Bob Bog 12 July

Compiled Sun 12 July 2020

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.

ENSO (El Nino /Southern Oscillation) update

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 strengthens the trade winds, and the El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator, weakening the trade winds. 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.

The Atmosphere:

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.

Since a brief El Nino late in 2019 the ENSO has been roller coasting above and below zero, in a neutral away . Recently, the subtropical ridge in the southern hemisphere has been remaining in the same position, slightly south of normal.

The SO can be seen at


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

The Ocean:

The sea surface temperatures in the equatorial eastern Pacific has been is now showing a cooling (La Nina) trend. It has recently stalled , but climate models show this stall is likely to be temporary


Sea temperatures, as seen at

Sea temperature from


This image shows the Humboldt current moving onto the Galapagos, cooling the sea.

Here are the anomalies from CPC


Waters just beneath the surface are slightly cooler than normal. These trends are towards a La Nina.


The International Research Institute of the Climate Prediction Centre compiles data from several ENSO prediction models.

The model forecasts shows ~45% near-equal chance of La Nina or neutral over next few months, with a slightly more chance of La Nina from Sep to Jan then neutral. It may be a weak La Nina.


CPC/IRI predictions from

So the outlook is for a near Neutral window, but we on watch for La Nina trends.


The Tropics

The latest cyclone activity report is at and Tropical Cyclone Potential is from

CHRISTINA is traveling west in the north Pacific but should fade before reaching Hawaiian longitudes. However, it is likely to be followed by another cyclone during the next week.



Weather Zones (see text) as expected Wednesday 00UTC showing isobars, winds, waves(magenta), Rain (Blue), CAPE (pink), STR, and SPCZ.)


SPCZ=South Pacific Convergence zone.

The SPCZ is active around Solomon Island, over northern Vanuatu, and Tuvalu/Tokelau/Samoa with a trailing trough further SE.

A brief trough should cross New Caledonia to Fiji slowly from Tuesday to Saturday.


Accumulated rainfall for next week from

Subtropical ridge (STR)

HIGH 1026hPa over northern NZ moving off to east along 40-45S,

Next HIGH 1034hPa expected south of Tasmania by Tuesday to get south of NZ as 1028hPa by Thursday the go NE and rise to 1030hPa. It also should leave a ridge back into Tasman Sea for a few days 1024hPa to 1018hPA.

Active trough in Tasman sea to cross northern NZ around Wed followed by a Low until Sunday then SE/S winds.

From Panama: Squally due to ITCZ, but often light winds. 10 to 15kt W to SW winds from 80W to 90W. then southerly winds to 95W, then SE winds. Tropical features likely to form off west of central America, but not a bother on voyage to Marquesas.


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