Bob Blog 9 Aug

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 9 Aug 2020


State of the ENSO

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. 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 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.

Since a brief El Nino late in 2019 the ENSO has been roller-coasting between weak positive and negative values. For the last three weeks it has been positive (a La Nina trend). The sub-tropical ridge has recently been (on the average) rather static,


As 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 were warmish early this year but has been showing a cooling (La Nina) trend since May.


Sea temperatures, as taken from

Sea temperature anomaly map shows a spreading of cooler than normal seas.


Note how the cool Humboldt current has extended to the Galapagos.


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 AVERAGED model forecast is going for a weak La Nina during Sep Oct Nov (with a 55% chance) and it may linger to early next year


CPC/IRI predictions from

So, it looks that a weak LA NINA is coming soon


The Tropics

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

There are no active storms at present, but several areas of high potential for formation., especially west of central America.



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 likely to shift a little north this week and be from Solomons to northern Vanuatu to Tuvalu touching Samoa at times.

Passing trough over New Cal on Monday and S Tonga /Minerva on Tue/Wed and another over S Tonga /Minerva on Friday developing into a Low near Niue on Sat (local Friday).


Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH 1020hPa north of NZ travelling east along 30S on Mon/Tues.

Next High 1020hPa East of NSW on Tues gets to South Island by Thu then is expected to intensify to 1034hPa off eastern NZ by Sun/Mon.

Tasman troughs

Low 1008hPa east of Sydney tonight is expected to travel east to Northern NZ on Tue/Wed dropping below1000 hPa, followed by strong southerly over NZ on Wed.

Another low 1008 is expected to form east of NSW by Sat 15 Aug.

From Panama: ITCZ 10N to 5N, NW winds in Panama for starters until Wed, but W/SW winds from 7N 80W to 90W.


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