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

07 April 2013

BOBGRAM issued 7 April 2013



Issued 07 Apr 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 SOI (30 day running mean) is based on the difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number. SOI has been erratic this year and on 31 March it was plus 0.78.

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 when this vapour rises then it cools into cloud. This region hosts the warmest sea on 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.

Well, in the past month the SST in this target zone has been warming in the east and about neutral near the 180 line. There is possibly a trend towards El Nino (warm seas) starting in the ocean. This is at odds with what is happening in the atmosphere.

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 early March an MJO moved into the Coral Sea and helped trigger TC SANDY and TIM. This MJO has since faded away and the next cycle may not arrive until mid or late April—so Island- hopping in the South Pacific over next few weeks may be OK.


Panama to Galapagos: The NE winds that were around Las Perlas last week have gone now, and winds this week south of the continental shelf at 7N are mainly light and from the SW. Not good for sailing. The International Convergence Zone ITCZ is weak and mainly located along 3 to 4N and one part of it is active at present near 4N 82W, so stay east and south of there.

Galapagos to Marquesas: There is still some remains of a convergence zone along 5South, with one area of convergence near5S 90W. The best sailing path at present is to head off to the SW to around 4S 95W and around there to latch into trade winds all the way to Marquesas.

South Pacific Convergence Zone SPCZ
The South Pacific Convergence Zone SPCZ has increased during the last week, and now is an amalgam of several convergence zones, with most of them being in the Coral Sea and one of them extending across Tuvalu to Southern Cooks and another from northern Vanuatu to Fiji/ northern Tonga. There are a few more CZ convergence zones in the Coral Sea and a tropical low is expected to form there off the north Queensland coast by Thu/Fri 11/12 April and then travel southeast out of the Coral Sea and across the Tasman Sea on Sun/Mon 14/15 April and central NZ on or around Tue 16 April. Avoid.

Sub-tropical Ridge STR
The Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is showing signs of drifting north, as is normal after the equinox. It is oscillating between 40/45S and 35S/40S. The high which is in the Aussie Bight tonight is expected to travel south of Tasmania and around the south coast of the South Island on Tue 9 April. This high is then expected to move NE across NZ on Wed to Fri 10/12 April and then off to the east/SE.

Roaring 40s and New Zealand
On Tuesday and Wednesday a S/SE wind change with a cold front is expected to move north over NZ. The next front is likely on Thurs/Fri/Sat 11/12/13 April preceded by a strong NW flow and followed by west/SW winds.

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