Bob Blog 28 Oct



Compiled Sun 28 Oct 2018

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 = trending towards an El Nino

The Ocean:

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 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.

Latest SST anomaly map shows warmer yellow/orange waters appearing around the Easter Equatorial Ocean between the Galapagos and the dateline.


Sea surface temperatures across the Pacific on 25 Oct from

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 2015, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, then a warm period until July 2017, and then a cool period until last June . The current warm period is intensifying and may soon reach the El Nino threshold.


Trend to an El Nino as seen at

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 last May the SOI has been mostly negative, consistent with a weak but rather persistent El Nino. In early October it reached -1 for a week, but it needs to meet or exceed -1 for a month to meet the El Nino threshold. The blocked high that affected New Zealand over the past few weeks has been one of the factors that has relaxed the SOI, but this is about to be replaced by passing troughs, and that should help the SOI to become more negative.


SOI trend as seen at

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

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to be near the +1.0C anomaly satisfying the definition of an El Nino during the next six months.


CPC/IRI predictions from


Latest cyclone activity as seen at


Super Typhoon YUTU ripped thru the Northern Mariana Islands during the past week.

And OSCAR may be going westwards for now but is expected to remain offshore.


Looking at the weekly rain maps, last week’s shows a relaxing of convection around Mexico and just north of Fiji, but an increase in activity about Papua New Guinea to the Mariana Islands (where YUTU formed). Not much change elsewhere.






Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), STR, and SPCZ. Pink area = lightning likely (high CAPE)

SPCZ=South Pacific Convergence zone.

The SPCZ is sitting over northern Coral sea and northern parts of Vanuatu across Wallis and Futuna to northern Tonga, and part of it may travel southeast to Southern Cooks this week.


Accumulated rainfall for next week from CZ= convergence zone.

Subtropical ridge (STR)

The remains of last week’s BFH are travelling eastwards along 30S well to east of NZ.

Next High is expected to form in the Tasman Sea near 30S by Tuesday and fade by Thursday as another forms near 40S. This other High is then expected to travel NE to 25S and to north of NZ over the weekend.

Tropics to New Zealand

Troughs are expected to cross northern NZ on Mon29/Tue 30 / Wed 31 Oct and then sun 4 Nov and then Wed 7 Nov and maybe Sunday 11 Nov. Arrange to arrive in-between troughs.

Between Tropics and Australia.

For those headed to Queensland or Coffs with the Down Under Go West Rally, now in its last week: Highs in the central Tasman Sea should maintain and useful easterly flow from New Caledonia to Australia this week. Winds may get over 20 knots this Thursday and again Wed 7 November.

For more info about the Go West Rally

From Tahiti to Tonga

There is a passing trough over Tonga on Monday UTC reaching Austral Islands by Wed UTC then moving off to the south. Should be a useful SE flow after this passing trough.


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