Bob Blog 26 March

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 26 Mar 2023

Switching between La Nina and El Nino.

Autumn is usually the season when the El Nino/ La Nina swing tends to occur,
possibly triggered by the equinox as the overhead sun switches from south of
the equator to north of the equator.

La Nina is the name given to the period when sea surface temperature SST
over the eastern equatorial Pacific is below normal. And during an El Nino
the SST is above normal.

In the atmosphere we monitor ENSO = El Nino/Southern Oscillation and its
impact on the weather map by using. the Southern Oscillation Index SOI (30
day running mean) This 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 index is
stays more than 10 for more than a month we call it a LA NINA event.
Since 2021 we have had three La Nino summers in a row.

And recent data is showing we are no longer in a La Nina and are now near
zero.
See www.longpaddock.qld.gov.au/soi/

The Ocean:
The parameter used from the ocean is based on the sea surface temperatures
in the equatorial eastern Pacific and called the NINO 3.4 SST anomaly.
The February observation shows a rising trend in the Nino 3.4 index.
International research Institute IRI have compiled the predictions for all
the models as shown here from iri.columbia.edu. The consensus is that
NINO3.4 will keep rising and be warm enough to start an El Nino period by
the start of May.

Expected impact in the South Pacific
An El Nino is associated with weaker trade winds, and this tends to shift
the subtropical ridge north allowing for stronger westerly winds south of
40S.

TROPICS
There are no named cyclones around. There is some potential for development
in the Indian Ocean.

WEATHER ZONES
South Pacific Convergence zone SPCZ

The SPCZ is from Solomons to Fiji. Low L1 now over New Caledonia is expected
to travel ESE and spread squalls to Tonga and Niue. Another convergence zone
is expected to linger between Pitcairn and Easter Islands.

HIGHS and LOWS
A trough over North Island tonight is expected to travel off to the east and
in the SW flow that follows by Tuesday low L2 is expected to develop south
of Chathams and then linger there until end of the week.
High H1 expected to travel east past south of Tasmania on Monday and then
along 50S across south Tasman and past south of NZ on Wednesday with a ridge
of light winds over Aotearoa NZ
Front and L3 expected to form off Sydney on Wed and then travel east across
the Tasman Sea on Thursday and Friday and then across Aotearoa NZ on
Saturday and Sunday .,

OK for sailing so long as avoid L1, L2 and L3.

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