Bobgram 28 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 28 September

A SSW (sudden stratospheric warming) in Antarctica.

The following notes are from https://www.weatherzone.com.au/news

The Polar vortex

The polar vortex extends from the surface up to the stratosphere, although
meteorologists typically refer to it in two distinct sections:

The stratospheric polar vortex sits about 10 to 50 km above Earth's surface.
This section of the polar vortex is strong and typically shaped like a
circle or oval. The stratospheric polar vortex does not directly affect
weather near Earth's surface.

The tropospheric polar vortex extends from the surface up to the bottom of
the stratosphere, spanning the lowest 8 to 12 km of the atmosphere. The
tropospheric polar vortex can directly influence weather patterns across the
southern and northern hemispheres by changing the shape and behaviour of the
polar jet stream.

Sudden stratospheric warming

The stratospheric polar vortex can become weakened and displaced by a
phenomenon called Sudden Stratospheric Warming (SSW), which refers to a
rapid increase in temperature in the polar stratosphere triggered by sinking
air. These stratospheric warming episodes sometimes filter downwards through
the atmosphere and can then   disrupt the tropospheric polar vortex, causing
it to weaken and change shape.

When stratospheric warming causes the tropospheric polar vortex to weaken,
we usually see the polar jet stream become wavier and expand towards the
equator, which allows nodes of cold air to spread away from the polar region
into the mid-latitudes.

SSW episodes are common in the northern hemisphere, but they happen less
frequently in the southern hemisphere, where the polar vortex is stronger.
However, an episode of stratospheric warming is currently occurring above
Antarctica.

The animation below shows the evolution of the current stratospheric warming
in the southern hemisphere.

Video: Temperature and geopotential height of the 10 hPa pressure level
above Antarctica over the last three months, showing a previously stable
polar vortex being disrupted by stratospheric warming in September.

If the current episode of stratospheric warming filters downwards through
the atmosphere, it could cause the tropospheric polar vortex to weaken,
allowing southern hemisphere's polar jet stream to drag cold air away from
Antarctica. This jet stream pattern is associated with a negative phase of
the Southern Annular Mode (SAM)

I use the AAO (AntArctic Oscillation) as a proxy for SAM and indeed the GFS
ensemble patterns are showing it may well be negative next few weeks:

www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/aao_index_en
sm.shtml

This would make the following conditions more likely in Australia/ South
Tasman Sea/South Island of New Zealand during late September and early
October:

More westerly winds= wet over the Southern Alps and hot and dry for
Canterbury   plains.

Higher chance of abnormally hot days in southern Australia

Weatherzone's meteorologists will be keeping a close eye on the polar vortex
over the next couple of weeks.

TROPICS

The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu
and Tropical Cyclone Potential is
from https://www.ospo.noaa.gov/products/ocean/tropical/tcfp/

• Super Typhoon Ragasa became the world's strongest tropical cyclone of the
year, leaving at least 17 people dead in Taiwan and 10 others in the
Philippines. It later pounded Hong Kong.

 • Typhoon Neoguri churned the waters of the northwestern Pacific, while
Hurricane Narda remained far off Mexico's coast.

 • Hurricane Gabrielle and Tropical Storm Humberto passed over the Atlantic.

Weather Zones

 Mid-week GFS model showing isobars, winds, waves (purple), rain (red), MT
(Monsoonal trough), STR (Subtropical Ridge), SPCZ (South Pacific Convergence
Zone) CZ (Convergence Zone)

 Rain accumulation this week from Windy.com below is very similar to last
week.  It shows well defined SPCZ between Solomon Islands and Funafuti and
again east of Samoa. It may shift south onto Samoa late in the week.  A
passing trough is expected over the Southern Cooks and Austral Islands

There are several passing fronts in the roaring 40s.

Wind accumulation from windy.com below shows disturbed westerlies south of
30S, typical of spring.  Some wind in the Coral Sea but no squash zone s in
the tropics. Maybe a lull around Suwarrow Island.

LOWS and HIGHS

There is a steady easterly migration with Low 1 going SE and High 1 slowly
following.

L2 and L3 and a series of associated fronts cross the NZ area.  The main
fronts over Northland are expected on Monday night and on Friday/Saturday,
then Wed 8 oct. (avoid arriving then).

As for Australia HIGH H2 is expected to spend the second half of the week
traveling east along 32S onto South Australia.

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