Weathergram

WEATHERGRAM

YOTREPS

Issued 14 Apr 2013

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.

Click on images to enlarge them.

Jeanne Socrates, the 70 year old solo circumnavigating grandma, is now moving into the central Tasman Sea on her way back to Vancouver. I’m watching the weather with her and we think that a low in the Tasman can be avoided OK. Here is a photo she sent on her web site at http://synereida.livejournal.com/

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SE Cape of Tasmania

The Atmosphere: The Southern Oscillation Index SOI (30 day running mean) is based on the difference in the barometer readings between Tahiti and Darwin. It sums up the weather pattern over the South Pacific as one number. SOI has been erratic this year and in mid-April it was plus 0.53, and trending down or relaxing.

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SOI since 30-day running means on a weekly basis since January 2010 from

http://weather.farmonline.com.au/climate/indicator_enso.jsp?c=soi&p=weekly

showing the ups and downs this year .

The Ocean: Sea surface temperatures SST across the equatorial Pacific may be thought of as a thermostat for the planetary weather engine. The warmer the sea the quicker it evaporates, tossing water vapour into the air, and when this vapour rises then it cools into cloud. This region hosts the warmest sea on the planet so its abnormalities tend to influence changes in clouds around the equator and consequent changes in the latitude zones of weather across the whole Pacific.

During March there was a warming trend in the SEEP (surface of the eastern equatorial pacific) but this has revered during April. So there is a neutral signal also coming from the Ocean, neither LA NINA nor EL NINO.

Madden Julian Oscillation MJO

The MJO is a cycle of enhanced tropical convection that occasionally moves from Indian Ocean across Australia into the Coral Sea, and it can trigger the formation of Tropical cyclones, as happened in early March.

The next MJO cycle is about to move into the Coral Sea area over the next few weeks… but is much weaker than the last one. This MJO has already been associated with one cyclone in the Indian Ocean- IMELDA now weakening near La Reunion Island, and there IS TIME for one more in the Pacific before we close this Cyclone season.

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MJO cycle appears as the blue area in this time-longitude plot from Australian Bureau at

http://cawcr.gov.au/staff/mwheeler/maproom/OLR_modes/h.6.MJO.S.html

Panama to Galapagos: The N/NE winds that are around Las Perlas at present are expected to fade away early in the week followed by light head winds- no good for sailing. Next lot of NE winds is, at this stage, expected to start arriving around Sunday 21 April- but be less than 10 knots. The International Convergence Zone ITCZ is weak and mainly located between 5 and 2N and mainly to west of 85W, easy enough to avoid.

Galapagos to Marquesas: Not much left of that convergence zone that was around 5S 90W last week. Still light head winds for starters, motor off to SW until trade winds are found for sailing near 2S 95W then sail to around 5S 112W then ride a surface current to 7S125W then go direct. From around next weekend there may be a period of SE trade winds over Galapagos- good for sailing the whole trip!

BLOCKING

MetService and NIWA have pointed out the preponderance of blocking anticyclones in the NZ area so far this year as being associated with some weird seasonal weather (wet over eastern Australia and dry over NZ). See http://metservice.com/rural/seasonal-forecast-north-island and http://www.niwa.co.nz/climate/sco/seasonal-climate-outlook-april-june-2013 The intensity and permanence of these anticyclones are both beginning to weaken as can been seem from a time-longitude plot of the Southern Hemisphere blocking index, so outlook is for things during the slide into winter to be closer to normal than summer was.

image

http://www.cpc.ncep.noaa.gov/products/precip/CWlink/blocking

/real_time_sh/real_time_index_nrm.shtml

Blocking index has been strong at times near 180 so far this year and is now weaker.

WEATHER ZONES

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Weather Zones (see text) as at Sunday 14 April 0400UTC

South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ has sorted itself out during the past week and is now extending from Papua New Guinea to Vanuatu to Fiji to Samoa to Southern Cooks, with another CZ Convergence Zone over French Polynesia.

Sub-tropical Ridge STR

The Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is showing signs of drifting north, as is normal after the equinox. Compare its current positi0n with where it was in February (see https://metbob.wordpress.com/2013/02/).

A new HIGH is expected to move east across the Australian bight and then across Tasmania on Friday 19 April and then across central NZ on Mon 22 April. There MAY BE some good voyages from NZ toi the tropics as this High moves off to the east of NZ

Roaring 40s and New Zealand

There are two subtropical lows, L1 and L2. L1 is moving Se across the Tasman Sea and is expected to cross central NZ on Tuesday and Wednesday 16/17 April. Then some chilled air on the west side of a Low in the Southern Ocean (L3) is expected to spread northeast-wards across New Zealand and when this air meets with sub-tropical air over the North Island a secondary low should deepen on Friday and Saturday 19/20 April, , along with a cold shock of southerly winds.

L2 is near 30S 170W and expected to have an easier track to the south then southeast, peaking on Wednesday like L1 but fading before it gets south of the STR.

See my yotpak at http://www.boatbooks.co.nz/weather.html for terms used.

Weathergram with graphics is https://metbob.wordpress.com

Weathergram text only http://weathergram.blogspot.co.nz

Website http://www.metbob.com

Feedback to bob@metbob.com