Bob McDavitt's ideas for sailing weather around the South pacific

08 January 2017

Bobgram issued 8 Jan 2017

Compiled Sun 08 January 2017

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.

Are we having a La Nina?

The Atmosphere:
The Southern Oscillation Index SOI (30 day running mean) sums up the weather
pattern over the South Pacific as one number. It is based on the standardized
difference in the barometer readings between Tahiti and Darwin. A La Nina event
occurs when the SOI is more than plus one (standard deviation from its mean) for
more than a month.
Over the past few weeks the SOI has been positive, but has been in "neutral
This can be seen at

The Ocean: The target area for measuring the sea surface temperature as an index
is the eastern equatorial Pacific, and the index is called NINO3.4. This has
been cooler than normal for the past seven months but it now relaxing.

The monthly numbers for NINO3.4 show how the 2016 EL NINO compares with the 1998

SO, in summary, we are in neutral territory at present. Even so, the subtropical
ridge in the southern Hemisphere has been hogging 35S recently and is expected
to drift SOUTHWARD over New Zealand for the next month or so, and may visit
central NZ and the South Island mid to late February.

The Tropics are quiet at present. There are a couple of tropical depressions
near Asia and their possible tracks this week may be as seen at

Rain maps show that activity is on the increase around Indonesia and SE Asia,
steady around the Coral Sea, and decreasing elsewhere, as seen at

SPCZ=South Pacific Convergence zone.
SPCZ is active between the Coral Sea and Tonga. A small depression may move
along this zone and visit New Caledonia on Monday and be at 30S to South of
Tonga by Friday 13 Jan UTC.

Subtropical ridge (STR)
High in North Tasman sea on Sunday is expected to slip along 33S, north of NZ
mid-week, and then further east from Thursday.
A new High is expected to spread into the mid/south Tasman Sea on Thursday and
spread east along 37S across NZ on Friday UTC, followed by a prefrontal NW flow
on Sat 14 Jan

Tasman Sea/NZ Area troughs.
Disturbed westerly flow until Wednesday, then a FRONT is expected to cross NZ on
Thursday, followed by SW change on Friday 13 Jan, then another FRONT on Sunday.

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