Issued 29 Sep 2013

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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

Background influences

SOI: The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardised difference in the barometer readings between Tahiti and Darwin.

It maintained a value over 1 for most of June, hinting at a La Nina. Since then it has dropped to just below zero in early September and bounced back to 0.4 by 29 Sep.


SOI, as shown at, showing a bounce in the trend over the past few weeks.


TC  WUTIP is heading westwards across the NW Pacific heading to make landfall in Vietnam.



Weekly rain signatures for past fortnight, as shown at



Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday

South Pacific Convergence Zone SPCZ

The SPCZ is active between Solomons and Tuvalu and is expected to extend towards Niue by Wednesday UTC

There is another convergence zone which is very active mainly along 10S from Tuvalu to Northern Cooks. This zone is a mirror of the ITCZ that tends to form for a few weeks around the equinox and is expected to remain active this week, occasionally spreading as far south to Samoa and Suwarrow.

Sub-tropical Ridge STR

The STR is still weak around the NZ longitudes and this is allowing lows to spread over the North Island. One is expected to cross this Auckland early on Monday and continue ENE, and then turn SE on Wednesday.

Over Australia and east of 180 the STR is strong along 30S as is normal for this time of the year.

Roaring 40s and New Zealand

A disturbed westerly flow, typical of spring, covers the Aussie Bight and southern NZ.

Route Briefings

Tahiti to Tonga:

Weak trough is expected to affect Niue on or around Wed/Thu UTC and another may affect Tonga from Sat to Sun 5/6 Oct. Try and time your voyage to avoid these troughs.

Between NZ and Fiji/Tonga

There is a good opportunity to travel south from Mon 30/Tue Sep/Oct as this should go nicely around the back end of a High.

See my yotpak at for terms used.

Weathergram with graphics is

Weathergram text only and translator is


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