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

12 August 2018

Bob Blog 12 Aug

WEATHERGRAM

YOTREPS

Compiled Sun 12 AUGUST 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.

 

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, as if it guards the eastern entrance to the South Pacific like a protective dragon, 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.

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 (it is quasi stationary, just like the High between California and Hawaii, and 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/

There are easterly winds on the north side of this “Andes” High: they are dry due to continental outflow from off South America. 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. The easterly winds travel well to west of the dateline around 10 to 15S, and the South/SE winds come and go according to the migratory high and are usually found around 15 to 25S. 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, and by the strong annual cycle which repeats each year, 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

 

TROPICS

There is a regular procession of tropical features off the western Mexico coast.

The Atlantic remains somewhat quiet. YAGI is travelling NW to China, and LEEPI to south Japan, and HECTOR is heading for south of Hawaii, with another feature following.

Looking at the weekly rain maps we can see that the Asian monsoon is active over Indonesia and the Philippines, and the ITCZ is active across the Pacific. Not much change in the past week.

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Fiji and spreading onto northern Tonga/Niue.

A weak passing trough is likely over Society Island and Tahiti on Monday/Tuesday UTC.

A passing trough, associated with a Low travelling east along 30S, should reach Niue on Mon UTC, and fade between Southern cooks and Tahiti on Wed/Thu UTC.

 

Subtropical ridge (STR)

HIGH is expected to move off Australia into northern Tasman Sea on Tue/Wed UTC and then travel east along 25S

 

Around Tasman/NZ

Front moving onto NZ on Mon expected to deepen into a Low over central NZ on Tuesday and then move off to the east, followed by W/SW winds until Friday.

 

Tahiti to Tonga

The best-looking date for departure is Wednesday local. This voyage is likely to encounter a weak passing trough near160W on Saturday and then should be able to go onto Tonga . >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> 

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 (offers link to FOLLOW)

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

Contact is bob@metbob.com or txt 6427 7762212

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05 August 2018

Bob Blog 5 Aug 2018

WEATHERGRAM

YOTREPS

Compiled Sun 05 AUGUST  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.

REVIEW OF JULY 2018

Sea Surface temperature anomalies as at beginning of August  may be seen www.ospo.noaa.gov/Products/ocean/sst/anomaly/index.html

The eastern equatorial Pacific around Galapagos is the focal region for ENSO and is now on a slowly warming trend. Temperatures around Australia and into the Tasman Sea are becoming below normal, a possible indicator of drier than normal conditions in the next month or so.

The Gulf Stream off the east coast of North America and the Kuroshio current off Japan still stands out as warmer than normal.  These may help steer tropical features to the northeast.

Warm anomalies continue off west side of Mexico indicating a busy cyclone season for next few months.

 

To see how the annual weather cycle and the seasons are working out, take  a quick look at the average isobar maps from www.esrl.noaa.gov/psd/map/images/fnl/slp_30b.fnl.html showing average isobars for past 30 days and their anomaly for JULY.

The isobar maps show that subtropical ridge in the southern hemisphere is looking robust. The North Atlantic and Northeast Pacific HIGHS are looking stronger than this time last month. The July lows in the Southern Ocean have been favouring three positions: south of Australia, SW of South America, and SE of South America.

Lower than normal pressures between Europe and Philippines show a more active monsoon. 

Zooming into the NZ area, the 1015hP (between blue and white) isobar has stayed put in the tropics and to east of NZ.  There has been a radical change south of Australia and in the Tasman with a DROP in pressure allowing the 1015 to shift from Invercargill to Auckland.  This explains the increase in westerly winds onto western NZ, and can be taken as an early start to SPRING weather patterns.

 

The last 30 days of rainfall, and its anomaly, are seen at TRMM at trmm.gsfc.nasa.gov/trmm_rain/Events/thirty_day.html

The rain map shows extra convergence in the ITCZ from Bangladesh to Philippines , and across  the North Pacific Ocean, but it has been drier than normal over the Caribbean..

In the Southern Hemisphere a large dry region stretches from Australia to northern NZ, and the South pacific Convergence zone has been weak around Vanuatu and : Fiji.  It seems that the SPCZ has shifted northwards (an El Nino trait).

 

In summary, it seems that, although the atmospheric and oceanic parameters are “neutral” at present, as seen in my last week’s blog, there are early signs of an El NINO weather pattern.

 

TROPICS

There is a regular procession of tropical features off the western Mexico coast. 

The Atlantic remains somewhat quiet, and SHANSHAN is heading for Japan. As seen at tropic.ssec.wisc.edu/

 

Looking at the weekly rain maps we can see that the Asian monsoon is active over Indonesia and the Philippines, and the ITCZ is active across the Pacific and to central America. 

See: trmm.gsfc.nasa.gov/trmm_rain/Events/big_global_accumlation.gif

 

WEATHER ZONES

SPCZ=South Pacific Convergence zone.

The SPCZ is expected to stretch from PNG and Solomon Islands to north of Vanuatu to the Tuvalu/Tokelau then Southern Cooks area.  

A passing trough is likely over Southern Cooks on Mon 6 Aug UTC (squally) and Wed 8 Aug (not so squally), These troughs should weaken east of the Cooks and not affect Society Group (apart from some northerly winds).

Another passing trough is expected to develop around Fiji/Tonga around 11 Aug UTC This may turn into a  Low near 22Z next week, Avoid.

 

Subtropical ridge (STR)

Weak HIGH is expected to appear in north Tasman Sea from Mon to Wed 6-8Aug, and another should cross northern Tasman Sea /North Island from Thu 08 to sat 11 Aug UTC.

A polar HIGH is expected travel east along 50 to 45S this week, east on NZ, there is likely to be a squash zone of gale easterly winds east of NZ between around 30 and 40S. Avoid.

 

Around Tasman Sea, NZ to tropics.

Passing troughs on Monday and Wednesday. Best date to depart NZ for the tropics this week is Thursday (local). 

 

Tahiti to Tonga

With the passing troughs, perhaps the best on offer this week is a Friday local departure to Palmerston or Aitutaki. Then again, if you don’t mind encountering a trough over the open sea, can depart any day, but get some waypoints to reduce the squalls. .

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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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