Bob Blog 19 June

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 19 June 2022

The State of the ENSO
La Nina is lingering but has weakened
Since last August the Pacific trade winds have been stronger than normal and
dragging sun-warmed sea to the west, encouraging upwelling of cooler deeper
water around the Galapagos. This "La Nina" episode shifts the subtropical
ridge poleward
However, the Tasman Sea has recently had a series of lows and these this
week affecting the area east of NZ.

The Atmosphere:
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 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.
SOI can be seen at
www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi&p=weekly
The weekly SOI was as high as over +2 in May (making this May the 2nd equal
highest year on record since 1876) and is relaxing during June but still
well over +1.

The Ocean:
The sea surface temperatures in the equatorial eastern Pacific Ocean are the
action centre for ENSO (El Nino Southern Oscillation). This area has been
blue (cooler than normal) for LA NINA shows clearly in a time-longitude plot
from http://www.cpc.ncep.noaa.gov shown here. The rippled pattern is an
artifact of west-moving eddies. There seems to have been a relation during
the last month
These cool waters are surrounded by a ring of warmer waters across central
North Pacific, around Indonesia and then across central south Pacific. A
warmer than normal Tasman Sea has been producing extra lightning for NZ
recently.

According to the International Research Institute of the Climate Prediction
Centre,
at iri.columbia.edu/our-expertise/climate/forecasts/enso/current/, reviewing
all of the different models for forecasting the future of this La Nina, it
is expected to strengthen in a few months and then weaken by end of the
year.


Impact on South Pacific
Trade winds are expected to be reliable between Galapagos and Marquesas.
Having the subtropical ridge further south than normal during winter should
keep the winter lows south of normal as well, but this hasn't been the case
in June.

TROPICS
TBLAS formed south of Acapulco and is travelling west.
CELIA formed offshore of El Salvador and is travelling west northwest.

WEATHER ZONES
The SPCZ stretches across the Coral Sea to northern Vanuatu to Samoa.
A convergence zone/trough is expected to trave east across Fiji on
Monday/Tuesday and reach the Cooks by end of the week.
The best route this week from Tahiti westwards this week seems to be the
northern route, and may need to deviate around the trough

HIGHS and LOWS
The upper trough from last week is now east of NZ and expected to form L1
that deepens as it travels SE.
L2 has also formed between New Caledonia and New Zealand in tandem with L1
and is expected to travel of to the ESE then East.

H1 is the surface sign of a long wave ridge and expected to travel northeast
across central NZ by mid-week.
Between H1 and L1 , ice-chilled air is expected to be strongly shovelled
northwards onto eastern NZ

Weather pattern looks OK this week for crossing the Tasman Sea.
For those wanting to sail from NZ to the tropics, this may be done on the
backside of L2 once the swell has eased.
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If you would like more detail for your voyage, then check metbob.com to see
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