Bob Blog 13 May



Compiled Sun 13 May 2018

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 state of the ENSO =  neutral

The Atmosphere:

El Nino and La Nina are opposite ends of the swing of an identifiable tropical influence on our seasonal weather: the La Nina, caused by cooler than normal seas along the equatorial eastern pacific. shifts the subtropical ridge away from the equator, and the El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator. Their comings and goings can last several months, maybe over a year, and so their status can be used to help forecast the weather for the coming season.

ENSO = El Nino/Southern Oscillation. The main parameter we watch from the atmosphere is the Southern Oscillation Index SOI (30 day running mean) as it sums up the whole weather pattern over the South Pacific in one number. It is based on the standardized difference in the barometer readings between Tahiti and Darwin, in other words it counts the average number of isobars between them on the weather map. When the SOI is more than plus one (standard deviation from its mean) for more than a month we call it a LA NINA event, and when it stays more than minus one we call it an EL NINO event.

For the past year the SOI has been mostly around plus 0.5 to plus 1.0, consistent with a weak but rather persistent La Nina. The subtropical ridge line has been further from the equator than normal, and trade winds were stronger than normal, but are now close to normal.

However, over the past month, the SOI has settled into a near zero state, and has a NEUTRAL status.


Neutral conditions as seen at

(Note that in this graph on the vertical axis 10= 1 standard deviation)

The Ocean:

NINO3.4 is a region in the eastern equatorial Pacific Ocean that acts as a heat storage area during an El Nino or becomes cooler than normal during a La Nina. This plays with the heat budget of the atmosphere and thus with the weather patterns.

At the farmonline web site we can see the trend in the sea surface temperature in the NINO3.4 area. The diagram shows the weekly temperature anomalies since Jan 2015, with the El Nino of 2015 looking like a hump on a camel. Since then there has been a cool period late 2016/early2017, then a warm period until July 2017, and then a cool period. That cool period seems to be coming to an end now, and the sea surface temperature are near normal.


Weak La Nina as seen at

Waters beneath the surface are slightly warmer than normal

The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models. The model predictions for the Nino 3.4 SST anomaly is that the seas ae likely to gradually WARM during the rest of this year, but the mean of the predications has only warming to 0.5 above normal— not enough to be called an El NIN event (but closer to it that we have been for a while).


CPC/IRI predictions from

Latest SST anomaly map shows the remains of a large blue pool of cooler water across the central equatorial Pacific. Also, there are warmer yellow waters appearing around the Galapagos.


Sea surface temperatures across the Pacific on 10 May from


There are a few tropical depressions in the north Pacific, but they seem to be weak this week.


The rain map for the past week shows peak rainfall occurring around Indonesia and Micronesia – see



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, sea (magenta), and current. STR, and SPCZ. Pink area = lightning likely (high CAPE)

SPCZ=South Pacific Convergence zone.

No tropical depressions or troughs this week. The SPCZ is expected to remain strong over Solomon Islands and then weak across Tuvalu and Tokelau, then strong from around Niue to Southern Cooks.

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Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH to east of NZ is expected to remain quasi-stationary near 30S shifting from 160W on Monday to 140W by end of the week.

Next HIGH is expected to remain quasi stationary around Aussie Bight this week and may poke out a tongue of high pressure across the north Tasman sea late in the week. There should be a squash zone of strong SE winds forming in the northern coral sea from Tues 15 Nay, in response to this HIGH, and this squash zone may extend to New Caledonia/ southern Vanuatu next week.

Around Tasman Sea

Low 1 in mid Tasman Sea on Monday is expected to travel onto central NZ on Wednesday.

Low 2 should form southeast of Lord Howe on Wednesday and then travel onto northern NZ on late Thursday. After that there may be a trough passing over NZ on Sunday, and then, with luck, a swing to SW winds.

New Zealand to Tropics

There may be a gap between passing troughs lows with a departure from Northland on Tuesday, or late Friday, otherwise may need to wait until SW winds arrive next week.

New Zealand to French Polynesia

Northwest to westerly winds or an offer for a good start this week.

OK for sailing east, but don’t depart in the passing troughs on late Thursday.

Panama to Galapagos /Marquesas

For sailing from Panama, the upcoming week looks to have head winds from SW /S around 15 knots. May be better to stay put and hope for better next week.

From Galapagos area to Marquesas, departure can be any time this week. Best path for wind and current is to motor/sail to 4 South 95W, then sail to 7S 130W and then go direct


If you would like more detail for your voyage, then check to see what I offer.

Feedback to or txt 6427 7762212

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Published by metbob

Pattern and Chaos

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