Weathergram

WEATHERGRAM

YOTREPS

Issued 29 December 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.

SOI The Southern Oscillation Index SOI (30 day running mean) sums up the weather pattern over the South Pacific as one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin.

SOI has dropped recently and is now hovering around zero. Its 30-day running mean was minus 0.02 on 29 Dec.

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SOI trend (x10) since 2010 showing the start of the December decline as seen at http://www.farmonlineweather.com.au/climate/indicator_enso.jsp?c=soi

MADDEN JULIAN OSCILLATION MJO

The MJO is the name given to a detectable pattern of increased convection ion the tropics that travels from the Indian Ocean to the Pacific Ocean.

See http://en.wikipedia.org/wiki/Madden%E2%80%93Julian_oscillation

Its arrival in the western Pacific is associated with Tropical cyclone formation. The amount of tropical convection can be traced by measurements (from satellite) of the Outgoing Longwave Radiation OLR. Latest pattern in the OLR seems to show a quietning of the convection in the South Pacific AFTER A possible burst of activity mid this week around Fiji/Samoa/Tonga

Basically a clear sky = lots of OLR, and is coloured yellow or orange in the diagram. Lots of convection reduces the OLR and this is coloured BLUE. The following diagram is a time line of the OLR on the south side of the equator covering all longitudes. Our focus us in the middle at 180. Say to yourself Yellow= mellow (quiet) and Blue= bubbly (active).

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OLR for past 6 months on south side equator from http://cawcr.gov.au/staff/mwheeler/maproom/OLR_modes/h.6.MJO.S.html

Shows the recent increase in activity near 120E and expected decrease in activity during January, after a possible burst of activity around Fiji/Samoa/Tonga mid this week

TROPICAL TOPICS—

Tropical Storm SIX is to the NE of Madagascar moving to the south, and Tropical Storm CHRISTINE is near the NW coast of Australia and expected to travel S and SE taking rain into the interior of Australia /reducing the recent heat wave in Western Australia.

In the recent past some models were picking a possible tropical LOW to develop over Fiji by 1 Jan. However, the convection in the Coral Sea has dried out over the weekend and the current expectations are this development nay be delayed until 8 January (GFS model)—or may occur  south of Tonga/Niue near 25S (ECWMF model) . Things are in flux so keep watching the local daily updates.

 

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Weekly rain signatures for past two weeks, as shown at

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

WEATHER ZONES

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Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing isobars, wind, waves, rain, SPCZ and STR . Purple lines are upper flow.

South Pacific Convergence Zone SPCZ

The SPCZ seems to have dried out over the Coral Sea recently and is in a northern position over Tuvalu and Tokelau with a weaker zone over Samoa and Tonga. It is expected to activate over Samoa/ Tonga/ Niue by 1 January.

Sub-tropical Ridge STR

The blocked HIGH that is near 40S to south of French Polynesia and should stay put this week. Another High is expected to move into the Australian Bight, but will now have to wait until after 4 Jan as the remains of CHRISTINE work their way out to the ESE. This leaves the Tasman Sea and NZ in a troughy area, between anticyclones, this week.

Tasman Sea /New Zealand

Troughy. The front that knocked 5 yachts out of the Sydney-Hobart race is expected to cross NZ on Mon followed by a weakening trough on Wednesday and then another active front/low on Fri/Sat, and then, early next week, the remains of TC CHRISTINE combined with another Low from the southern ocean.

There may be the odd day here and there worthy for fishing, sailing, swimming, especially in the north, but it does not look like there will be any extended period of settled weather this week for holidaying.

 

However, please let me give you an you a not-so-familiar verse to the familiar refrain for the New Year

We two have paddled in the stream,
from morning sun till dine;
But seas between us broad have roared
since auld lang syne.

 

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

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