Issued 25 Aug 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) 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 relaxed to around 0.1 (25 Aug).


SOI as shown at, showing a hint of a June hump and then a gradual relaxing.

The Ocean: The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. An index for this is NINO3.4 and its abnormalities tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet.

This parameter has been near average so far this year, slightly on the cool side.


Sea temperatures along the equatorial Pacific as shown at, showing things to be near average.


The flooding in Manila last week has been caused by a double whammy of monsoon rain and a tropical storm (TRAMI) dumping more than an average month of rain in a day. Rain in the area returned to normal last week. TC PEWA is also making its way to the NW in the NW Pacific. The Cyclone season is getting busy and TC IVO is fading off the Baja Californian coast—unlikely to interfere with the final race of the Louis Vuitton in San Francisco.

Trade winds have weakened in the South Pacific between Australia and the dateline recently and the South Pacific Convergence Zone SPCZ has been stronger than normal between Papua New Guinea and Fiji/Tonga.



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 currently extends from Solomons to Fiji/Tonga and is increasing in intensity. It is expected to stay in much the same position during this week, but SE trade winds over Vanuatu and Coral Sea are expected to become strong in a squash zone as a slow-moving High forms in the Tasman Sea. There is a smaller Convergence Zone CZ along 10S around Northern cooks.

Sub-tropical Ridge STR

The new High moving into the Tasman Sea is a slow-moving one – the first we have had in several weeks, and this provides a good pattern for sailing between NZ and the Tropics.

Roaring 40s and New Zealand

Low that crossed northern NZ on Sunday is expected to move off quickly on Monday. Next trough is expected to arrive over South Island from southern ocean on Wednesday and over North Island on Thursday and then deepen into a low east of the North Island on Friday and doesn’t move off to the east until Sunday. This is a classic setup for an “old man southerly” in Wellington that lasts three days. And it introduces another week if winter to NZ after a mild few weeks.

Route Briefings

Tahiti to Tonga:

As was the case last week, this week’s weather pattern is looking Ok for sailing from Tahiti to Tonga or Samoa via the northern route (passing Suwarrow/Suvorov). There are not many troughs or convergence zones to worry about; however it is not expected to be entirely squall-free.

The southern route — going via Southern Cooks /Niue—is second best this week because there is a trough that is expected to visit Southern Cooks on Tuesday/Wednesday (Monday/Tuesday local)

Between NZ and Fiji/Tonga

The slow-moving high entering the Tasman Sea offers a good pattern for travelling north with departures on Monday and Tuesday. After that the voyage is likely to be affected by that trough crossing the North Island on Thursday.

See my yotpak at for terms used.

Weathergram with graphics is

Weathergram text only and translator is


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