Bob Blog 19 June

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.

Compiled Sunday 19 June 2022

The State of the ENSO

La Nina is lingering but has weakened

Since last August the Pacific trade winds have been stronger than normal and dragging sun-warmed sea to the west, encouraging upwelling of cooler deeper water around the Galapagos. This “La Nina” episode shifts the subtropical ridge poleward

However, the Tasman Sea has recently had a series of lows and these this week affecting the area east of NZ.

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 into 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.


SOI can be seen at

The weekly SOI was as high as over +2 in May (making this May the 2nd equal highest year on record since 1876) and is relaxing during June but still well over +1.

The Ocean:

The sea surface temperatures in the equatorial eastern Pacific Ocean are the action centre for ENSO (El Nino Southern Oscillation). This area has been blue (cooler than normal) for LA NINA shows clearly in a time-longitude plot from shown here. The rippled pattern is an artifact of west-moving eddies. There seems to have been a relation during the last month


These cool waters are surrounded by a ring of warmer waters across central North Pacific, around Indonesia and then across central south Pacific. A warmer than normal Tasman Sea has been producing extra lightning for NZ recently.

According to the International Research Institute of the Climate Prediction Centre,

at, reviewing all of the different models for forecasting the future of this La Nina, it is expected to strengthen in a few months and then weaken by end of the year.


Impact on South Pacific

Trade winds are expected to be reliable between Galapagos and Marquesas. Having the subtropical ridge further south than normal during winter should keep the winter lows south of normal as well, but this hasn’t been the case in June.

Here is the ICU Island Climate Update Rainfall outlook until August from



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


BLAS formed south of Acapulco and is travelling west.

CELIA formed offshore of El Salvador and is travelling west northwest.


Weather Zones Mid-week GFS model showing isobars, winds, waves (green to red +arrows), Rain (Blue), STR (Subtropical Ridge), SPCZ (South Pacific Convergence Zone) and CAPE (pink)


CAPE mid-week as seen by ECMWF and GFS from


Different models seem to produce different values.

SPCZ=South Pacific ConvL1ergence zone and STR (Sub tropical ridge).


Rain Accumulation next five days from

The SPCZ stretches across the Coral Sea to northern Vanuatu to Samoa.

A convergence zone/trough is expected to trave east across Fiji on Monday/Tuesday and reach the Cooks by end of the week.

The best route this week from Tahiti westwards this week seems to be the northern route, and may need to deviate around the trough


The upper trough from last week is now east of NZ and expected to form L1 that deepens as it travels SE.

L2 has also formed between New Caledonia and New Zealand in tandem with L1 and is expected to travel of to the ESE then East.

H1 is the surface sign of a long wave ridge and expected to travel northeast across central NZ by mid-week.

Between H1 and L1 , ice-chilled air is expected to be strongly shovelled northwards onto eastern NZ

Weather pattern looks OK this week for crossing the Tasman Sea.

For those wanting to sail from NZ to the tropics, this may be done on the backside of L2 once the swell has eased. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

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

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