Bob Blog 17 Jan

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 Sun 17 January 2021

The state of the ENSO

La Nina is at its peak

The tropics drive our weather pattern. El Nino and La Nina are opposite ends of a 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.

The El Nino, with warmer than normal seas, draws the subtropical ridge closer to the equator, weakening the trade winds, and allowing the westerly winds of the roaring 40s to get further north. 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 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.

The SOI has been positive since last August and is now reaching as high as plus 2.


SOI can be seen at

The Ocean:

The sea surface temperatures in the equatorial eastern Pacific zone NINO3.4 is used as our oceanic parameter. This started going negative in May 2020 and reached a peak in coolness last November and is no slowly relaxing.

Looking at the two graphs shown here, we can conclude that the El Nino was strongest in the ocean a few months ago and is now strongest in the isobars, but seems to be at its peask as the oceanic temperature are relaxing.


Sea temperature anomaly, taken from


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 weaken to neutral by May 2021.

Impact on South Pacific

Trade winds should stretch further south than normal and may be more robust than normal.

As for tropical cyclones, well looking at the sea surface temperature, the most likely area for formation is around Coral Sea/New Caledonia.

The Tropics

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

The southern Indian Ocean remains the basin of attention , with TC JOSHUA drifting to the west. There are some zones of high potential for tropical cyclone formation around north Australia and Philippines.



Weather Zones Mid-week GFS model showing isobars, winds, waves(magenta), Rain (Blue), STR (Subtropical Ridge), and SPCZ (South Pacific Convergence Zone).



Predict wind CAPE mid-week gives two possibilities for SPCZ

SPCZ=South Pacific Convergence zone.

SPCZ is spread out from Coral Sea to Vanuatu/New Caledonia to Fiji with a weaker convergence zone from Samoa to the east.

A low south of Fiji is expected to move east along 25S to 30S, followed by a string of lows forming south of Fiji then moving off to the southeast.


Rain Accumulation next five days from

Subtropical ridge (STR)

HIGH 1016 in north Tasman sea s expected to fade away by Tuesday.

New High 1020 over Tasmania on Wednesday should move northeast across Tasman sea and reach northern NZ over the weekend— in time for the next two Round Robins in the Americas Cup in the Hauraki Gulf (very light winds on the Saturday).

NZ/Tasman troughs

Strong westerly flow over southern NZ on Monday turning NW on Tuesday.

Deep low 970hPa expected to travel across south of South Island on Tuesday/Wednesday with associated trough over NZ. Then there is expected to be strong SW flow and large swells on Thursday, easing on Friday.


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