Bobgram 8 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 Sunday 8 September 2024

The disturbed westerlies of spring

The annual climate of Hs (Significant wave height) around the planet
follows the seasons with a winter peak and summer minimum. As seen at
www.researchgate.net/figure/Climatological-distributions-of-the-averag
ed-Hs-MAS-significant-wave-height-simulated-by_fig3_370153387

However, in springtime there is the disturbed westerly pattern .

Since mid-August we have been having several examples of the weather
pattern termed "disturbed westerlies". On the weather map over
Australia /NZ and the South Pacific (south of the subtropical ridge)
we usually have a High and ridge followed by a Low and trough then
another High and so forth.

As we approach the equinox, the sun dawns again around the Antarctic
circle. Just as the coldest time of the day is around dawn, so it is
that the coldest time of the year around Antarctica is around the
equinox. This then is the time of the year when there is the strongest
temperature difference (delta T) between tropics and pole in the
southern hemisphere. And it is simple physics that the stronger the
delta T the stronger the wind speed and hence the higher the swells.

With the stronger delta T the weather pattern in the midlatitudes
turns more topsy-turvy, and instead of the High -low-High-low parade
there may be three or four troughs between the highs. This is the
pattern called "disturbed westerlies"

This year the disturbed westerlies seem to have come early as the NW
winds from Australia bump into the southerlies from the Southern
Ocean. The extra friction has been producing extra static and extra
lightning in the Tasman Sea. This is consistent with an incoming La
Nina, and if that happens then there may be an early end to these
disturbed westerlies, maybe during November.

I asked Chat GPT to write a sonnet about disturbed westerlies.

The Winds of Spring in Tasman's Rolling Tide

When westerlies, disturbed, sweep through the sea,
They bring the breath of storms to ocean's flow,
As winds twist in wild uncertainty,
And the light of spring begins to grow

From warmer lands the northern breezes fly,
While polar chills rush in to clash and churn,
These winds contend beneath the shifting sky,
When orbit of earth and season steadily turn.

Across the Tasman waters swirl and rise,
Restless westerlies their course ignite,
A dance of air and sea before our eyes,
In spring's embrace, both fury and delight.

So: sailors watch these winds with cautious eyes,
For springtime's song is sung in stormy skies.

Not bad, but I had to edit some bits, Chat GPT came up with Spring's
dimming light????

TROPICS
We are having a lull at present after a busy period. Typhoon Shanshan
left seven people dead and around 130 others injured as one of the
strongest storms to hit Japan in decades. . Downpours in the northern
Philippines triggered by Tropical Storm Yagi killed at least 14 people
in landslides, floods and swollen rivers. Tropical Storm Asna spun up
over the Arabian Sea.

WEATHER ZONES
The South Pacific Convergence zone is sitting over Solomon Islands and
extends to Samoa and then to the southeast across Southern Cooks. This
zone has been producing small squally lows that bring over40kt for
between a few hours and a few days. Avoid.

HIGHS and LOWS

HIGH H1 is moving steadily east along 30 to 35S off to east of NZ.

Low L1 belongs to a trough that is crossing NZ on Monday then moving
off to the east followed by H2 on Wednesday

Low L2 is expected to belong to a trough that crosses NZ on Friday,
followed by a "disturbed westerly flow" which blurs the details of
next week.

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