Bob blog 9 July

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 9 July 2023

Weather regimes - SAM

Us humans love finding patterns in things. Our orbit around the sun is so
repeatable and predictable that we have "seasons". Watching weather maps
long enough, one can see that sometimes the same weather system may repeat
in clusters. We like to seek signs that predict changes.

Tonight, I'll blog about SAM, the southern annular mode. This is a measure
of the average speed of the westerly winds around the Polar vortex (the ring
of westerly winds that circle the planet between 50S and the Antarctic
circle (66 S). The value of SAM alters the north-south movement of this
vortex. A high positive value of SAM occurs when the air pressure over
Antarctica is lower than normal, and westerly winds in the polar vortex are
stronger than normal (note, the actual isobars over Antarctica are always
higher than those in the polar vortex, but SAM works with the anomaly
values, not the actual values). So, in a high positive SAM the polar vortex
is shifted southwards, and pressures is the mid-latitudes are higher than
normal, with weaker winds and settled weather.
Read more in blog.metservice.com/Southern-Annular-Mode

However, when SAM is negative, the polar vortex is weaker, and may spread
outwards, so that west to southwest winds over the mid-latitudes are
stronger than normal--- the chances are that there may be a POLAR BLAST.
Negative SAM phases can lead to a displacement of the westerly winds and
allow cold air outbreaks to extend further northward into the mid-latitudes.
Therefore, monitoring SAM can provide valuable information for forecasting
polar outbreaks.

SAM refers to surface winds 50S-66S. A proxy of SAM is available, namely the
AAO or Antarctic Annular mode (average 700hPa Z wind component 20S to 90S)
at
www.cpc.ncep.noaa.gov/products/precip/CWlink/daily_ao_index/aao/new.aao_inde
x_ensm.html
This jumped negative on 28 June and a polar blast brought much-wanted snow
to NZ ski fields. SAM has since stayed negative and there have been decent
snowfalls over SE Australia the last few days. Another minor polar blast is
expected to reach NZ around Wednesday as seen here on windy.com.

TROPICS
Cyclone BEATRIZ dissipated after skirting the Mexican coast near the port of
Manzanillo.
There are areas of potential development eat of Philippines and west of
Mexico, and two point of interest being closely watched, but no cyclones
tonight.
The MJO, a burst of extra energy in the tropics, remains weak and near the
Indian Ocean.

WEATHER ZONES
The South Pacific Convergence zone is expected to slowly rebuild over the
north Coral Sea ran extend eastwards to Samoa. A trough is expected to move
east across Tonga around 14 or 15 July and then further east the next few
days. If sailing from Tahiti to Tonga then maybe find safe harbour by time
the trough arrives. The southerly swells that reached as far as 17S last
week should be mostly gone this week.

HIGHS and LOWS
Low L1 is 980 off east of North Island and moving slowly southeast. Tues and
Wed may be the only OK days to depart NZ for tropics this week.
Low L2 is expected to travel across South Tasman Sea at 50S and on Wednesday
to go northeast towards Chatham Islands, followed by a cold southerly flow
over NZ. There
High H1 over central Australia is expected to slowly move into central
Tasman Sea after mid-week. There is expected to be a SQUASH ZONE of enhanced
SE trade winds on its northern side that may spread as far as Southern Cooks
by 20 July. AVOID.

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If you would like more details about 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
at bottom).
Weathergram archive (with translator) is at weathergram.blogspot.co.nz.
Contact is bob@metbob.com or txt 64 277762212
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