Fourth La Nina event, if it occurs, will probably first be noticed underwater in the Pacific Ocean

Climate scientists will be watching the subsurface water temperatures in the central and eastern Pacific Ocean to determine if a fourth year of La Nina weather conditions could emerge.

The CSIROs Data 61 Senior Research Scientist Dr Nandini Ramesh, says while three-year La Niña events have happened before, there has never been a four-year event. Nonetheless “it is very unlikely, but not impossible, that it will continue for another year”.

La Niña events typically bring cooler temperatures and higher rainfall levels over eastern Australia and droughts in the west, and are being blamed for the weather effects currently flooding vast areas of inland Victoria and NSW. The forecast is for another wet summer over eastern Australia, expected to persist until about February.

El Niño and La Niña climate phenomena form on a year-to-year timescale; each lasts a few months with global effects. The current La Niña event Australia is experiencing is also responsible for the drought in the western US and excessive rainfall and flooding across South Asia this year.

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Dr Nandini Ramesh

La Niña years were also responsible for flooding in Brisbane in 2011 and 1974 and the record Murray River flooding of 1974.

Australia has never had four La Nina weather conditions in a row. The closest it came was in the 1950s, when two La Niñas were followed by a relatively weak El Niño, and then a third La Niña.

The previous rainfall record in Sydney ­– which was smashed this year – was in the 1950s in this La Niña–Neutral/El Niño–La Niña period.

According to Ramesh, a fourth La Niña “would certainly be unprecedented”.

Explainer: What is La Nina?

Ramesh says scientists are watching for signs in the eastern and central equatorial regions of the Pacific Ocean. “La Niña events arise when the eastern-central equatorial Pacific is colder than usual. These can turn into longer La Niña events, when the ocean’s currents and atmosphere’s winds reinforce each other, which means a La Niña state can perpetuate for years at a time.”

“We’ll be watching surface temperatures and temperatures under the surface … the underwater temperatures will give us the longest lead time in predicting the next event.”

“Conditions over the eastern-central Pacific are relatively unpredictable in March to May because of a phenomenon called the Spring Predictability Barrier (because it occurs in spring in the Northern Hemisphere). So we’ll watch carefully in about July or August next year to understand what will unfold. We’ll also be watching the pattern of surface winds in that region as they influence surface temperatures.”

Because the pattern of La Niña–La Niña/Neutral–La Niña has occurred in the relatively recent past, Ramesh, who is also a Research Affiliate in the School of Geosciences, University of Sydney, says scientists are not associating it with climate change at this stage. “Whether La Niña years are becoming more frequent is part of a hot debate right now.”

“We can’t be certain yet whether climate change is playing a role in the excessive rainfall over eastern Australia; that will take a few months of scientific study. However, we do know that the Australian region has warmed by over 1.4 degrees due to climate change, and in a warmer atmosphere, storms produce more rainfall.”

Dr Ramesh will deliver the Roderick Lecture hosted by the School of Civil Engineering on 28 October, discussing rainfall events in the face of climate change. Register here.

Engineering on 28 October, discussing rainfall events in the face of climate change. Register here.

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