Compiled Sun 12 AUGUST 2018
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.
The South Pacific Convergence Zone explained:
Now that many yachts are about to travel west from Tahiti to Tonga, and are thus about to sail thru or around the SPCZ, as if it guards the eastern entrance to the South Pacific like a protective dragon, 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 meteorological teaches concentrate on the pattern. In tropical meteorology the first idea given is the Hadley cell.
Because the sun is most directly overhead at the equator, that’s where the warmest seas are, and this causes rising air. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out. 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. 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 (it is quasi stationary, just like the High between California and Hawaii, and has a gyre that is collecting a rubbish heap just as badly (see Henderson island). blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-in-paradise/
There are easterly winds on the north side of this “Andes” High: they are dry due to continental outflow from off South America. 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. The easterly winds travel well to west of the dateline around 10 to 15S, and the South/SE winds come and go according to the migratory high and are usually found around 15 to 25S. 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, but sometimes may have large gaps or be very quiet.
It is affected by many things: the PDO which takes many years to switch, by the El Nino/La Nina which last a year or so, and by the strong annual cycle which repeats each year, and by 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
There is a regular procession of tropical features off the western Mexico coast.
The Atlantic remains somewhat quiet. YAGI is travelling NW to China, and LEEPI to south Japan, and HECTOR is heading for south of Hawaii, with another feature following.
Looking at the weekly rain maps we can see that the Asian monsoon is active over Indonesia and the Philippines, and the ITCZ is active across the Pacific. Not much change in the past week.
SPCZ=South Pacific Convergence zone.
The SPCZ is expected to stretch from PNG and Solomon Islands to north of Fiji and spreading onto northern Tonga/Niue.
A weak passing trough is likely over Society Island and Tahiti on Monday/Tuesday UTC.
A passing trough, associated with a Low travelling east along 30S, should reach Niue on Mon UTC, and fade between Southern cooks and Tahiti on Wed/Thu UTC.
Subtropical ridge (STR)
HIGH is expected to move off Australia into northern Tasman Sea on Tue/Wed UTC and then travel east along 25S
Front moving onto NZ on Mon expected to deepen into a Low over central NZ on Tuesday and then move off to the east, followed by W/SW winds until Friday.
Tahiti to Tonga
The best-looking date for departure is Wednesday local. This voyage is likely to encounter a weak passing trough near160W on Saturday and then should be able to go onto Tonga . >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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