Bob Blog 16 June

WEATHERGRAM

YOTREPS

Compiled Sun 16 June 2019

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.

The South Pacific Convergence Zone explained

Now that many yachts are about to travel west from Tahiti to Tonga, and are thus about to sail thru or around the SPCZ, this obstacle has become a talking point…as if it guards the eastern entrance to the South Pacific like a protective dragon, and some have asked what is it, why is it there, how does it differ from the ITCZ and what makes it tick.

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.

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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. 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 (see Henderson island). blogs.fco.gov.uk/lauraclarke/2018/04/10/henderson-island-plastic-pollution-in-paradise/

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.

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

Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone

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From www-gte.larc.nasa.gov/pem/pemt_flt.htm

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 windy.com.

Read more about it at en.wikipedia.org/wiki/South_Pacific_convergence_zone

THE TROPICS

TC VAYU is in the Indian ocean and weakening and about to make landfall over NW of India.

The latest cyclone activity report is at tropic.ssec.wisc.edu and TCFP tropical Cyclone Formation Potential at www.ssd.noaa.gov/PS/TROP/TCFP/index.html

There are areas of potential development in China sea and North of PNG, also off the west coast of central America. This week.

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WEATHER ZONES

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

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SPCZ=South Pacific Convergence zone.

The SPCZ stretches from Solomons to Tuvalu and Samoa this week.

A trough is expected to travel across Southern Cooks and Tahiti late in the week.

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Accumulated rainfall for next week from windyty.com.

Subtropical ridge (STR)

HIGH over South Tasman Sea is expected to cross New Zealand on frosty Tuesday and then travel to east of NZ.

Next High is expected to spread into Tasman Sea next week.

Tasman Sea

Low is expected to deepen east of Coffs on Monday and then travel east and weaken, with associated front crossing NZ on Thursday followed by a westerly flow on Friday.

This Tasman trough is likely to replace the trade winds over northern Tasman Sea with light winds or SE winds, good for getting from Australia to Noumea, but not the other way.

That Tasman trough is expected to cross NZ on Thursday so the last good day to depart NZ for the tropics is Tuesday, and even that will need waypoints to zigzag across a period of northerly winds on Wednesday and Thursday.

Tahiti to Tonga

A passing trough is expected to affect Southern Cooks on local Tuesday and maybe the Tahiti area on local Thursday. Departures from Tahiti until Wednesday should be able to avoid that trough and have trade winds for a while. Passing trough expected to reach Tonga by local Tuesday 25 June.

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If you would like more detail for your voyage, then check metbob.com to see what I offer.

Or Facebook at /www.facebook.com/metbobnz/

Weathergram with graphics is at metbob.wordpress.com (subscribe/unsubscribe at bottom).

Weathergram archive (with translator) is at weathergram.blogspot.co.nz.

Contact is bob@metbob.com or txt 6427 7762212

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