Bob Blog 30 July




Compiled Sun 30 July 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 the ENSO

The Atmosphere:

You may have heard about El Nino and its opposite, La Nina. They are tropical influences on the weather: the La Nina shifts the subtropical ridge away from the equator and the El Nino 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 forecast the weather for the coming season.

A month or so ago we were on the brink of a possible El Nino, but latest data shows things are “unsteady”.

The main data from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words the placement of the 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.

For most of the period from May 2015 to May 2016 we had an extreme El Nino. Then there was a weak La Nina in October 2016, and since then we have had several fluctuations, plus then minus, each lasting around a month. For want of a better word, I’ll describe this as “UNSTEADY”. The latest SOI, last week, is plus .5 (or plus 5 in the units used in the farmonline site), and shows a continuing of this “unsteady” trend.

An unsteady SOI is seen at


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 the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2014, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017 , and a warm period so far in 2017. This warmth is getting less over last few weeks. It seems that the trend in towards a neutral state.

Neutral territory as seen at


The International Research Institute of the Climate Prediction Centre compile data from several ENSO prediction models. The overall trend is towards little change later this year, with most of the models on the side of a weak El Nino.


CPC/IRI predictions from

One seasonal prediction that can be made is that the number of tropical cyclones forming in the North Pacific Ocean over the next few months is likely to be more than normal, due to warmers than normal seas. It is too early at this stage to comment on the incoming South Pacific cyclone season starting later this year.


People have been noting warmer summer temperatures in the Northern Hemisphere, and Dr. Hanen and two colleagues have been extracting some bell curves for various decades to compare with the 1951-1980 epoch. See or /

— the New York Times doesn’t explain much about the methods used, but the PNAS paper gives a full account.


This is the peak of the heat for the Northern summer, and, sure enough , it’s busy with cyclones as we can see from, showing 5 cyclones = HILARY and IRWIN east of Hawaii, and NORU, HAITANG, and NESAT between Hawaii and China. Also an “invest” area. These storms are mostly oceanic, but for NESAT heading for Taiwan with HAITANG on its heels.


Last week’s rain map, compared with the week before, shows continuing high rainfall over parts of India and across SE Asia to the Philippines and an increase in rain around Solomon Islands. The path of cyclone NORU stands out.



Rain for the past fortnight from


Weather Zones (see text) as expected mid-week on Wednesday (GFS model) showing wind, isobars, current, lightning, Sig wave height purple lines, SPCZ and STR.


SPCZ=South Pacific Convergence zone.

The SPCZ starts this week draped from Solomon Islands to Tuvalu and Tokelau Islands, and slowly increasing in intensity.

A convergence zone /trough tonight stretches from Vanuatu to Fiji to Tonga containing variable winds and a few squalls. This is expected to travel southeast and deepen into a low near 30S on Tuesday UTC, that may then bomb (rapid deepening) to 972 hPa at 40S 40E by Thursday. Too far away to bother anyone, but may generate southerly swells to over 4 metres for the Gambier Islands by the weekend. Long period swells that are loved by surfers. Accompanying convergence zone is expected to pass to east of Rarotonga on Tue UTC and then stall and then return to south across Rarotonga on Sat 5 Aug UTC.


Subtropical ridge (STR)

High over NZ tonight is expected to travel northeast to 30S then eastwards at around 1020 hPa- not strong enough to enhance the southeast trade winds on its northern side.

Next High is expected to form over south Tasman sea on Tuesday and travel around the south end of NZ and then along 50S to 170W by the weekend.

Between Australia /NZ and the tropics:

Low is expected to deepen in Tasman Sea off New South Wales on Monday and then travel east across Northland on local Thursday and then travel east along 33S to east of NZ to the end of the week. It may be followed by another low deepening off the New South Wales coast on local Thursday. Avoid these lows/.

French Polynesia to the west:

The main challenge for going west this week is the trough/convergence zone that is expected to be crossing the Tongan area on Monday UTC. This zone is expected to continue southeast past Rarotonga on Tue/wed UTC and then stall and travel back southwards across Rarotonga around Sat UTC. Avoid.

This zone is not expected to have much impact any further north—a trip via Palmerston Island is likely to be mostly unaffected.

The SPCZ is expected to be confined to the Solomon Islands to Tokelau Islands area.

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


See my website for more information

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