Bob McDavitt's ideas for sailing weather around the South pacific

24 March 2013

BOBGRAM issued 24 March 2013

Issued 24 Mar 2013
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 of weather maps, so please fine-tune to your place. Dates are in UTC unless otherwise stated.

The Atmosphere: The Southern Oscillation Index or SOI (30 day running mean) is based on the barometer readings from Tahiti and Darwin and sums up the weather pattern over the South Pacific as one number. It has been behaving erratically so far this year – down and up, then down and now up again, with a 30 day running mean at plus 0.7 on 24 March.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and if this vapour rises then it cools into cloud. This region hosts the warmest sea ion the planet so its abnormalities tend to influence changes in clouds around the equator and consequent changes in the latitude zones of weather across the whole Pacific.

AT this time of the year, just after the equinox, if there is one definite trend in the SST anomaly in the target area then it usually clicks in and stays for much of the coming year. In those circumstances a swing to El Nino (positive anomaly) or La Nina (negative anomaly) can be picked.
Well, in the past month the SST in this target zone has been warming in the east and cooling in the west-central Pacific. No definite trend to comment on.

Madden Julian Oscillation MJO
The MJO is a cycle of enhanced tropical convection that occasionally moves from Indian Ocean across Australia into the Coral Sea, and it can trigger the formation of Tropical cyclones. In the past few weeks an MJO has moved into the Coral Sea and tropical cyclones SANDY and TIM have formed. This MJO has faded away now and looking at the cycle, the next one may not arrive until mid-April—so Islands hopping in the South Pacific over next few weeks may be OK.
Note that tropical cyclones CAN form even if there is no MJO around. Some computers models are picking that a disturbance may form on SPCZ by Tuesday in Timor Sea and then go south making landfall near Darwin later in the week and the into the North Australia interior by the weekend

Panama to Galapagos: Light winds. Next patch of useful NE winds is expected for a few days from 28 to 30 March.

Galapagos to Marquesas: Around the equinox for a few weeks there is the strange "twinning " of the International tropical Convergence zone ITCZ- another convergence zone may form along 5South between 90 and 150W . This can produce too many squalls for those planning to take off to the southwest of Galapagos and then go direct when they encounter trade winds near 5 or 6S. For the early birds that want to go soon, the alternative route to just north of the equator and a ride on the west-going equatorial current is recommended.

South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ is sitting mainly between 10S and 5S at present, and is shrinking after its more active time last week.
There is a Low L in the sub-tropics to south of Tonga and it is expected to fade off to the southeast.

Sub-tropical Ridge STR
Meanwhile the Sub-tropical ridge-STR, the zone dividing the SE trace winds from the roaring 40s, is oscillating between 45S (H1 to east of South Island) and 35S (H2 on Aussie South Coast).
H1 and H2 are both expected to travel east, with H2 over central NZ on Tues-Thurs 26 to 28 March.
Over the next few weeks the STR id expected to drift north into the interior of Australia- since the nights are now longer than the days the deserts are moving into the cool half of the year and that helps make the air denser and isobars higher.

Roaring 40s and New Zealand
One consequence of post-equinox weather is that the troughs in the roaring 40s can spread further to the north. The trough that is expected to cross the South Tasman Sea on Sat/Sun 30/31 March (Easter weekend) is likely to be preceded by N/NW winds over all NZ on Friday and bring a cold southerly change to all NZ on Easter Mon/Tues 1/2 April.

This trough is likely to induce light winds in the north Tasman Sea during the Easter weekend. Also it may help a low to form between Tonga and New Zealand, and that Low may be pushed southwards so that it side swipes the eastern North Island during the annual Auckland to Tauranga Yacht race.

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