Bob Blog 11 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 11 September 2022

La Nina or not?

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

The La Nina, caused by cooler than normal sea surface temperatures along the
equatorial eastern pacific, (STEEP), shifts the subtropical ridge away from
the equator and strengthens the trade winds.

The El Nino, with warmer than normal seas, draws the subtropical ridge
closer to the equator, weakening the trade winds.

Their comings and goings can last several months, maybe years, and so their
status can be used to help forecast the weather for the coming season.

Isobars
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 across the South Pacific into one number,
based on the number of isobars between Tahiti and Darwin, The SOI is the
normalised Tahiti minus Darwin barometer reading. When this is more than
plus one there is usually a La Nina event.
See www.bom.gov.au/climate/enso/wrap-up/

LA NINA EVENTS: We had it strong from Dec 20 to March 21, weaker peaks
July/Aug 2021, and Nov 21 to Jan 22, and a strong again May to July 22. We
are now "in-between events" with the SOI hovering around plus one. The SOI
has been mostly positive since Aug 2020, and some meteorologists are
wondering what this means --- see www.nature.com/articles/d41586-022-01668-1
and reliefweb.int/report/world/el-ninola-nina-update-august-2022

The Ocean:
The sea surface temperatures in the equatorial eastern Pacific have been
cool for a while but are currently mixed with a warm tongue between
Galapagos and Peru.

In passing, the warm seas in the northern hemisphere are remarkably red.
territory.

The warmer than normal ocean between Indonesian and Australia is a NEGATIVE
IOD (Indian Ocean dipole). This is associated with a wet Spring for eastern
and southern Australia and usually tugs the monsoon eastward off India (but
maybe not at present).

Sea temperature anomaly is at psl.noaa.gov/map/clim/sst.shtml

NINO3.4 is the parameter mostly used from the ocean for watching ENSO and
recently this has been hovering on the threshold,

Other factors now considered are the sub-surface temperature, the height of
the sea, cloudiness, and the trade winds. The combination of several
parameters and models are then summarised into a forecast for NINO3.4 for
the months ahead. The trend is looking neutral. See
www.bom.gov.au/climate/enso/wrap-up/

TROPICS
The latest cyclone activity report is at tropic.ssec.wisc.edu and Tropical
Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

EARL has wandered along the Gulf stream to the North Atlantic. MUIFA is
travelling north to east of Taiwan and FIFTEEN is also in the NW Pacific.

WEATHER ZONES
The SPCZ stretches from PNG across Solomons to Samoa and fades further east
by southeast. A trough in the Coral Sea is expected to deepen into a Low
(L2) over New Caledonia by mid-week and then travel southeast towards
Northland by Saturday. Avoid arriving in NZ around 23/24 Sep.

HIGHS and LOWS
HIGH H1 east of Aotearoa NZ expected to travel east-northeast from 40S to
35S.

Low L1 expected to travel southeast across central Aotearoa NZ on Monday
followed by strong southerly flow on Tuesday with rough seas.

High H2 is expected to travel along 45s into Tasman Sea by mid-week and then
across southern Aotearoa NZ by end-of-week, with a squash zone of easterly
winds with L2 on its northern side.

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If you would like more detail for your voyage, then check metbob.com to see
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