This NASA enhanced satellite illustration from December 26, 2010 shows La Niña by the large (blue and purple) water stretching from the eastern to the central Pacific Ocean, reflecting lower than normal sea surface heights. Photo: AFP
Australia's Bureau of Meteorology (BOM) is in discussions with international atmospheric agencies to overhaul how La Niña and El Niño are defined, potentially rewriting the record books of global weather history.
The potential revamp of the world's most dominant driver of weather variability follows a year of occasional confusion when meteorological agencies released contradictory declarations on the state of the Pacific Ocean.
Declaring La Niña and El Niño episodes
The BOM's current definition of Pacific oscillations has been in place for about two decades and includes an analysis of sea surface temperatures compared to the average in a small region, called Niño 3.4, of the tropical Pacific Ocean.
Andrew Watkins, the bureau's manager of climate prediction services, admits this method is too simplistic.
"Just hitting a particular number doesn't necessarily mean you have locked in a pattern that will have global impacts," he said.
"We had a workshop with the National Oceanic and Atmospheric Administration (NOAA), the Japanese Meteorological Agency (JMA) and others, and came to the conclusion that no one is quite convinced we know exactly the best way of doing this."
One key area of contention with the current method is what baseline average should be used for the Niño 3.4 calculation, considering climate change has warmed the Pacific by about 0.5 degrees Celsius in the past 50 years.
As a result, a simple glance at the bureau's Niño 3.4 anomaly graph shows a clear trend towards the warm episode El Niño in recent decades when in reality the reverse is true - La Niña has dominated during this century.
"We started a project last year about, are we monitoring El Niño Southern Oscillation (ENSO) correctly," Dr Watkins said.
"Is the Niño index the right thing to be using … how do we account for global warming?"
This confusion was no more apparent than in August last year when a survey of seven agencies, including NOAA, JMA and the World Meteorological Organization (WMO), showed six declaring the Pacific in La Niña territory, while only one, the Australian BOM, had the Pacific as neutral.
Behind the discrepancy was partly what decades were used to calculate the average temperature, but also the bureau's rigorous threshold of 0.8C below average, compared to a less strict 0.5C for most other agencies.
"The point eight is up for investigation. We haven't really done this since the early 2000 and stepped back, and looked at the whole thing and thought, is there a better way of doing this?" Dr Watkins said.
Factoring in climate change and size of Pacific Basin
Another limitation of the current method is the Niño region surveyed only makes up a tiny fraction of the Pacific Basin, and therefore fails to capture the complete picture of how the Pacific is influencing global weather patterns.
An improved process of analysis would take a more holistic approach, along with abandoning using ocean anomalies, since climate change is continually altering the figures.
Dr Watkins explained a new method could focus on comparisons across the Pacific at any one point in time, similar to the method currently used to monitor the Indian Ocean.
"One of the things which seemed to get a little bit of momentum was the idea of a relative Niño index, so rather than just being relative to a base period like 61 to 90 you have it relative to the tropical mean."
He is even open to the idea of the bureau allowing the WMO to take ownership of datasets, while still allowing nations autonomy over declarations.
"There have been discussions around that topic of the WMO making declarations which the met [meteorological] agencies weren't that keen on, but we're more keen on a standard dataset we all use and then decide what's best for Australia."
One repercussion of a definition upgrade would be a retrospective reanalysis of past events, which would essentially rewrite the record books.
Past La Niña and El Niño events could be reclassified as non-existent, while new unknown events could enter the almanacs.
Forecasts now more accurate
So what is best for Australia? Regardless of criteria and thresholds, will a change improve forecasting?
Dr Watkins has shifted his philosophy in recent years on the best way to make long-range forecasts, trusting physical weather models over the older statistical methods or basic ENSO episode declarations.
"it used to be, we would go, 'there's an El Niño, it's going to be dry', now I go, 'the models are going dry, so there's the chance of El Niño'.
"Throwing your trust in dynamical models, we know from weather forecasts how much they have improved, that's the name of the game these days. I have a fair bit of faith in what the models tell us."
In other words, Dr Watkins's hope is Australians don't just assume it will be wet or dry based on hearing the words La Niña or El Niño, considering every event is unique, with its own nuances and interactions with other global climate drivers like the Indian and Southern Oceans.
A global change to defining ENSO events is realistically at least a few years away, but the BOM could adjust their baseline average to compensate for climate change as early as this year, helping to align their published data with other agencies.
"It's been front of mind ever since 2019, but unfortunately we have been slammed by fires and floods. Once we hit this neutral period, I think it's a good time to make this change," Dr Watkins said.
- ABC
