Bob Blog

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 11 February 2024

SUBTROPICAL RIDGE
Occasional in summer we get today's wonderful pattern when the MJO takes the Monsoonal trough out from Australia and into the central Pacific:
During an El Nino period this helps feed the Hadley and Walker circulations to intensify the Subtropical ridges

I often mention how we can work out the rhythm of weather in the South Pacific by watching the Subtropical ridge STR Recently I have encountered a paper from the boffins at CSIRO in Tasmania who are watching the STR as a way to calculate likely rainfall trends in SE Aus. They have introduced parameters to measure the STR along their local longitude near 150E STR-I for Intensity (hPa) and STR-P for position (Latitude).
www.cawcr.gov.au/projects/vicci/wp-content/uploads/2015/11/Grose2.pdf
Their investigations show that the STR is usually at its southern -most latitude of 38S in February with an average isobar of 1015. Their modelling shows that in the next few decades it is expected to remain close to this positional cycle (with a nudge poleward) but increase its intensity.... Thereby, as a rule of thumb, reducing the rainfall and increasing the surrounding winds.


TROPICS
As expected, it has been a busy week in the South Pacific
Cyclone Nat skirted parts of French Polynesia after forming to the east of American Samoa.
Tropical Storm Osai also formed near Samoa but drifted southeast toward the Cook Islands and unwound.
The Tropical depression now S of Fiji (12P/05F) may blossom into cyclone if it shifts to Samoa
but then enters an area of strong winds aloft that will likely help it unravel.
The Radiosonde from Nadi airport (as seen on Windy) shows these strong winds:
This vertical cross section of the atmosphere (done by good olde weather balloon)
shows very warm moist and unstable air in the dangerous Quadrant of that Low.

WEATHER ZONES
The South Pacific Convergence zone is strong, active, squally and still breeding tropical depressions,
but there may be too much wind shear aloft to allow cyclones to form. L1 southeast of Tahiti is moving off.
L2 near Fiji is the main feature to watch and may still blossom into a cyclone near Samoa mid-week.

HIGHS and LOWS
East of NZ the STR reigns, with a squash zone of enhanced easterlies near 30S.
Over NZ, a cool dying southerly flow on Monday
then a passing HIGH Tuesday to Thursday,
followed by a fading front reaching Southland late Thursday
and Chatham Islands on Saturday.

Panama to Marquesas: the Northerly winds in the gulf may fade on Tuesday and again this weekend.
Avoid departing then.

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
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 text 64277762212.
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>