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

17 March 2013

BOBGRAM issued 17 March 2013

Issued 17 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 was almost minus 1 during December, relaxed to near zero during January, fell to minus 0.9 during mid-February and has jumped up to plus 1.09 on 17 March. So it is down and up like the proverbial ---.

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. When the sea temperature is abnormal we get a change in clouds around the equator and consequent changes in the latitude zones of weather across the whole Pacific.
Well, in the past month the SST in this target zone has been slightly below normal, but relaxing toward normal, indicating a tendency towards neutrality.

Interestingly there is a zone of slightly cooler than normal SST in the north Tasman and around New Caledonia, probably partly thanks to the stirring done by last week's TC SANDRA. This renders the region slightly less likely to support the growth of any topical system. There is a similar cyclone cooler around Samoa to Southern Cooks.

Panama to Galapagos: Some useful NE winds for departure until Tuesday. Follow the surface current clockwise around Malpelo Island and carry enough fuel to motor in the light winds from there to Galapagos.
Galapagos to Marquesas: Probably still too early to get good weather for this trip due to light winds around Galapagos. Motor to just north of the equator and hitch a ride of the west-going equatorial current to around 110W and then take the most direct route.

Coral Sea
Ex-TC Tim in the central Coral Sea was circling clockwise and keeping its distance from Willis Island. It is a weakening system and has been downgraded to depression status and is expected to travel towards the Townsville area later this week.
Another tropical low might form in the Coral Sea later this week and travel west this weekend 23/24 March.

South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ1 is sitting mainly along 10S at present, with a second branch SPCZ2 across Vanuatu, Fiji and Tonga and currently connected to a Low near 32S 162W (L1).
L1 is expected to wander off to the southeast leaving SPCZ2 behind.

Sub-tropical Ridge STR
Meanwhile the Sub-tropical ridge-STR, the zone dividing the SE trace winds from the roaring 40s, has been finally knocked to north of NZ by a trough that combined in the Tasman Sea with the remnant cloud from last week's TC SANDRA – This trough was crossing NZ tonight, and has brought some welcome rain. The STR had been sitting over NZ since early February—and six dry weeks is abnormal for NZ.
The STR is still well south of normal in other longitudes. This is a feature of La Nina episodes but that cannot really be used as an explanation for the current abnormality.

A large anticyclone tonight in the Australian Bight, H1, is expected to track across Bas Strait on Monday and then across Tasman Sea on Tuesday and over central NZ on Wed and Thursday, then off to the east of NZ along about 40/45S. A trough following H1 is expected to move across New South Wales on Sat 23 March and then NZ on Sun/Mon 24/25 March.

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