Issued 21 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 maintained a value over 1 for most of June, hinting at a La Nina. During early July it dropped to 0.64 by mid-month and rose to 0.77 by 21 July.


SOI as shown at, showing a hint of a hump and now 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. So far this year NINO3.4 has been on the cool side, but rather weak.


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


Strangely quiet in the tropics as we enter the dog days of northern summer, the hottest time of the year.

Over Asia, SW/W Monsoon wind and rain are crossing Indonesia and have reached the southern Philippines. In a week or so as these equatorial westerlies stretch to Micronesia , Tropical cyclones are likely to form.




As shown at



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

South Pacific Convergence Zone SPCZ

The SPCZ is lingering around the Solomons and has a weak branch in the Coral Sea. It was very active over Vanuatu last week and over just north of Fiji this weekend and expected to be active over Samoa /Tonga next few days along with a branch that stretches along 15S to French Polynesia. This branch is expected to drift south and weaken late in the week.

A low is forming over Minerva/Kermadecs region tonight, between NZ and Tonga, and going quickly south on Tuesday, leaving behind a trough on Wednesday, but the easterly winds should reform in this area late in the week.

Sub-tropical Ridge STR

Large High well east of NZ is expected to expand over next few days with a squash zone of enhanced easterly winds on its northern side from French Polynesia to Southern Cooks. Avoid.

Another HIGH is expected to start moving from South Australia to central Tasman Sea on Wednesday and cross central NZ on Thu/Fri/Sat 25/26/27 with light winds. There may be a squash zone of strong easterlies on the northern side of this High over northern NZ on Sun/Mon 28/29 July as a low forms to NE of NZ near 30S 180.

Roaring 40s and New Zealand

LOW/trough crossing Tasmania tonight is expected to cross the South Island on Tuesday allowing a SW change to spread over NZ on Wed/Thursday.

Route Briefings

Tahiti to Tonga:

Too much wind and waves in the squash zone this week. Just spend the time on other cultural activities.

To/from NZ:

After that Low near the Kermadecs comes on Monday and goes on Tuesday, there is good weather for departure from NZ with the SW winds on Wednesday and Thursday, but get north of 30S before the easterly winds get strong south of there by Saturday.

Coming south, the weather is not looking the best this week and its OK to delay.

Circumnavigation news

Jeanne Socrates made it — solo nonstop circumnavigation for 70 year old, breaking all sorts of records – reaching Vancouver, Canada on Monday 8 July—I should have mentioned this last week.

See . Endeavour such as this should be marked appropriately, and I ask my friends to record their appreciation by dropping off a small donation to Jeanne’s charity page for fighting cancer at

See my yotpak at for terms used.

Weathergram with graphics is

Weathergram text only and translator is


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