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

10 December 2017

Bob blog 10 Dec 2017



Compiled Sun 10 December 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 ENSO – We are having a LA NINA—but for how long?

El Nino and La Nina are opposite ends of an identifiable tropical influence on our weather. La Nina comes with cooler than normal seas along the equatorial eastern pacific, and is associated with stronger than normal trade winds and shifts the subtropical ridge (and its jetstream) away from the equator. El Nino has weaker than normal trade winds and warmer than normal seas draws the subtropical ridge closer to the equator. So, they are like opposite tilts of a seesaw. 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 next month or so.


BoM, Australia, has now declared this to be a LA NINA.

The central to eastern tropical Pacific Ocean has cooled steadily over last few months and are now at the La Niña thresholds (0.8 °C below average). See

However, the BoM have pointed out that this La Nina may be weak and short-lived—weaker than the last one in 2010-2012. It looks stronger than the La Nina we had around a year ago.


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 since August there has been a cool trend for NONO3.4, and a few weeks ago the seas were over 1-degree cooler than normal.

Weak La Nina is seen at



The 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.

Since August the SOI has been more than 0.5, and for two weeks in October and one week (so far) in December it was over the +1 threshold.

This all ties in with “early start to summer” experienced in NZ, with anticyclones lingering over the Tasman Sea and New Zealand mainland, bringing periods of sunny weather and light winds. After New year, these HIGHS may shrink, but still stay near central NZ, so that there may be periods of strong easterly winds over Northland. In the South Pacific, convergence zones should be further south than normal, allowing tropical lows to form near 15South latitude rather than 10S.

La Nina is seen at

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


The Models

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The mid-November edition shows the the cluster of data from the models for NINO3.4 temperature may have bottomed out in DJF (Dec Jan feb) and start rising a little in JFM 2018, and more so in FMA.

CPC/IRI predictions are at



No cyclones around at present.

There continues to be a burst of equatorail westerly winds from east of Malaysia to north of Papua New Guinea.

Looking at the weekly rain maps from and comparing last week and the week before, we can see pockets of intense convection around Thailand and Indonesia, but weaker than last week. This cluster of extra convection is expected to make its way eastwards and start appearing in the Coral Sea area ovr next few weeks. This is called an MJO (Madden Julian Oscillation).



SPCZ=South Pacific Convergence zone.

The SPCZ is hovering over the Coral Sea to south of Solomon Islands to Samoa to Southern Cooks. The low that is tonight at the end of the SPCZ and south of the Southern Cooks is expected to travel slowly south and deepen.

By mid-week a new convergence zone and Low is expected to form in the Coral sea, and by next weekend this Low may deepen as it travels from Vanuatu to Fiji, with SPCZ over Tonga. Avoid these developments.


Subtropical ridge (STR)

HIGH H1 is expected to travel east along 40S from south of Southern cooks tonight to south of Pitcairn island by end of this week.

HIGH H2 is forming in South Tasman Sea by Tuesday, and expected to travel onto southern NZ on Wednesday and stretch from North Tasman Sea across central NZ to south of Chathams by Friday. Then it may stall for a few days, or perhaps weaken away.


Tasman Sea/NZ

I think the sailors are now resting. If you would like more detail for your voyage, then check to see what I offer.


“I think my whole life has been one of sort of daring,

and sort of sailing against the wind instead of just going with the wind.”

John Lewis


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