Bob Blog 15 May

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.

Compiled Sunday 15 May 2022


AN interesting study has been pointed out to me at

which investigates the shape od a city (either 1) edged like a triangle in a valley or 2) rectangular around a river or 3) edgeless and sprawling like a circle) and its impact on the intensity of its rainfall,


Comparing edged-shaped cities with circular-shaped cities, clouds find it easier to rise over circular -shaped cities and this helps intensify downpours. For inland cities the maximum rain is usually in the morning and for coastal cities it is usually in the afternoon. It would be interesting to extend this study to cities like Auckland which are on an isthmus and get converging sea breezes.

The South Pacific Convergence Zone explained

Now that many yachts are about to travel west from Tahiti to Fiji and are thus about to sail thru or around the SPCZ, this obstacle has become a talking point…it is something like a rite of passage for crossing the South Pacific, and some have asked what is it, why is it there, how does it drift and what makes it tick. Stand by for lots of jargon,

Weather is a mix of pattern and chaos, and meteorological teaches concentrate on the pattern. In tropical meteorology the first idea given is the Hadley cell.


Because the sun is most directly overhead at the equator, that’s where the warmest seas are, and this causes rising air. Once the rising air reaches high enough it spreads outwards and sideways to the north or south, where it sinks at dries out. The sinking air reaches the surface again around 30N or 30S (subtropical ridge) and then recirculates back to the equator as surface winds know as trade winds (so reliable they can be used to determine trade routes). The trade winds from each hemisphere converge together in a zone, and this convergence narrows the zone of rising air into a feature called the Intertropical Convergence Zone or ITCZ

But in the Southern Hemisphere, the Andes of South America cause a split in the trade winds. They block a HIGH near 30S around 90 to 110W, or near Easter Island. It is quasi stationary, just like the High between California and Hawaii, and also has a gyre that is collecting a rubbish heap just as badly (Henderson island).

1. There are easterly winds on the north side of this “Andes” High: they are dry due to continental outflow from off South America. These easterly winds travel well to west of the dateline along around 10 to 15S.

2. And there are migratory Highs that travel east along the subtropical ridge from Australia to east of NZ, with a zone of south to southeast winds on their northern side. These South/SE winds come and go according to the migratory high and are usually found around 15 to 25S.

3. The convergence zone between these easterly and Southeasterly winds is called the South pacific Convergence Zone, or SPCZ.

It is typically located from the Solomon Islands southeastwards to the Southern Cooks, but sometimes may have large gaps or be very quiet.

Read more about it at



It is affected by many things: 1) the PDO which takes many years to switch, 2) the El Nino/La Nina which last a year or so, 3) the strong annual cycle which makes the seasons, and 4) the MJO which comes for a week or so every six weeks or so.

So far this year we have been having a LA NINA, and this helps shift the SPCZ south and slightly west of its normal position and intensifies the strength of the southeast trade winds.

However it is normal for troughs to travel eastwards along the SPCZ, connected to travelling lows in the Southern Ocean , and sometimes these passing troughs induce the formation of deepening lows near 30S , usually south of the Southern Cooks.

I have found that the easiest way to determine the position and severity of the SPCZ is to use satellite imagery, and the easiest way to decide what it may do over next few days is to use the 5day rain accumulation parameter on


The latest cyclone activity report is at and Tropical Cyclone Potential is from


All quiet at this time of the year,


Weather Zones Mid-week GFS model showing isobars, winds, waves (green to red +arrows), Rain (Blue), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (pink)


CAPE mid-week as seen by ECMWF and GFS from

Different models seem to produce different values.


SPCZ=South Pacific ConvL1ergence zone.

The SPCZ stretches from Solomons to Vanuatu to Samoa A passing trough is expected to move east across Tahiti around mid-week.


Rain Accumulation next five days from


Low L1 is forming in a trough over central NZ tonight and expected to track to the northeast this week all the way into the tropics. It’s on the backside of its upper trough.

High H1 east of L1 is also expected to travel NE this week in tandem.

Low L2 is expected to form mid-week over Vanuatu, then fade by end of the week

High H2 in the North Tasman Sea is expected to slowly travel east along 30S.

Low L3 is expected to travel from 50S and deepen over southern NZ late this week followed by strong SW winds that may bring 4m swells as far as 30south.

H2 and L3 make trans-Tasman travel difficult this week.

For departing westwards from Tahiti may be an idea to wait until the passing trough has gone.


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