Bob Blog 19 March 2023

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 Mar 2023


AUTUMN EQUINOX =START OF INDIAN SUMMER
The subtropical ridge is a belt of high pressure around the globe at about
30° south and is formed by the sinking of air that has risen in
thunderstorms around the equator. This overturning of the atmosphere is
powered by the heat of the sun, in a chaotic fashion because clouds
intermittently block sunshine. You can find it on any weather map by
drawing a line along the main ridges, connecting the centres of the main
highs. The subtropical ridge gets only passing mention in meteorology
textbooks, as the zone that divides the trades winds from the 'roaring 40s'.
But in the Australia/South Pacific weather kitchen the subtropical ridge is
the top chef. And, like any moody chef, it has rhythms.

As the sun appears to go north in our winter and south in our summer, the
subtropical ridge follows it. This explains our seasons. The roaring 40s get
their strength from the temperature difference between subtropical ridge and
pole. This difference is strongest at the spring equinox (late September),
when Antarctica is at its coldest and the flywheel of the roaring 40s
extends furthest north, covering Tasman Sea/New Zealand with 'equinoctial
gales'. The autumn equinox is this Monday 20 March at 21hr24min UTC=
Tuesday Morning in Oz/Nz, when the overhead sun crosses the equator. In the
month following this, the subtropical ridge tends to lap the South
Australian coast and is generally found near central New Zealand, giving
long periods of light winds and settled weather which some call 'Indian
Summer'.

The phrase 'Indian Summer' comes from a similar series of long periods of
light winds and settled weather in North America in their autumn months
(October/ November). There is no agreement to the origin of the expression
but the one I like the best is that when their subtropical ridge visits the
prairies the local Indians would, after the first frost, light fires in the
long grass when hunting bison and the calm conditions produced a smoke haze.


El Niño weakens the subtropical ridge and tugs it north. La Niña strengthens
the subtropical ridge and keeps it in the south. I think we are now
transiting from La Nina to El Nino, and that should encourage an 'Indian
Summer' during April.

In temperate parts of South America, the phenomenon is known as "Veranico,"
"Veranito" or "Veranillo" (literally, "little summer"), and usually occurs
in early autumn, between late April and mid-May, when it is known as
"Veranico de Maio" ("May's little summer") or as "Veranito de San Juan"
("Saint John's little summer"). Its onset and duration are directly
associated with the occurrence of El Nino.

TROPICS
For a change there are no named cyclones around. There are two small
tropical depressions L1 west and L2 well-east of Vanuatu but they seem to
be more likely to fade than develop. L2 may deepen as it travels southeast
and peak at 30S on Wednesday.

The next MJO is expected to start forming in the Indian Ocean during the
next few weeks and, with a lot of hand-waving, if this season's rhythm
continues that means it may reach northern Australia around start of April.
So, there is still time for another cyclone to form in the Pacific before
the send of the season

WEATHER ZONES
South Pacific Convergence zone SPCZ
The SPCZ is from Solomons to Fiji but expected to weaken this week. Another
weak zone should linger to south of Austral Islands. There is also a
convergence zone likely along 4S from 100 to 160W.

HIGHS and LOWS
High H1 that has been lingering near 40S and south of Tahiti is expected to
move off to the east this week.
A trough and low L3 is crossing South Island on Monday fand North Island on
Tuesday, followed by a cold southerly with large swells by Wednesday.
The Large High H2 should follow this trough and cross NZ on Thursday and
Friday
The next trough and Low L4 is expected be crossing New South Wales on
Wednesday and then travel east across the Tasman and reach Aotearoa /NZ by
the weekend.
OK for travelling west across the Tasman Sea if north of 25S.
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If you would like more detail for your voyage, then check metbob.com to see
what I offer.
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at bottom).
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Contact is bob@metbob.com or txt 64 277762212
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