Bob Blog 18 Nov



Compiled Sun 18 Nov 2018

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.

SAM and the Polar Vortex

The Southern Annular Mode (SAM) is a parameter that measures the westerly winds that circle Antarctica. When it is positive the belt of strong westerly winds contracts towards Antarctica—so there a higher pressures and weaker winds over southern Australia/ New Zealand. When the belt moves north towards NZ, SAM becomes negative, and storms from the Southern Ocean can reach NZ.

One way of thinking about SAM is that it is a measure of the strength of the dam that walls the cold air over the Antarctic. When positive this wall is strong, and when negative this wall is weak and polar air can make be diverted outwards and northwards, maybe affecting New Zealand.

A light-hearted and animated presentation of SAM as one of the climate dogs that affect New South Wales may be seen at . To meet the whole dog pack see


When SAM drops suddenly from positive to negative, there is an increased possibility of a “polar outbreak” somewhere in the southern hemisphere.

A good proxy for SAM is the Antarctic Anomaly Oscillation (AAO) as seen at ,this shows that SAM is forecast to dip negative this week


Its not a huge dip, but does make SAM as negative as it was in mid-October, and if we look at the air temperature maps on a polar stereographic projection using we can see cold air spreading north between last Thursday and tonight, turning all NZ dark green.


Last Thursday, above

Tonight, below :


This helps explain the coldness of the front crossing New Zealand early this week.

The abrupt temperature change helped form a tornado over the Canterbury Plains this afternoon:




Latest cyclone activity as seen at


GAJA continues west after crossing southern India during the past week.

There is also BROCHRA near 12S 84W going WSW.

Looking at the weekly rain maps, last week’s shows a split in activity over the Indian Ocean away from the equator, an easing in the activity over the South Pacific and an increase in activity over the tropical Atlantic.





Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), STR, and SPCZ. Pink area = lightning likely (high CAPE)


SPCZ=South Pacific Convergence zone.

The SPCZ is expected to weaken during the coming week and extend from east of the Solomon Islands to between Fiji and Samoa. It is likely to produce a trough that may visit Tonga mid-week and Niue area on Wed UTC , fading over Southern Cooks on Thu UTC.


Accumulated rainfall for next week from CZ= convergence zone.

Subtropical ridge (STR)

Since SAM is going negative , the Subtropical ridge is retreating to the north.

A High is expected to form near 25S between New Caledonia and New Zealand by mid-week and then travel eastwards to fade over Southern Tonga by the weekend.

Tropics to New Zealand

A series of trough and lows are expected around New Zealand this week. There should be an interlude of relative calm on local Friday.

Between Tropics and Australia.

OK at first, but maybe a trough travelling from Australia interior across the coast and into the Tasman sea on or around late local Thursday followed by west to southwest winds.

From Tahiti to Tonga

Weak trough passing over Tahiti on local Sunday, then looks Ok to go with eastly wind that may last over a week to north of 18S. Further south there is another passing trough to take into account.


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