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

14 July 2013

Bobgram issued 14 July 2013

Issued 14 July 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 rose to over plus 1 briefly in April, dropped to minus 0.39 in early May, and has since then bounced back positive, maintaining a value over 1 for most of June. By 14 July the SOI relaxed to 0.64 .

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. So far this year NINO3.4 has been on the cool side, but rather weak.

Remains of TC SOULIK, after clipping north end of Taiwan, are making landfall into China, and it is currently quiet elsewhere.
Over Asia, Monsoon rain has had a wet week over India and now is arriving over western Indonesia.
ITCZ is returning to its normal position and starting to cover the entire Pacific Ocean as a continuous band.

Lows in the Southern Ocean occasionally deepen and widen out so that they polar blasts northwards, and allow southern ocean SW swells to reach the tropics. A good parameter for monitoring the likelihood of these lows occurring is called the SAM or Southern Annular Mode, as studied at

A good proxy for SAM is the AAO or Antarctic Oscillation (something like the NAO parameter which explains a lot about European seasonal weather. When SAM or AAO drops negative quickly, as happened around the solstice /June 21st, there is a strong likelihood of a polar blast somewhere in the southern hemisphere—and last month it was New Zealand's turn.

SAM/AAO is currently positive, indicating that the southerly gale over NZ tonight is just a passing feature typical for the time of the year and not really a polar outbreak. It is forecast to be variable for the rest of July with a 50% chance of a polar outbreak (or not) between 20th and 30th July.

South Pacific Convergence Zone SPCZ
The SPCZ is expected by mid this week to split into three branches. One by then is from Papua New Guinea to Fiji, another from Solomons to Wallis/Futuna, and a third between Samoa and Suwarrow. The north end of a transient trough in the roaring 40s/mid-latitude sis expected to move across French Polynesia on Wednesday. By the end of the week the SPCZ is expected to stretch from Wallis/Futuna to Southern Cooks and further SE with a low 1008 forming on it near Borabora by Monday 22 July, and strong winds on its southern and eastern sides.

Sub-tropical Ridge STR
Large High already over central Tasman Sea is a slow-move rand should finally cross central NZ on Thursday/Friday (frosty, but with westerlies on south if STR warming Otago for a change). There is a squash zone of enhanced SE trade winds on the north side of this High. It has been in the Coral Sea for the past week and should travel east with the High this week and get to Fiji Tonga by end of the weak (somewhat weakened as the High weakens).

Roaring 40s and New Zealand
There is a vigorous south/southwest flow over NZ tonight between lows that are north and south of Chatham Island and the High I the Tasman Sea. The High should bring a period of light winds to northern New Zealand. The next trough is expected to breed a series of Lows. It is expected to cross Australian Bight this week, Tasmania on Thursday/ Friday, mid-Tasman Sea on Sat/Sun and NZ on Monday/Tuesday 22/23 July preceded by strong NW winds over NZ on Sat/Sun.

Route Briefings

Tahiti to Tonga:
The challenge on this voyage is to go through or around the SPCZ. The trough crossing French Polynesia on Wednesday is a nuisance as it replaces the trade winds with a period of fluctuating NE then NW winds. SE winds should return during Thursday UTC, but the likelihood of a SPCZ coming and a low forming near Borabora by Mon 22 July makes it a difficult time to go west.
As for going to Suwarrow along the northern route, this is looking OK so long as you get there by Sat 20 July UTC.

To/from NZ:
Departing from northern NZ looks OK on Tuesday and Wednesday – after the SW flow eases and getting north of 30S before the NE/N winds arrive there by Saturday.
Coming south, the weather is looking good for arrival from Thursday to Sunday 21 July. On Monday 22 the front end of the next trough is expected to start arriving.

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