MetBob

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.

The tilt and rotation of the Earth relative to the sun cause huge global circulations.(ABC Weather: Kate Doyle)

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

The latest cyclone activity report is at zoom.earth and tropic.ssec.wisc.edu and Tropical Cyclone Potential is from www.ssd.noaa.gov/PS/TROP/TCFP/index.html

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

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) and CAPE (lime)

CAPE maps mid-week GFS and EC from Predictwind, showing chance of lightning.

Rain Accumulation next five days from windy.com

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 tp 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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