Bob Blog



Issued 29 March 2015

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 Atmosphere:

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. It switched briefly blue and positive at the start of March but has now gone very negative again, as Tahiti drops below normal atmospheric pressure and Darwin returns to normal pressure after recent weather.


SOI as shown at


The recent cluster of tropical cyclones may be related to a recent Madden Julian Oscillation MJO that travelled eastwards across the South Pacific. This can be seen as the blue/purple spread in the Outgoing Longwave radiation anomalies observed by satellite ad graphed in this longitude-timeline. Basically in periods of very active weather there is more cloud, and the more white cloud there is, the lower the amount of radiation that can escape to space / the lower 9bluer) the OLR.


OLR from shows a recent MJO.

Note that yellowish-orange zone is forecast to travel east across the South Pacific over the next week or two. This is a zone of positive OLR, meaning less cloud than normal, and that suggests a weaker South Pacific convergence zone.

Note that another MJO event is being forecast for mid to late April—that will be the last chance for cyclones in the South Pacific this Cyclone season.

In the Northwest Pacific TC MAYSAK has formed over Palau and is expected to travel west towards Philippines – it may peel off to the north or it might possibly make landfall around 5/6 April. Too early to tell, so be watchful.


The Cyclone that I mentioned the models were predicting to form over Northern Cooks last Friday/Saturday never actually developed at all. AT present it is between Niue and Southern Cooks and it does has some squally showers, but nothing worth writing a book about.

The rain map for the last two weeks show a shift eastwards and weakening of the strongest convective activity in the South Pacific, and a build-up of convection in the NW pacific.



Weekly rain signatures for past two weeks, as seen at



Weather Zones (see text) as expected mid-week at 0000UTC on Wednesday (GFS model) showing wind, isobars, Sig wave height green lines, swell direction orange arrows, current red arrows, SPCZ and STR.

Panama to Galapagos

May be a good idea to wait for the onset of the next period of NE winds on local Tuesday. If you do this then you can take a direct trip, and no need to take the Isla Mapelo diversion. Good west going current from 2N 85W to Galapagos.

Galapagos to Marquesas:

Light winds at the start— head for 5S to 6S ASAP to get into the trade winds. Best day to depart this week could be local Monday/Wednesday (northerly winds for starters).

SPCZ=South Pacific Convergence zone.

SPCZ is weakening away and mainly over Samoa to Southern cooks at the beginning of the week. Another new branch may form over central Coral Sea to Vanuatu by end of week.

STR= Sub-tropical Ridge

The STR was knocked to north of NZ during the past week, and the northwards-knock is expected to be taken east this week as the remains of TC RUEBEN travel ESE into the mid latitudes. The low that has been over NZ this weekend is expected to go east and fade by Wednesday UTC.

HIGH that follows that Low should travel across central Tasman Sea on Monday/Tuesday and central/northern NZ on Wednesday/Thursday.

Next HIGH is shaped like an Easter Egg– and should travel east across Tasmania on Thursday and then across southern Tasman Sea on Friday and then travel NE across NZ on Sat/Sun/Monday.

Over NZ

The incoming HIGH on Thursday is shoveling cold air from the southern Ocean onto NZ ahead of it. Thus a cold front, preceded by strong NW winds and followed by chilling southerlies, is expected to travel NE across the South Island on Thursday and the North Island on Friday. Then clearing condition by Saturday. In Northern NZ expect an increasing NW flow for the annual Auckland-Tauranga yacht Race starting on Thursday, and strong and squally SW change during Friday, and a change to easing SE winds by Saturday.


The first full moon after the equinox occurs on Sat 4 April—this is the Jewish Passover: the Sunday after it is the Christian Easter Sunday. In AD33 the Passover Full moon was on a local Friday, this year it is on a Saturday and there is a lunar eclipse as well visible from the Pacific (full eclipse at 1am Sunday for Auckland). Rather strange to see the Easter moon come and go so quickly.

The late Augie Auer would study the position and intensity and trend of all the weather zones around the planet at the March equinox very carefully, for he thought of this as the start of the meteorological year and that any trends happening now will not wobble or fade, but will continue and mark the year ahead.

Indian Ocean

You may have heard of the loss of a Catamaran (3 People on board) in the Indian Ocean last month. A possible life-raft has now been observed at 24.317346S, 72.127445E. If you can help by spending time scanning incoming real time hi-resolution satellite data then the Facebook page

Many thanks

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