Bob Blog 12 Sep

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

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 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 longpaddock.qld.gov.au 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 relaxation in August.
There has been a positive swing so far in September.

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 NIWA is
forecasting 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.

TROPICS
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
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.

HIGHS and LOWS
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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