Bob Blog 12 Sept 2021

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.

Compiled Sunday 12 Sept 2021

The Status of the ENSO

La Nina may be coming back

What is a La Nina?

When the Pacific trade winds are stronger-than-normal they drag sun-warmed sea surface to the west, encouraging upwelling of cooler deeper water around the Galapagos. This can go on for months and is called a “La Nina” episode. It shifts the subtropical ridge away from the equator.

The reverse of this, with less trade winds and warmer-than-normal seas around the Galapagos, is called “El Nino”. The comings and goings of these episodes 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:

The sea surface temperatures in the equatorial eastern Pacific are near-normal, but there is a zone of cooler-than-normal sub-surface water that indicates conditions are starting to edge towards La Nina.

The graphs below are from



The Atmosphere:

ENSO = El Nino/Southern Oscillation. The Southern Oscillation Index SOI (30 day running mean) sums up the whole weather pattern over the South Pacific into 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. These graphs from multiply the SOI by 10, and show a LANINA episode (>10) from Nov 2020 to Feb 2021., then near-zero March to June then a strong positive swing in July and a relation in August.


The next graph zooms in on data for the past month showing a positive swing .


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

The NCEP CFSv 2 and North American Multi-Model Ensemble show that a weak LA NINA is likely in the next few months with a 70-80% chance of La Nina from Oct 2021 to Feb 2022.


So we seem to be on the edge of a new La Nina. At this time of the year the Tasman Sea / NZ area is having its usual spring weather pattern with a ridge /trough cycle and strong NW and SW winds, with each trough followed by a burst of SW swell reaching into the subtropics. If this La Nina clicks in as forecast then these troughs may weaken and shift south bringing this area an early start to summer-type conditions. NZ’s climate agency NIA is fore casting larger than normal HIGH pressure systems about and east of NZ for the next few months, and this is consistent with an incoming La Nina.


The latest cyclone activity report is at and Tropical Cyclone Potential is from


CONSON and CHANCHU (Kiko) formed over the Philippines. CHANCHU as Cat 5 for a while and is now travelling northwards off Taiwan and heading for Shanghai.


Weather Zones Mid-week GFS model showing isobars, winds, waves(magenta), Rain (Blue),

STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (in pink)


CAPE mid-week as seen by ECMWF and GFS from


SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomona to Tuvalu to Samoa to Southern Cooks.

Trough to north of L1 is expected to reach New Cal by Wednesday and Tonga area by Saturday.


Rain Accumulation next five days from


HIGH1 above 1024 to NE of NZ is moving ESE along 30 to 40S followed by H3 travelling along 30S. this maintains useful trade winds this week between Tahiti and Niue.

LOW1 is expected to form offshore of Coffs by Tuesday and deepen to 995 by Wednesday in mid Tasman then travel ESE and weaken east of Northland by Saturday. Avoid.

LOW2 below 970hPa at 55S is bringing a NW  gale to Canterbury tonight and its trough is travelling east of NZ next few days.

HIGH2 over 1024 over Tasmania by Wednesday is expected to cross the South Tasman and get east of the South Island by Saturday, combining like an eggbeater to produce a vigorous SE flow over eastern North Island and thru Cook Strait.


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