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

26 August 2018

Bob Blog



 Compiled Sun 26 AUGUST 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 = neutral

 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 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 May 2018.  Ove the last three months we have been having another warm period. 


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 gradually WARM another half a degree during the rest of this year, and then relax slowly next year.  The most likely period for this parameter to reach +1 and be called an EL NINO is NDJ (November December January).  And that is the early part of our South pacific cyclone season.

 CPC/IRI predictions are 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 ofisobars 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.


For the past year the SOI has been mostly around plus 0.5 to plus 1.0, consistent with a weak but rather persistent La Nina. However, there have been several brief periods of negative SOI over the past six months and we are having another of these at present, indicating that there is a tendency towards an El Nino.


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

 Latest SST anomaly map shows the remains of a large blue pool of cooler water across the central equatorial Pacific. Also, there are warmer yellow waters appearing around the Galapagos.  The  Australia/Tasman Sea is getting colder than normal at the coldest time of the year, and so is the Humboldt current off western South America.

Sea surface temperatures across the Pacific on 23 Aug is at


In Summary: We are neutral but watching as parameters trend towards an el Nino. There is a good chance of a weak EL NINO for the turn of the year.  This should weaken the trade winds, allowing the South pacific Convergence zone and subtropical ridge to shift to the north a little--- this may disrupt cyclone formation to the east of the norm, somewhat lessening the likelihood near Australia and increasing it in the Fiji /Samoa area.  Still too early to be sure of this, so just keep it on the backburner for now, and don’t make any decisions based on that idea just yet.



TC LANE brought wind and rain to Hawaii, much as forecast. There are a couple of tropical depressions that seem to be vaguely moving westwards or north-westwards.

 After a week of heavy rain over Japan and Korea, the North Pacific is relaxing now,  and the Atlantic continues to remains quiet.

 Looking at the weekly rain maps we can see that the Asian monsoon is active between India and the Philippines, and the ITCZ is active across the Pacific and around Panama. South Pacific Convergence zone is weakening.





SPCZ=South Pacific Convergence zone.

 The SPCZ is expected to stretch from PNG and Solomon Islands to Samoa and from there to the southeast.  

 Broad trough to east of NZ is expected to travel eastwards and associated passing trough should reach Society Island around local Tuesday and Tuamotu Archipelago around local Wednesday.

 There is a jetstream over New Caledonia tonight and this is expected to combine with a trough deepening east of Australia on local Monday to form a trough over Vanuatu on local Tuesday, reaching Fiji on local  Thursday and southern Tonga on local Friday.

Subtropical ridge (STR)

 HIGH crossing NZ tonight is expected to is expected to move east along 35S to 40S this week and intensify to over1040hPa by end of the week. A squash zone of enhanced trade winds on the north side of this high is expected to peak along 12 to 20S between Tahiti and Tonga from local from local Tuesday to Saturday.  Avoid.

 This squash zone detracts from a comfortable voyage from Tahiti to Tonga this week., unless  you skirt around it via Suwarrow and avoid its peak on local Thursday.

 From Thursday to end of the week, Another HIGH is expected to travel east from Australia to South of Fiji along 30 to 20S.  Small squash zone is expected in Coral Sea on the northwest side of this High.


Around Tasman Sea, NZ to tropics.

Passing High with light winds on Monday.    Low is expected to deepen in Tasman Sea bringing strong northerly winds to northern NZ on Tuesday, then a front on Wednesday, and then the LOW over central NZ on Thursday and Friday.   It’s a stay at home week for NZ yachts, but there are good voyages from Australia to New Caledonia.   If sailing to NZ avoid the Wednesday front.


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

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