Compiled Sun 23 Sep 2018
Bob McDavitt's ideas for sailing around the South Pacific.
Disclaimer: Weather is a mix of pattern and chaos; these ideas are from the=
As an addendum to last week's summary of time zone changes and national hol=
Samoa also changes to Daylight saving on 30 Sep, from UTC +13 to UTC+14. Th=
is is so that Apia remains 1 hour ahead of Wellington all year round. (The =
only other place on the planet that uses time zone UTC+14 are the Line Isla=
nds, including Kiritimati, near equator 160W,almost south of Hawaii).
The state of the ENSO =3D neutral, with a weak hint of an El Nino
El Nino and La Nina are opposite ends of the swing of an identifiable tropi=
cal influence on our seasonal weather. During La Nina we have cooler than n=
ormal seas along the equatorial eastern Pacific , and this shifts the subtr=
opical ridge away from the equator. In El Nino, with warmer than normal sea=
s, the subtropical ridge shifts closer to the equator- trade winds are weak=
er and the disturbed westerly wind so the Southern Ocean can penetrate furt=
her north. Each episode may last several months, sometime over a year, and =
so their status can be used to help forecast the weather for the coming sea=
ENSO =3D 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 in one number. =
It is based on the standardized difference in the barometer readings betwee=
n 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 (stand=
ard deviation from its mean) for more than a month we call it a LA NINA eve=
nt, and when it stays more than minus one we call it an EL NINO event.
Since May we have mainly had a negative SOI, consistent with a weak but rat=
her persistent El Nino signature.
BoM states that "model outlooks continue to indicate El Ni=F1o is possible =
from late spring 2018."
Weak El Nino conditions is seen at www.farmonlineweather.com.au/climate/in=
(Note that in this graph on the vertical axis 10=3D 1 standard deviation)
NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a =
heat storage area during an El Nino or becomes cooler than normal during a =
La Nina. This plays with the heat budget of the atmosphere and thus with th=
e weather patterns.
At the farmonline web site we can see the trend in the sea surface temperat=
ure in the NINO3.4 area. The diagram shows the weekly temperature anomalies=
since Jan 2015, with the El Nino of 2015 looking like a hump on a camel. S=
ince then there has been a cool period late 2016/early2017, a warm period u=
ntil July 2017, then a cool period until June 2018. Since then we have been=
having a warm period, but remaining less than 0.5 degrees above normal.
Weak El Nino is seen at www.farmonlineweather.com.au/climate/indicator_enso=
The International Research Institute of the Climate Prediction Centre IRI/C=
PC compiles data from several ENSO prediction models. The model predictions=
for the Nino 3.4 SST anomaly is that the seas ae likely to gradually WARM =
during the rest of this year, but the mean of the predications has only war=
ming to 0.9 above normal--- not enough to be called an El NINO event (but c=
loser to it that we have been for a while).
CPC/IRI predictions are at iri.columbia.edu/our-expertise/climate/forecasts=
Latest SST anomaly map shows a large blue pool of cooler water west of Sout=
h America. Also, there are warmer yellow waters appearing around the Galapa=
gos. Not much ice (white) left in the Arctic at the equinox.
Sea surface temperatures across the Pacific on 20 Sep are at www.ospo.noaa.=
The number of tropical features is reducing after that early September peak=
Map of current storms is at tropic.ssec.wisc.edu/
TRAMI is heading for Taiwan
And KIRK may skirt Venezuela
Looking at the weekly rain maps, last week doesn't have the noticeable cycl=
one rain tracks that show in the previous week but shows more intense activ=
ity in the ITCZ across the Pacific and Atlantic. It also shows a resurgence=
of activity around the Bay of Bengal. NZ had some passing fronts last wee=
k, but Australia... remains... dry.
SPCZ=3DSouth Pacific Convergence zone.
The SPCZ has been hovering around the region between Solomons Island and no=
rth of Fiji for a few weeks, and is expected this week to drift south acros=
s northern Vanuatu and across Fiji/Tonga. By the end of the week a low may =
form south of Niue and travel off southeastwards, taking a passing trough e=
astwards along 20S reaching Southern Cooks by local Friday/Saturday.
And over French Polynesia a convergence zone is expected to help form a low=
near 20S 135W that moves off Southeastwards.
Subtropical ridge (STR)
HIGH often over 1030hPa to travel east across Tasmania on Tuesday and linge=
r over northern NZ from Thursday to Saturday then fade to NE of NZ next wee=
k. eats of NZ travelling east along 40S. Squash zone of enhanced winds on i=
ts northern side mainly north of 20S from Wednesday to Saturday.
Between Tropics and Tasman/NZ.
Avoid that SPCZ travelling south over Fiji by Wednesday and the squash zone=
that follows. SO, there are OK voyages from Tonga/Fiji on Monday, or for f=
ast boats on Tuesday, then it's a case of staying put.
Trough crossing NZ on Monday and then a Low deepening east of NZ by Tue mov=
ing off on Wednesday leaving NZ bathed in SW winds.
After the High, next trough for NZ is expected over southern NZ on Sunday/M=
onday, then a trough from the Tasman after Tuesday next week.
From Tahiti to Tonga
OK to depart after convergence zone ha gone. Anticipate a passing trough al=
ong the way late this week.
If you would like more detail for your voyage, then check metbob.com to see=
what I offer.
Or Facebook at /www.facebook.com/metbobnz/
Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe=
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is firstname.lastname@example.org or txt 6427 7762212