By Tom Saunders, ABC
Photo: 123rf
The great rainmaker, La Niña, could be back for the fourth time in five years, increasing the prospect of a soaking start to 2025 across most of Australia.
La Niña refers to a cooling of the tropical Pacific Ocean and the subsequent shift in global weather patterns, including a strengthening of moist easterly winds blowing towards Australia, and a subsequent increase in cloud development and rain over the longitudes.
Your average La Niña forms in winter, peaks in late spring, and then gradually weakens through summer. However, the current edition has not played by the rule book - for only the second time in 75 years, its onset has arrived in the middle of summer.
Pacific showing multiple La Niña signatures
La Niña was forecast by modelling all through 2024, however, after initial development back in autumn, the cooling of the Pacific stalled through winter and spring.
While hopes of La Niña had mostly diminished by December, waters along the equator took a major dip in the last weeks of the year and have now breached La Niña thresholds.
The key value used to monitor the state of the Pacific is called the Niño3.4 Index, and its latest measurement was reported by the Bureau Of Meteorology (BOM) at 0.9C below average, just breaching the BOM's La Niña threshold of 0.8C below.
The Southern Oscillation Index (SOI). Values below -1.0 correspond to El Niño conditions while values above 1.0 correspond to La Niña conditions. Photo: Supplied / NIWA
What is critical though for any climate driver is a response by the atmosphere to the shift in the ocean, and there are multiple signs the weather across the Pacific is also now in a La Niña phase, including:
- Trade winds are stronger than normal.
- The Southern Oscillation Index in December was around +10, above the La Niña threshold of +7.
- Cloudiness has shifted from the International Date Line to Indonesia.
- The tropical Pacific sub-surface is cooler than normal.
The signals, both atmospheric and oceanic, would need to be sustained for at least three months for a fully-fledged La Niña episode, however typically weather patterns have already shifted well before official declarations.
If La Niña conditions are to persist for another two months, it would result in the fifth occurrence since 2020, a regularity seen twice over 125 years of data, from 2007 to 2011, and in the early 1970s.
How a January-forming La Niña will impact our weather
The more intense La Niña events can last for years and lead to widespread flooding, as seen recently from 2020 to 2022 when record rain fell across much of New South Wales.
But what happens when La Niña forms in the middle of summer?
As the only other occasion, since 1950, of an onset this late was in 2008/09, using historical comparisons is futile.
Photo: RNZ / Mary Argue
If we instead analyse the impact of all 34 La Niña events since 1900 but only look at summer and autumn weather, the data shows Australia has received a 20 percent boost in rainfall between December and May.
While this represents a clear and noticeable increase, the impact is less significant than the 49 percent boost observed in spring.
While historical trends were used exclusively as predictions just decades ago, in recent years mathematical modelling now outperforms averaging past events to forecast the seasons ahead - and most are pointing towards a wet start to 2025 for Australia.
The BOM's own model, ACCESS, shows a medium-to-strong swing to wetter than normal conditions for the next three months, apart from in South Australia, Victoria and Tasmania.
Looking globally, the well-renowned ECMWF model is tipping up to a 90 percent chance of above median rainfall across Australia's tropics and east coast and extends the likely wetter region as far south as Adelaide and Melbourne.
This satellite illustration from 2010 shows La Niña by the large (blue and purple) water stretching across the eastern and central Pacific Ocean. Photo: AFP
The arrival of La Niña can also dampen temperatures, as an increase in cloud and rain reduces the intensity of incoming sunlight.
However, most modelling still forecasts average-to-above-average temperatures across Australia for the first few months of 2025 - a result of very warm sea surface temperatures surrounding our country, climate change, and in the short term, an absence of monsoon rains.
Will the BOM declare La Niña?
The BOM made a major change to how they communicate climate indicators last month, moving away from reporting on individual drivers, and instead encouraging the community and media to follow their official forecasts.
Photo: RNZ / Mary Argue
The shift includes a retirement of the ENSO dial, meaning the BOM will no longer issue La Niña Watches or alerts, a move designed to prevent overly simplistic headlines or notions of the upcoming seasons that don't convey the nuances of a complex climate pattern.
A likely catalyst of the change was the "hot and dry summer" headlines following the declaration of El Niño in September 2023, even though historically El Niño has not normally reduced summer rain.
Competing climate drivers can also compromise the value of declarations since interactions can both enhance but also soften the impact of each factor.
- ABC
