.
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If you would like more details about your voyage, then check metbob.com to see what I offer.
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is bob@metbob.com or text 64277762212.
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MetBob 
