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

01 December 2013

BOBGRAM issued 1 Dec 2013

Issued 01 December 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 Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin.
It reached a high of 0.52 on 22 September, and since then has dropped and bounced. On 24 November it has a 30-day running mean of plus 0.3, and is trending upwards.
The International Research Institute of Columbia University have now produced data that looks at the plume of 24 ENSO models and shows that probability of an El Nino event is likely to increase to over 50% by mid-2014. This may well impact on your sailing voyages next year so be aware of this as you do your preparations.

The Ocean:
The warmer the sea the quicker it evaporates, tossing water vapour into the air, where is rises and cools into cloud. The equatorial Pacific region hosts the warmest sea on the planet. Thus its sea surface temperatures SST may be thought of as a factor in the running of planetary weather engine. Anomalies tend to influence changes in clouds along the equator and thus tweak the latitude zones of weather around the planet.
In September and October all seemed to be around normal, but at present there seems to be a warming trend west of the dateline and a cooling trend further east.

TC Alessia went inland and then west across Northern territories where it dropped its rain and ran out of puff. It reminds us that the Cyclone season for the South Pacific is now OPEN.

South Pacific Convergence Zone SPCZ
The SPCZ continues to be north of its normal position, rather weak and somewhat disjointed. It is expected to gain intensity over the northern Coral Sea this week is in its northern most position at present and may spread south to visit Fiji and Tonga from 7 December.

Sub-tropical Ridge STR
The STR continues to be strong and south of normal in the zones east of NZ and over Southern Bight of Australia with large blocking Highs that have been a feature all during November. However it continues to be weak in the Tasman Sea/NZ area. There is a natural tendency for lows to form on the NW shoulder of a blocking high, and sure enough one has formed today in between New Caledonia/ New Zealand /Lord Howe/ It is under an upper trough that is about to bud off and so it is expected to slowly go SE and then south.

Tasman Sea/NZ.
That low should slide southwards to west of NZ in a dying state on Wednesday and Thursday, and then be followed by a more aggressive trough/Low combo from the west crossing NZ on Friday and Saturday.

Between tropics and NZ
Slack winds for starters this week as the Tasman Low moves slowly off, stealing the wind. Then a useful northerly flow from Thursday onwards around the developing High east of NZ as it travels off to the NE.
The next trough to more onto NZ is currently expected maybe around 12-13 Dec. This system may only affect central and southern NZ, but be aware that could affect northern NZ and if so try not to arrive there at the same time.

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