LAST week the Bureau of Meteorology issued a declaration that the upcoming summer would be wetter and cooler than normal, the result of observations of tropical Pacific Ocean temperatures and conditions.

The current model outlooks suggest this La Niña will persist until the late southern hemisphere summer or early autumn 2022.

All models surveyed by the Bureau indicate SSTs will meet NINO3.4 La Niña thresholds in December and January with a majority also predicting thresholds will be met in February 2022.

Atmospheric indicators including the Southern Oscillation Index (SOI), trade wind strength, and equatorial cloudiness have demonstrated a response to this oceanic cooling and are typical of La Niña conditions.

For a La Niña event to be declared, any three of the following criteria – sea surface temperature, winds, SOI and models - need to meet certain markers.

The current situation has all four criteria affirming the weather pattern.

 Sea surface temperatures in the NINO3 or NINO3.4 regions of the Pacific Ocean are 0.8 °C cooler than average, trade winds have been stronger than average in the western or central equatorial Pacific Ocean in three of the last four months, the three-month average SOI is +7 or higher (currently +8.6) and a majority of surveyed climate models show sustained cooling to at least 0.8 °C below average in the NINO3 or NINO3.4 regions of the Pacific Ocean until the end of the year.

The term La Niña describes a particular phase of the El Niño–Southern Oscillation (ENSO) climate cycle, with the opposite El Niño event usually meaning drier and warmer conditions for Australia.

ENSO is a coupled atmosphere-ocean phenomenon, which means that the transition between La Niña, El Niño and neutral conditions (neither El Niño nor La Niña) is governed by interactions between the atmosphere and ocean circulation.

In the atmosphere, ENSO is monitored via the Southern Oscillation Index (SOI), a measure of atmospheric circulation that takes the difference of atmospheric pressure between Darwin and Tahiti.

In the ocean, ENSO is most commonly monitored through observed sea surface temperatures within a region of the central and eastern tropical Pacific known as NINO3.4.

El Niño and La Niña are not turned on and off like a switch - rather, El Niño and La Niña are a function of the strength of departures from average in NINO3.4 and the SOI.

This means that if conditions are close to La Niña (El Niño) thresholds, one might expect to see some La Niña-like (El Niño-like) effects on Australia.

La Niña events are typically defined when SOI values are sustained above +7 and NINO3.4 temperatures are more than −0.8 °C below average.

Events that maintain index values close to these thresholds are generally classified as moderate to weak, while those that greatly exceed them are referred to as strong.

The increased rainfall and cloudiness in the western Pacific associated with La Niña usually means above-average winter–spring rainfall for Australia, particularly across the east and north.

The six wettest winter–spring periods on record for eastern Australia occurred during La Niña years.

In the Murray–Darling Basin, winter–spring rainfall averaged over all 18 La Niña events (including multi-year events) since 1900 was 22 per cent higher than the long-term average, with the severe floods of 1955, 1988, 1998 and 2010 all associated with La Niña.

Culla farmer, Geoff Amery said that although he didn’t put much stock in long-term forecasts, there might be some farmers who would welcome the conditions.

“Generally speaking, you don’t want rainfall in summer,” he said.

“But if you had a summer crop, that would offset the damage that happens on the dry part of your farm.”

Bureau climatologists will continue to closely monitor conditions in the tropical Pacific as well as model outlooks for further changes to this La Niña event.