Bob Blog 11 Aug



Compiled Sun 11 August 2019

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 = neutral

The Atmosphere:

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.

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 March 2019 the SOI has been mostly negative, and in past few weeks it has been on the verge of an El Nino, but now it is relaxing.


As 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 that NINI3.4 has been warmer than normal for the past year , and verging on El Nino values in last October and March, but recently has been mostly trending downwards. Atmosphere and Ocean are getting back in phase,


Moving away from El Nino as seen at

The warm waters in the NINO3.4 area are just near the surface, and a temperature/depth graph shows cooler waters below. This speaks against any swing to El Nino in the near future.


Below sea level temperatures and anomalies may be seen at

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 stay as they are for the rest of this year. But three  models are predicting a swing to the cool side.


CPC/IRI dated 19 July from

Latest SST anomaly map shows swirls of cool blue water over the eastern Equatorial Pacific. The Northern Hemisphere is mostly above normal. In the Sothern hemisphere there are areas of cool water around Indonesia, Western Australia and Aussie Bight (Positive Indian dipole), the Eastern Pacific Ocean, and the Southern Ocean. From Fiji to east of NZ New Zealand, seas are continuing warmer than normal. However, Tasman Sea is returning to normal.


Sea surface temperatures across the Pacific on 8 August from



The latest cyclone activity report is at and TCFP tropical Cyclone Formation Potential at

It’s been a busy week in the Western Pacific with LEKIMA making landfall in China and now KROSA seemingly heading for Japan.

There are areas of potential development west of central America and near the Equatorial Eastern Atlantic.

Activity in the SE Asia area is expected to increase during the next week or two due to a Madden-Julian Oscillation event transiting the region.



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 expected to stay over Solomons and travel north of Samoa into the Tuvalu/Tokelau area.

Passing trough over New Caledonia today going southeast, expected to reach Minerva on local Monday night/Tuesday.


Accumulated rainfall for next week from

Subtropical ridge (STR)

The STR remains north of NZ this week, but a new High is expected to spread into the Tasman Sea near 30S from Wednesday, passing across the north of NZ on the weekend.

Tasman Sea /NZ/Aus

Disturbed westerlies for eastern Tasman/northern NZ until Wednesday. Should be able to depart for Tonga on Thursday or for Noumea by Saturday, but it is a no-go week again for NZ to Australia.

Looks Ok for Australia to New Caledonia with a Monday departure (going east at first and then motoring a lot), and for a Noumea to South Port departure from Wednesday.

Tahiti to Tonga

Looks good to go this week with trade winds.


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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