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

21 June 2015




Issued 21 June 2015

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.

As you read this give yourself a pinch and punch– not for the first of the month but the turn of the year, the Solstice is at 21 1648UTC, that’s when the sun’s declination stops going north and starts coming south, hooray. 

And the quote of the week goes to Pope Francis in his encyclical on Our Common Home saying “The biblical texts tell us to "till and keep" the garden of the world (Gen 2:15). "Tilling" refers to cultivating, ploughing or working, while "keeping" means caring, protecting, overseeing and preserving. This implies a relationship of mutual responsibility between human beings and nature.”

  I somehow think that cruising yachts, of necessity, appreciate the relationship between humans and nature.

Now, what is nature up to???

The Atmosphere:

The Southern Oscillation Index SOI is based on the standardized difference in the barometer readings between Tahiti and Darwin (30 day running mean) and thus sums up the weather pattern over the South Pacific in one number. The weekly update was lower than -10 for four weeks in May, enough to convince the meteorological community that we are now having an EL NINO episode. And in the past week it has gone slightly positive.   This may possibly show that the current El Nino is fickle – or that troughs around Tahiti are variable.

SOI is shown at


The Ocean

The amount of heat that is being stored in the sea in the Eastern Equatorial pacific has been above normal since late 2014, as measured by the NINO3.4index, and is currently well over the EL NINO threshold.

NINO3.4 is shown at, confirming an El Nino.


El Nino’s impact:  During an El Nino episode, weather patterns tend to be drawn closer to the equator. The South Pacific Convergence zone tends to be tugged north and east of its normal position and there tend to be more troughs over Tahiti.  The subtropical ridge in the southern hemisphere tends to be north of its normal position and this weakens the trade winds overall, but doesn’t stop the squash zones that can be found on the north side of the migratory HIGHS.

El Nino encourage the disturbed westerlies of the Southern ocean to be very active, especially in the southern spring (September to November).  It raises the planet’s atmospheric temperature overall but places like New Zealand have cooler air and sea temperatures.



During the past week, between my blogs. TC BILL came and went along east coast of USA.

A rather subdued TC KUJURA is about to make landfall on south China coast to west of Hong Kong.

The rain map for the last two weeks shows the Indian monsoon at its height and extreme rain to left and right of Solomons. The one-week-average over Tasman Sea looks climatologically tame but fell fast enough to swamp Whanganui, Western North Island, New Zealand.  Weekly rain signatures for past two weeks, may be  seen at




SPCZ=South Pacific Convergence zone.

SPCZ starts this week in normal El Nino position STTT= Solomons Tuvalu Tokelau Tahiti and by mid-week should have lifted south to Samoa.  The section over French Polynesia is expected to spread out and diversify, spreading light wind and odd squalls across much of the Tuamotu Islands. So if you are there then wait until late in the week for more steady winds. Tahitian troughs are an El Nino trait.


STR= Sub-tropical Ridge

STR has weakened since that 1-38hPa whopper last week.

The High entering the Tasman Sea today should take its time and fade and spread northeast to be between Fiji and NZ by Sun 28 June UTC. 

There should be strong squash zones on north side of his HIGH and another one travelling east along 45S to south of Tahiti.


Mid latitudes

Cold southerly is spreading north and likely to trigger the development of a low near Minerva on Monday UTC. Associate trough should travel east across Fiji on Monday, Tonga on Tuesday and reach Tahiti on Sunday UTC.


Departing from Australia to the tropics this week:

High crossing the Tasman Sea makes it too difficult. Maybe OK after trough moves off New South Wales on Thursday.


Departing from Northern NZ going north.

Low near Minerva is likely to result is a squash zone with strong winds and heavy swells at first.  Best to delay departure until at least Thursday.


See my yotpak at for terms used.  See my website at for information on tailor-made voyage forecasts– Feedback to Tell anyone you like that to subscribe they should email me.

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