Waning La Niña turns winter rainfall prospects to 50:50

Liz Wells March 31, 2021

THE La Niña weather pattern, which began in September 2020 and led to widespread heavy rains throughout the eastern seaboard in early March, is beginning to fade as the El Niño–Southern Oscillation (ENSO) returns to neutral.

The Bureau of Meteorology’s ENSO Outlook has moved from ‘La Niña’ to ‘Inactive’ as most ENSO indicators have now returned to neutral levels.

Senior climate scientist with the Bureau, Greg Browning, said with ENSO returning to neutral and climate signals at their weakest in autumn the outlook for rainfall over the coming winter was 50:50.

“This time of year there are no strong climate drivers like La Niña or El Niño, so we are going back into a neutral ENSO phase which means we don’t have seasonal drivers that favour either above or below average rainfall,” he said.

“It comes back to short term climate drivers. We are heading into the time of year when the Southern Annular Mode will be more of an influence. It is not much of a factor in autumn, but as we go into winter it can be.”

Mr Browning said the Bureau was expecting a return to ‘normal’ rainfall.

“Based on the latest outlook, it is looking like it is going back to a neutral signal which means roughly equal chances of above or below average rain for most of south eastern Australia from the southern half of Queensland through to Victoria,” he said.

“There is some indication Victoria has a slightly greater than 50 per cent chance of above median rainfall, but it is not a strong signal.

“For all intents and purposes we are going back to a neutral climate signal which means we are likely to be dominated by short term weather influences. That is a similar story for the longer term outlooks.”

Mr Browning said Western Australia was normally not as strongly influenced by La Niña as the other states, but there had been some very warm waters develop off the WA coast in recent months.

“That has favoured wetter events. When a good weather system moves through it has more moisture in the atmosphere that it can tap into. Overall WA has done reasonably well,” he said.

La Nina developed late

Mr Browning said the latest La Niña that became active at the end of September was a ‘late developer’.

While La Niña is typically associated with wetter than average conditions over much of eastern and northern Australia throughout the spring and summer months, relatively dry conditions persisted across most of Australia into November.

“While La Niña is a seasonal driver and is very significant for Australian climate in general, there are other climate drivers that can come into play and can either enhance the impacts of La Niña or suppress them,” he said.

“What we saw in November was a strong episode of the Madden Julian Oscillation, which is a tropical wave that can have a big impact on Australian weather where it typically suppresses rainfall over Australia. That seemed to be the main driver in November.”

Mr Browning said things started to pick up in December and rainfall became more widespread into the new year through to the heavy falls that drenched much of the eastern seaboard in March.

“Ideal weather conditions tapped into the moisture in the atmosphere and caused the rainfall. It made a significant difference, mainly in NSW, and some parts of southern Queensland,” he said.

However, Mr Browning said Queensland did the worst out of all the areas for rainfall under the La Niña, apart from significant falls in the southern and south east corner.

“Queensland often typically does quite well in La Niñas, but it didn’t this time,” he said.

Neutral outlook

The Bureaus says climate model outlooks suggest the Pacific Ocean will remain at neutral ENSO levels through the remainder of the Southern Hemisphere autumn and winter.

Tropical Pacific Ocean sea surface temperatures have persisted at ENSO-neutral values for several weeks.

Below the surface, much of the tropical Pacific is now at near average temperatures.

Atmospheric indicators are also generally at neutral ENSO levels.

The Southern Oscillation Index (SOI) is close to zero, while trade winds are currently being enhanced by the Madden–Julian Oscillation (MJO).

Only cloudiness near the Date Line continues to show a weak La Niña-like signature.

The Bureau says these changes are consistent with climate model outlooks, which have indicated a return to ENSO neutral during the southern hemisphere autumn, with little indication of a return to La Niña patterns in the coming months.

A return to ENSO neutral conditions in autumn is also typical of the life cycle of ENSO events.

All models indicate ENSO will remain neutral until at least the end of the southern winter.

During this time, other drivers, such as the Madden-Julian Oscillation are likely to play a bigger role in influencing Australian rainfall patterns.

The Madden–Julian Oscillation (MJO) is currently the strongest climate driver influencing Australia.

The MJO has moved into the Australian region at moderate strength and is expected to bring increased cloudiness and rainfall to far northern Australia and the broader Maritime Continent over the next week or two. This also brings an increased risk of tropical low/cyclone activity.

The Southern Annular Mode (SAM) is currently neutral and expected to remain neutral for the coming fortnight. A neutral SAM has little influence on Australian rainfall.

The Indian Ocean Dipole (IOD) is also neutral. The IOD typically has little influence on Australian climate from December to April.

The Bureau says climate change continues to influence Australian and global climate.

Australia’s climate has warmed by 1.44 ± 0.24 °C over 1910–2019, while recent decades have seen increased rainfall across northern Australia during the northern wet season (October–April), with more high-intensity, short-duration rainfall events.

Southern Australia has seen a reduction of 10–20 per cent in cool season (April–October) rainfall in recent decades.




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