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

20 January 2019

Bob Blog 20 Jan 2019




Compiled Sun 20 Jan 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.


An addendum to last week’s blog about barometers is that there is a good website prepared by David Burch at that discovers web links to the ten closest barometers near your location and allows you to interpolate between them.


The state of the ENSO = on watch for an El Nino, but signs are fading.

The first sign to check when looking at the state of the ENSO index is the sea surface temperature

Latest SST anomaly map shows warmer-than-normal sea all the across the equatorial Pacific. Of note, there is also a warm stretch from Solomon Islands to French Polynesia and another form the Australia Bight to central New Zealand. Also of note; the Gulf Stream off the east coast of North America is much warmer than normal. Overall, the yellow/red outweighs the blue.

Sea surface temperatures across the Pacific on 17 Jan are at


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.


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 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 2016, show a cool period late 2016/early2017, and again in late 2017/early 2018. A warm period started in mid-2018 and briefly reached the El Nino threshold in late 2018, but is now fading.

Weak El Nino is seen at


The Atmosphere:

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.

Over the past year, the SOI has fluctuated up and down. There was a period of negative SOI back in September/October 2018, but during December the SOI went positive, indicating that atmospheric pattern is out-of-sequence with the oceanic pattern.

Uncoupled conditions is seen at

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


CPC/IRI predictions from

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 the same or slightly cool during the next few months, so that there is a decreasing chance of an El Nino event.

The performance of these statistical and dynamical models since 2017 may be seen from the IRI website and shows an interesting amount of variation.




Latest cyclone activity and TCFP tropical Cyclone Formation Potential as seen at or

There are two tropical depressions around at present: 01W over the Philippines and 10S between Madagascar and Mozambique.

Models now have the idea of a developing tropical lows off the northwest and northeast of Australia later this week.



SPCZ=South Pacific Convergence zone.

The SPCZ is active from Norther Australia to Solomon Islands and then stretches across the South Pacific almost all the way to Tahiti. A low is likely to form to south of Rarotonga after wed UTC and take some of the convective energy from the SPCZ southwards to higher latitudes.


Subtropical ridge (STR)

HIGH crossing Tasmania tonight is expected to travel slowly NE across the Tasman Sea by Wednesday UTC, and then fade as a ridge along 30S during Thursday UTC.

Next HIGH is expected to travel east along 45S past Tasmania on Wednesday UTC and then across central/northern NZ by the end of the week.


Australia/Tasman Sea / New Zealand

With the Highs crossing the Tasman Sea, the main wind flow north of latitude 35South is from the east, good for sailing from New Caledonia to Queensland. Any vessels wishing to get from Australia to NZ can possibly motor thru the High near 35 south or sail in disturbed westerly winds and sometimes large swells further south.

Arrange arrival in New Zealand between fronts. One front is expected to reach NZ on local Monday, another on local Wednesday/Thursday, and the next on local Saturday/Sunday.


Panama to Marquesas

Looks OK to go any day this week with northerly winds for starters and a good SW going current to 2N 85W. From there either peel off to the Galapagos Islands or follow the equatorial current to 120W and then head for Marquesas Islands.


King Tides

Note that there is full moon tonight (and a total lunar eclipse visible from North and South America. This month’s lunar perigee (moon closest to Earth) is Monday 21 Jan 1959hr UTC, so there are some King Tides later this week (more extreme high and low tides). Take this into account in your travels.


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

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