Bob McDavitt's ideas for sailing weather around the South pacific

19 July 2015

Bob Blog



Issued 19 July 2015

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.


Late autumn in New Zealand saw rain return to west coast of South Island after a very dry period.  This can be seen in the daily storage of the NZ hydro dams at


In The Atmosphere:

The Southern Oscillation Index SOI sums up the weather pattern over the South Pacific in one number and is based on the standardized difference in the barometer readings between Tahiti and Darwin (30 day running mean). The weekly update was lower than -10 for four weeks in May, enough to start a moderate El Nino episode. The value relaxed during June had dived rapidly during July, reaching -20 last week, as seen at  .


El Nino’s impact: During an El Nino episode, weather patterns tend to be drawn closer to the equator. The South Pacific Convergence zone tends to be tugged north and east of its normal position and there tend to be more troughs over Tahiti. The subtropical ridge in the southern hemisphere tends to be north of its normal position and this weakens the trade winds overall, but doesn't stop the squash zones that can be found on the north side of the migratory HIGHS.

During El Nino years, tropical cyclones in the South pacific enter middle latitudes anywhere between the Australian coast and about 150°W and typically weaken quickly as they move away to the south-east, due to strong winds aloft.

I am NOT aware of any association between SOI and an early onset to the southern hemisphere cyclone season.  And cyclone RAQUEL over Solomon Islands earlier this month was a “one-off” so there is no reason that I know of for sailors to speed up their visiting plans across the South Pacific,




Out west, HALOLA is now a tropical depression and heading for Japan .

East of Hawaii the remains of ENRIQUE are fading Further east, near the Mexican west coast, DOLORES is travelling north The weekly rain maps over the past two weeks show the intense rainfall from tropical cyclones.  There also seems to have been an increase in intensity in the convection across Tuvalu and Tokelau.

 Weekly rain signatures are at




SPCZ=South Pacific Convergence zone.

SPCZ this week is expected to stretch from south of Solomon Islands to Tuvalu to Tokelau to Northern Cooks.  A small convergence zone (on the north side of a Southern Ocean Low) is expected to travel eastwards (upper westerly winds) and cross the Palmerston Island on Tuesday/Wednesday UTC (local Monday/Tuesday).


STR= Sub-tropical Ridge

STR is along 40S in Tasman Sea and 30S in the SE Pacific, but is having a disjointed week in-between.

A cold HIGH is moving slowly across the Tasman Sea and expected to cross NZ on Thu Fri UTC.

There should be a squash zone of enhanced SE winds on the north side of this Tasman sea HIGH, affecting Minerva to Coral sea around mid-week.



The weekend low is now east of NZ and moving off to the eats,  but on Monday one more cold front is expected to skirt the east coast bringing a southerly gale (and alpine snow) until Wednesday morning (mainly Wellington to Gisborne).

For the remainder of the week the incoming High brings light winds and frosty nights.

This is the same low /cold High combo which brought snow to parts of Queensland late last week.


Travelling Westward from French Polynesia:

A good week for going west, but be aware of the strong SE winds expected over Tonga from Wed 22 to Sun 26 local.


Departing from Australia to the tropics this week:

The squash zone on north side of that Tasman High makes Brisbane to Noumea very difficult for first part of the coming week.  A trough is expected to travel east across the eastern seaboard on Thursday and then local Friday may be Ok for a departure.


Departing from Northern NZ going north.

A departure on Monday is likely to encounter strong tail winds at first.  A departure after Wednesday may encounter NE head winds by Friday.



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