Bobgram

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-pollu
tion-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 south-eastwards 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


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

It is a quiet time with no named cyclones.

WEATHER ZONES

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
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