Issued 26 August 2012
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 Ocean: Seas surface temperature anomalies in the equatorial mid Pacific have been increasing positive, and over much of the area they are now between 0.5 and 1C above. So it is trending towards an El Nino, but not there yet. Between Galapagos and South America a blob of cooler than normal water has welled up to the surface and is now slowly wandering westwards. Interesting.
The Atmosphere: The Southern Oscillation Index or SOI is steadying after a dip during past month. On 25 Aug its 30-day running mean was -1.01. So the atmosphere is now following the Ocean and trending El Nino, following the Ocean.
Tropical cyclones: ISAAC is at present attacking the tent city of ‘quake refugees in Haiti and is likely to visit the Florida Keys later this week. In the NW Pacific we have TEMBIN still skirting around Taiwan, and BOLAVEN is heading to southwest end of Japan.
Prognosis at Wed 29 Aug 00UTC showing items defined in this test.
In the South Pacific, the Convergence Zone SPCZ continues to be fairly quiet. The part of the SPCZ over the Coral Sea seems to have shrunk to a zone over the Solomons. There is still a lot of convection between the Equator and 7S from 160E to 170W— possibly due to the warmer than normal sea temperatures there. Another branch continues around Tokelau, and this seems to be lingering in place. A weak CZ is moving eastwards across the area around Niue and is likely to cross the Cooks on Monday UTC and fade over French Polynesia on Tuesday /Wednesday. In conjunction with this CZ a LOW is expected to form near 30S 155W/150W… this Low should quickly deepen and then move off to the ESE.
Anyone planning to go west from Tahiti this week should let that SPCZ pass by first.
A small CZ is expected to form over New Caledonia on Monday UTC. In conjunction with this CZ a low is expected to form near 30S 170-180E on Tuesday/Wednesday 28/29 Aug and this Low should then deepen a lot as it rolls off to the east southeast, expanding to dominate the weather in the Pacific next week to east of NZ.
These sub-tropical lows are able to feed off moisture from the SPCZ and energy in the nature of upper divergence from the subtropical Jetstream.
SUBTROPICAL RIDGE STR
The STR is mostly reverting to its Aug/Sep normal latitude of 30/35S this week. The High that crossed NZ late last week and is now just east of NZ is expected to slowly go north to 30S 165W by Wed 29 Aug and then should slowly make its way east. The High that is expected to cross the central Tasman Sea on Tuesday 28 Aug and the South island on Wed 29 Aug is likely to then be constrained to move east along 45S from Thurs 30 Aug to Sat 1 Sep.
A frontal system preceded by NW winds and followed by SW wind sis crossing NZ tonight/Monday. This system is at the intensity and latitude that is typical of spring, indicating that the Roaring 40s are now spreading onto NZ.
The next trough should start approaching NZ on Friday preceded by NW/N winds. This system is likely to form a low and then draw in a southerly change over NZ during Sat/Sun/Mon 1/2/3 Sep.
Traveling towards New Zealand this week:
In a simple roaring 40s scenario there is a rhythm in the weather with which you can arrange a reasonable sail to NZ. However this week there is a Low near 30S 170-180E on Wed/Thu 29/30 Aug and this poses a challenge. If you wish to approach NZ from the north this week, try and time your arrival day to be Sat 1 Sep.
Now that we are at the end of August we can start forming ideas about the coming cyclone season. The SOI is hovering around -1, and such a low SOI is associated with an El Nino (we are not there yet but the trend is in place). During an El Nino the SPCZ tends to be shifted to north and east of its mean position , and this tends to encourage cyclones to form around or east of the dateline with fewer than normal near Australia.
A study of the number of Australian cyclones is available at http://www.aoml.noaa.gov/hrd/Landsea/oz/index.html. Using Figure 4 from this study and -1 for this year’s value of the August SOI
we get an estimated count of 8 cyclones (for Australia) compared with the long term average of 11.
Fig 4. Scatter diagram of number of Australian Cyclones per season versus the August SOP as a precursor.
The Bureau advice at this stage on the cyclone season at http://www.bom.gov.au/wa/cyclone/seasonal/ is “The tropical cyclone season runs from 1 November to 30 April. The seasonal outlook for 2012/2013 will be issued on 15 October 2012.”
I shall try and do some more research on this topic over the next few weeks?
See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.
Weathergram with graphics is at https://metbob.wordpress.com/
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