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

17 September 2017

Bob Blog 17 Sep 2017



Compiled Sun 17 September 2017

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.

The state of the ENSO = weak La Nina trend now showing

The Atmosphere:

El Nino and La Nina are opposite ends of an identifiable tropical influence on our seasonal weather:  the La Nina, caused by cooler than normal seas along the equatorial eastern pacific. shifts the subtropical ridge away from the equator, and the El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator.  Their comings and goings can last several months, maybe over a year, and so their status can be used to forecast the weather for the coming season. 

The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words the placement of isobars on the weather map.  When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

So far this year there have been an unsteady swings either way, and in Aug/Sep the SOI has settled around plus 0.5 (or 5 units in the graph shown here), in the zone which some call a weak La Nina.  So, this is a hint that the subtropical ridge line may shift southwards faster than normal over the next three months.  The warming of spring/summer should reach Australia and NZ faster than normal.   Maybe this also means the tropical wet season may start earlier than normal.

Weak La Nina may be seen at

(Note that in this graph on the vertical axis 10= 1 standard deviation)

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino, or becomes cooler than normal during a La Nina.  This plays with the heat budget of the atmosphere and thus with the weather patterns.

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2014, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, and then a warm period until July 2017.  We are now in a cool period again. But only around half a degree below the old "normal"( maybe 1 degree below the new normal).

Weak La Nina is seen at

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models.  The early September probabilistic forecasts show the chance of La Nina is as high as 60% over next few months, dropping below 50% again early next year. CPC/IRI predictions from


The sea surface temperature anomaly map has been showing MOSTLY WARMER THAN NORMAL on planet earth for months now, so that climate change is beginning to warp out the seasonal influences. The warmer than normal seas in the Atlantic has been breeding stronger than normal cyclones, but it seem that HARVEY has managed to stir the Gulf waters so they are now cooler than normal.  Reminds us that tropical cyclones are one way this planet gets rid of excess energy. Without them, worse would happen.

Latest SST anomaly map shows the wonderful cooler eddies along the equator of a typical La Nina, but the ENSO values involved are only half a degree and so it’s only half-a-La-Nina, and not being called as such yet.  I’m wondering that if we could remove the climate change trend for the data, then maybe it’d exceed the La Nina threshold. 

Sea surface temperatures are at



IRMA has done its damage and JOSE is staying offshore,

LEE is Fading away,

Next in line is MARIA in the Caribbean,

And NORMA off Baja California,

and to west of NORMA is OTIS.

Meanwhile over in Northwest pacific, TC TALIM has turned away from Taiwan  and is now affecting Japan.




SPCZ=South Pacific Convergence zone.

The SPCZ starts this week from Solomon Islands to Samoa area, and is expected to drift South so as to visit Fiji and maybe northern Tonga this weekend sat/sun/mon 23/24/25 Sep.

A passing, mainly upper, trough, is expected to travel eastwards across Niue on local Monday night and Southern Cooks on local Tuesday/Wednesday, preceded by NE/N winds, accompanied by variable winds and showers, and followed by SW/S/SE winds.

Tropical accumulated rainfall for next week may be seen at


Subtropical ridge (STR)

HIGH moving off eastern Australia tonight is expected is expected to move northeast across the North Tasman Sea next few days, fading near the date line after Thursday.

Another HIGH is expected to move off eastern Australia on Thursday and travel across Northern NZ on Sunday 24 Sep and remain above 1022 and it travels east along 30S to east of NZ next week.

When these Highs are in the Northern Tasman Sea we can expect a weak squash zone in the Coral sea.


French Polynesia to the west:

The main obstacle this week is that travelling trough over Niue on local Monday and Rarotonga on local Wednesday. This trough may bring a day or two of variable winds south of 18S, but not much disturbance to the surface winds further north. Take into account that Tonga may receive some tropical showers early next week ( 25/26 Sep Local) from a visit from South Pacific Convergence zone.

Between NZ and the tropics:

LOW#1 is travelling Northeast across eastern NZ on Monday/Tuesday followed by strong squally SW winds and a burst of SW swell over 4 metres as far north as 28S. Avoid.

Low#2 is expected to travel northeast across central NZ on Thursday and Friday followed by a period of strong SW winds and a burst of SW swell over 4 metres as far north as 30S. Conditions easing Friday night/Saturday morning.

Best dates to depart/arrive is either Wednesday or Sunday 24 Sep.


If you would like more detail for your voyage, then check to see what I offer.



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