Bob Blog 13 Sep

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.

Compiled Sun 13 Sep 2020

LA Nina watch

ENSO (El Nino /Southern Oscillation) update

El Nino and La Nina are opposite ends of the swing 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 strengthens the trade winds

The El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator, weakening the trade winds. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

The Atmosphere:

ENSO = El Nino/Southern Oscillation. 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 it counts the average number of isobars between them 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.

There has been a positive trend I the SOI over the last few weeks , heading for the start of a La Nina event soon.

The SOI can be seen at


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

The Ocean:

The sea surface temperatures in the equatorial eastern Pacific has been showing a cooling (La Nina) trend . It looks like the Atmosphere and Ocean parameters are “coupled” or doing much the same thing, so a La Nina event may indeed soon be occurring.

. It has recently stalled, but climate models show this stall is likely to be temporary


Sea temperature anomaly, taken from


Waters just beneath the surface are slightly cooler than normal. These trends are towards a La Nina.


The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model forecasts shows a 55% chance of weak La Nina for Sep Oct Nov. This is the trend of the average of the models.


CPC/IRI predictions from

So, we are currently “neutral”, but we on watch for La Nina trends.

The Tropics

The latest cyclone activity report is at and Tropical Cyclone Potential is from

Three Cyclones in the North Atlantic, PAULETTE, TD20 , an d RENE an d one inn Gulf of Mexico, Also a high potential for development off west Mexico coast.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.)


SPCZ=South Pacific Convergence zone.

SPCZ is moderate and slightly north of normal from Solomon Islands to North Vanuatu to Tuvalu /Samoa. Passing trough over Southern Cooks on local Sunday/Monday.


Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH 1028 to 1030 hPa crossing Tasman Sea Tues/Wed an d the travelling along 32S between Fiji an d NZ on Wed night/ Thursday, with a squash zone of rough weather over Fiji, easing from Friday.

Next High is expected to cross Tasmania on Friday and grow to 1032hPa in South Tasman on Friday/Saturday, crossing central NZ during the weekend.

Tasman troughs

Strengthening NW flow south of 40S on Monday, followed by a cold front on Tuesday with heavy rain for Southern Alps, then another front for all NZ on Thurs night/Friday.



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

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Weathergram archive (with translator) is at

Contact is or txt 6427 7762212


Published by metbob

Pattern and Chaos

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