Issued 26 May 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

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

SOI rose to over plus 1 briefly in early April and dropped to minus 0.39 in early May and bounced back into the positive in May, was plus 0.53 on 26 May. So the signal coming from the Atmosphere continues to be erratic.


SOI from

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds along the equator and these in turn tweak the latitude zones of weather around the planet.

Over the past few months conditions have been near average, and in the past month there has been a growing area of cooler than normal conditions over the eastern equatorial Pacific. This is possibly a sign of a trend towards La Nina conditions.

ITCZ and Monsoon

ITCZ is strong over eastern Pacific and weakening over western Pacific. Not much sign of monsoon over southern India but active convection for Burma/Myanmar .


Rain for the last week as seen at



Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday 29 May

South Pacific Convergence Zone SPCZ

The SPCZ is expected to move south this week, reaching Fiji by Wednesday and maybe developing a low over Tonga by Sat 1 June. Avoid.

There is a left over convergence zone CZ along 10S between Samoa and half-way-between-Marquesas-and Tahiti. Rather inconvenient.

Sub-tropical Ridge STR

The STR straddles 30S this week. The High east of NZ is expected to travel along 30S and the High west of NZ should travel along 33/35S, crossing NZ on Thursday to Sat 30May to 1 June. There should be a squash zone of enhanced SE winds over New Caledonia and Fiji mid-week between the SPCZ and STR. Avoid.

Roaring 40s and New Zealand

Southerly outbreak AND large swells from southern ocean are expected to travel NE across NE on Monday and reach as far north as Tonga by Tuesday. Avoid.

Next front is expected to reach Tasmania on Fri 31 may and then NZ by Sun/Mon 2/3 June. Avoid.

Route weather picks

Galapagos to Marquesas: Good to go this week. Light to moderate South to southeast winds over Galapagos and moderate trade winds trade winds from 2S onwards, but light easterlies and a nearby CZ around Marquesas.

NZ to the Tropics.

Wait for the polar blast to get across NZ on Monday and past Tonga by Tuesday.

Wednesday looks to be best day to depart. Light winds on Thursday and Friday and a strong to gale northerly flow on Saturday and Sunday close the window of good weather quickly.

See my yotpak at for terms used.

Weathergram with graphics is

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