Bob Blog 20 Aug 2017



Compiled Sun 20 Aug 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:

El Nino and La Nina the opposite ends of an identifiable tropical influence on our seasonal 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.

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 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.

So far this year there have been an unsteady swings either way, and at present we are on the La Nina side of zero, but that has peaked so I think we are still in some more unsteady swings over the next few months, with the SOI basically hovering around zero. This means the subtropical ridge may wander a little north or south of its seasonal normal position, but not hover away from its norm long enough for us to change our cruising plans. Just keep an eye on that subtropical ridge.


An unsteady SOI is 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 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 then a warm period until July 2017. We are now in a near zero or 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 all of the models within -0.5 and +0.5.


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. This is likely to be more than normal, due to warmer than normal seas. It is too early at this stage to comment on the incoming South Pacific cyclone season starting later this year….. but the early indication is that, because of warmer than normal seas, there may be an increased risk.


Sea surface temperatures across the Pacific on 17 Aug from


At present tropical cyclone HARVEY is travelling west onto central america and KENNETH tavelling northwest off the west Mexican coas. There is a depression heading west into the China Sea and another on the Atlantic west of Africa that may travel west to Florida.

We can see these tracks at


Last week’s rain map, compared with the week before, shows a shift of intense rain from Myanmar to Indonesia, and an easing of rain in the NW Pacific. In the tropical South Pacific, the rain map shows a passing intense trough during the past week; as mentioned in last week’s weathergram. And there are continuing hotspots of intensity around Panama and Sierra Leone.

A link to the devastating mudslides in Sierra Leone is at



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 is expected to slowly spread southwards towards Fiji and Samoa this week.

Also a passing trough is expected to bring some rain to the Tonga and Niue area early this week and then fade away.


Accumulated rainfall from

Generically speaking, a passing trough in the South Pacific islands is preceded by a period of weakening northerly winds, accompanied by variable winds and showers and then followed by a period of S then SE winds. Satellite imagery shows that these troughs travel eastwards, and that may appear confusing at first to someone on the ground considering these wind changes. However, if we look at a time/height diagram of the wind flow over, say, Nuku’alofa, we can see there are lots more westerly winds aloft than easterly winds at the surface, and THAT is what explains the eastward travel of these passing troughs.


Time/height diagram over Nuku’alofa, as seen on, type in Nukualofa as the search box then select airgram.

Subtropical ridge (STR)

High is expected to build in the North Tasman sea on Monday and then travel onto northern NZ by Wednesday and then travel slowly to east of the North Island by the end of the week building to around 1029hpa. this should help maintain se trade winds between French

Between NZ and the tropics:

Low near Chatham Islands tonight is travelling off to the east. The southerly flow and swell on the backside of this low should weaken over northern NZ on Monday and Tuesday, then the outlook over northern NZ is for light winds on Wednesday to Friday, then we expect strengthening NE winds ahead of an approaching trough on Sat and Sunday

So, the best days for getting off to the north are Monday and Tuesday, and a departure on Wednesday may be OK as well.

Between Australia and the tropics:

A trough is expected to travel off the eastern seaboard and into Tasman sea on Tuesday and then travel east across the Tasman Sea and deepen into a low on Wednesday that should reach the South Island early next week, with associated trough reaching New Caledonia around Sunday and North Island around Monday 28 August.

Any travel form Australia to tropics should be on back side of that trough, and any travel from New Caledonia to Australia this week will need to encounter this trough.

French Polynesia to the west:

There is a passing rough over tonga tonight that is travelling east and expected to fade south of Niue during Monday UTC.

After that a slow-moving and not-very-intense High /subtropical ridge along 30 to 35S should ensure moderate trade winds and swells, good for sailing westwards.

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


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