Issued 23 Sep 2012

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.

The Ocean: Only small changes have occurred in the sea surface temperature anomalies in the equatorial mid Pacific during September so far. Generally things have stalled at around the plus 0.5C mark. So the recent trend towards El Nino is now on hold.

The Atmosphere: The Southern Oscillation Index or SOI is rebounding after August’s dip. On 25 Aug its 30-day running mean was -1.01, and since then it has risen to plus 0.46 on 21 September. So the atmosphere keeps oscillating about and is not ready yet to dive into an El Nino episode. Even so, computer models are indicating that El Nina may slowly arrive by end of October.

Tropical cyclones: Things are still ticking over with JELAWAT east of the Philippines and MIRIAM off the west coast of Mexico.

In the South Pacific, the main branch of the South Pacific Convergence Zone SPCZ is now deactivating as it is stretched out across Solomons to Northern Vanuatu to Fiji to Tonga /Southern Cooks. There is another convergence zone from Tuvalu (weak) to Samoa to between Northern and Southern Cooks to the SE.


High H1 east of NZ near Chathams tonight is expected to travel steadily eastwards mainly between 30 and 40S this week, and maintain a SQUASH ZONE of enhanced trade winds to its north at around 18 to 28S as it wanders eastwards.

High H2 is expected to move across the northern Tasman Sea from Tue 25 Sep. This is likely to divide into two. One part is likely to cross central NZ on Thursday and t’other is likely to go north of the North Island over the weekend.


Situation at 26 0000UTC, showing position of terms used in the text.

NZ/Tasman Sea

A broad trough with several small features should slowly cross the country from Tuesday to Thursday, proceeded by a northeast flow on Monday and followed by a SE flow over the North Island on Friday. One of the features in this trough is likely to bring large WSW swells onto the South Island west coast on Wednesday 26 Sep. Otherwise, it is an OK week for sailing towards NZ.


If you are planning to sail from Tonga/Fiji/Vanuatu/ New Caledonia to NZ in October/ November, and looking to buddy with someone, then you may be interested in checking out the ICA’s "All Points Rally" to Opua, see .


We had an equinox (Latin for equal night, but it is misnamed) yesterday 22 1449UTC. That’s a special moment in the earth’s orbit of our sun, and it fittingly picked by many to mark the start of the Southern hemisphere SPRING.

Many in NZ, for practical reasons, such as climatologists who measures averages, etc. on a monthly cycle, take the months of September, October and November to be Spring, but astronomers, for logical reasons, use the four ‘corners’ of the earth’s orbit as markers (equinoxes and solstices).

The best definition of the equinox is when the sun, as seen from earth, is directly overhead the equator.

As viewed from earth the sun is directly overhead a different latitude every day of the year. The difference between this latitude and the equator is called the sun’s declination, as all navigators know already.


On the day of the equinox some say it’s a 12 hour day everywhere—that’s true only if you measure sunrise and sunset using the mid-point of the sun. Sunrise and sunset tables actually use the top limb of the sun so that adds an extra 5 to 8 minutes of (rather dull) sunlight. The name given to the day with 12 hours exactly sunlight/darkness is the equilux – in Auckland that was 19 Sep (within 20 seconds).

From now on the days will lengthen. But the rate is not linear—Auckland’s dawn is getting earlier at the rate of 10 minutes per week at present, but dusk is only getting later at the rate of 6 minutes per week. At the equinox the delimiter (line on the earth dividing day and night) is shaped like a square wave and during the next few months or so it slowly reshapes into a sine wave. You can watch this by downloading the program Home Planet from and watching this in the animate mode.


The Delimiter is a square wave at the equinox

At this time of the year the days are getting longer at the rate of around an hour per month. It is thus a good time to do a switch to Daylight saving, and indeed in NZ we SPRING our clocks ahead an hour at 2am next Sunday 30 Sep. This rate starts easing from now onwards, but the extra sunlight is now affecting the weather patterns.

Our spin axis is NOT at a right angle to our motion around the sun: the 23 deg tilt influences our heating cycle. OK it nods the southern hemisphere closer to the sun than the northern hemisphere for the next six months – and we are now approaching the flatter side of our elliptical orbit for the next six months, but the MAIN FACTOR is to do with the concentration of sun beams over a smaller area (and, in the northern hemisphere, the spreading out over a wider area. A good explanation is given at


December Solstice, with the southern hemisphere hogging the sunlight.

There is a lag between the change in increasing sunlight and the spot with the warmest temperatures on earth. The lag in the ocean is a few weeks more than in the land. Currently the warmest SST band in the open part of the Pacific is at around 8 deg N.


Sea surface temperatures. The warmest isotherms are coming south. Click to enlarge

Note that the 28C isotherm is the South Pacific already extends as far as between Samoa and Fiji, and is extending southwards at a steady rate. This is the time of the year that the subtropical ridge—that zone which divides the trade winds from the roaring 40s- starts to drift southwards. As it drifts south, the isobars on its southern side may bunch closer together in places, and that increases the disturbed westerly winds in the parts of the roaring 40s. This period, known as equinoctial gales, is likely to last until mid-November.


See my yotpak at for terms used.

Weathergram with graphics is at

Weathergram text only


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Published by metbob

Pattern and Chaos

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