Bob Blog 29 July 2018



Compiled Sun 29 July 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, but leaning towards an El Nino.

The Atmosphere:

The tropics influence our seasonal weather, swinging from El Nino to La Nina, and occasionally hovering in-between (called neutral). The La Nina is caused by cooler than normal seas along the equatorial eastern pacific and it shifts the subtropical ridge away from the equator. The El Nino, with its warmer than normal seas in the target region, 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.

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 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.

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, over the past two months, the SOI has settled into a sub-zero state, but has not been strong enough to be called an El Nino, and so is deemed to be NEUTRAL.


Neutral conditions as 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 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 in mid-2017, and then a cool period until May this year. In the past two months NINO 3.4 has been around plus 0.5C, in NEUTRAL territory.


Weak La Nina as seen at

Waters beneath the surface are slightly warmer than normal, as seen at


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 over the next few months and then remain steady a few months and then ease back in 2019. SO there may be an El Nino event, and it should peak between October and February.


CPC/IRI predictions from

Latest SST anomaly map shows warm yellow waters across the central Pacific.


Sea surface temperatures across the Pacific on 10 May from

In summary, it seems that there may be an incoming El Nino, and it should peak late in the year. This is likely to cause the subtropical ridge between New Zealand and the Tropics to shift to the north, and thus to encourage the cold west to southwest winds to be more noticeable than normal around New Zealand and the Tasman Sea during the coming spring.


After a busy July in the NW Pacific, conditions are returning to normal for the start of August.


As seen at

Rain eased over India and Japan last week, but the monsoon is active from Pakistan to SE Asia and over the Philippines






Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves (magenta). STR, and SPCZ. Pink area (none this week) = lightning likely (high CAPE)

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Vanuatu to the Tuvalu/Tokelau area.

A passing trough is likely over Southern Cooks on Tue 31 July reaching Austral Islands on Thursday 2 August UTC,

A passing trough is expected to cross New Caledonia on Mon UTC and travel east along 20 to 25S reaching Tonga on Thursday UTC and then fade.

Trough crossing NZ on Thursday is expected to develop a subtropical LOW near 25S 180 around Friday 3 Aug UTC that may travel slowly along 25S and deepen to BELOW north of 1000hpa on Sun/Mom 5/6 August UTC and then further east. Associated trough should reach Cooks by Sun 5Aug followed by W/SW winds. Avoid.


Accumulated rainfall for next week from

Subtropical ridge (STR)

Weak HIGH is expected to travel east across northern NZ on Monday and then off to east.

Next HIGH is expected to travel across southern Tasman sea on Thu and build to east of South Island from Sat 4 August.

The squash zone between this HIGH and the LOW near 25S should produce an easterly gale with 5m+ swells near 27 to 30S next week. Avoid.

Around Tasman Sea, NZ to tropics.

Ridge over NZ on Monday and trough on Tuesday NW flow on Wednesday and a trough on Thursday. Then an easterly to NE flow for late in the week with Low to northeast and HIGH to southeast. A week to stay put.

Tahiti to Tonga

May be able to so some Island hopping north of 20S until Wed or Thu, then should stay put. Incoming trough reaching Cook Island, including Suwarrow, by Sun 2 Aug UTC followed by W/SW winds


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