Weathergram

WEATHERGRAM

YOTREPS

Issued 31 Mar 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.

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 on 31 March it was plus 0.88.

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

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 vapor into the air, and when this vapor 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.

AT this time of the year, just after the equinox, if there is one definite trend in the SST anomaly in the target area then it usually clicks in and stays for much of the coming year. In those circumstances a swing to El Nino (positive anomaly) or La Nina (negative anomaly) can be picked.

Well, in the past month the SST in this target zone has been warming in the east and cooling in the west-central Pacific. No definite trend to comment on.

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. In early March an MJO moved into the Coral Sea and helped trigger TC SANDY and TIM. This MJO has faded away now and looking at the cycle, the next one may not arrive until mid-April—so Island- hopping in the South Pacific over next few weeks may be OK.

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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 useful NE winds near Las Perlas are expected to fade by 1 April. Then the forecast is for light winds for the next week or more, mainly from south and southwest. No good for sailing.

Galapagos to Marquesas: There is a convergence zone along 5S from 110W to 140W and this extends southwards to 12S around 125W. This week the recommended strategy is to motor in the light winds around Galapagos to near 10s 105w and then going direct with trade winds for sailing. This deviation helps avoids many of the squalls in that convergence zone.

WEATHER ZONES

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Weather Zones (see text) as at Sunday 31 March 0400UTC

South Pacific Convergence Zone SPCZ

The South Pacific Convergence Zone SPCZ has weakened a lot in the past week and is now sitting mainly from Papua New Guinea to northeast of Vanuatu to Fiji/Tonga to Niue/Southern Cooks. This is its normal position for this time of the year.

There is a subtropical Low near the Kermadecs about midway between Tonga and NZ this evening. This is expected to stall there for a few days and then fade/move off to the S/SE over Thursday. next few days (similar to last week).

Sub-tropical Ridge STR

Meanwhile the Sub-tropical ridge-STR, the zone dividing the SE trade winds from the roaring 40s, is oscillating between 45S (H1 to east of South Island) and 35S (H2 on Aussie South Coast). Not much change since last week.

HI is moving off to the east, and H2 is expected to cross Tasmania on Thu 4 April and then be redirected around the south end of the South Island on Sun 7 April.

Roaring 40s and New Zealand

The Tasman Sea/NZ area is in-between anticyclones and so that makes for a troughy week (for a change). A trough is expected to fade overt the South Island on Monday. There is a low near Lord Howe Island until Tuesday and this then moves SE and mingles with a trough crossing NZ on Wed and Thu 3 & 4 April, followed by disturbed SW flow on Friday.

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

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