Back-to-back La Nina events – like the one which just helped deliver New Zealand one of its warmest, wettest episodes in history – may become more frequent as our planet heats, a new study suggests. Photo / Dean Purcell
Back-to-back La Nina events – like the one which just helped deliver New Zealand one of its warmest, wettest episodes in history – may become more frequent as our planet heats, a new study suggests. Photo / Dean Purcell
Back-to-back La Nina events – like the one which just helped deliver New Zealand one of its warmest, wettest episodes in history – may become more frequent as our planet heats, a new study suggests.
The research is the latest to indicate how climate change could drive important shifts inthe wider, large-scale pattern known as El Nino-Southern Oscillation (Enso).
A key driver of climate variability - and measuring the movement of warm, equatorial water across the Pacific Ocean - Enso is better known by the opposite, weather-influencing patterns at each end of its seesaw: La Nina and El Nino.
Researchers are still debating what global warming means for this seesaw-like cycle – and a National Institute of Water and Atmospheric Research (Niwa) climate scientist says the latest findings, published overnight in the major scientific journal Nature, warrant further investigation.
Led by a team of international scientists, it found “multi-year” La Nina events were likely to become more common under future greenhouse gas emission scenarios.
Under a high-emissions scenario, the frequency of multi-year La Nina increased from one event every 12.1 years in the last century to one event every 9.1 years this century.
El Nino events – such as one that’s been forming up over recent weeks – typically happen irregularly and peak within one winter season, while La Nina can last for two years or more.
That was the case with a rare “triple-dip” event that contributed to New Zealand’s latest warmest years on record (2022, 2021); three back-to-back record warm winters (2020, 2021, 2022); our wettest year (2022); and a summer marked by extreme deluges.
Compared with single-year La Nina, consecutive events were marked by broader easterly winds along the equatorial Pacific, which slowed down the recharging of heat and allowed cold anomalies to persist.
This graph shows La Nina and El Nino events over time, using the Southern Oscillation Index. Over a period of three months or more, values below -1.0 correspond to El Nino conditions while values above 1.0 correspond to La Niña conditions. Values between -0.5 and -1.0 lean toward El Nino, while values between 0.5 and 1.0 lean toward La Nina. Values between -0.5 and 0.5 are considered neutral. Image / Niwa
In the study, researchers used climate models with future greenhouse gas forcings to find consecutive La Nina events were projected to become 19 to 33 per cent more frequent, depending on different emission scenarios.
The warming caused by greenhouse gases led to changes in the Pacific Ocean’s temperature patterns, which further enhanced the northward extent of easterly anomalies.
This effect, the study suggested, increased the likelihood of consecutive La Nina events this century, resulting in more frequent climate extremes.
Its authors said their discovery of a “two-way interaction” between the tropics and subtropics that intensified under greenhouse warming marked an advance beyond recent findings.
“Our result of a probable future increase in multi-year La Nina frequency strengthens calls for an urgent need to reduce greenhouse gas emissions to alleviate the adverse impacts.”
Niwa climate scientist Dr Daithi Stone said studying the potential effects of climate change on Enso was fraught with complexity and uncertainty.
“It’s quite plausible that aspects of Enso could change under climate change, because it could be quite sensitive,” he said.
“On the other hand, it’s very difficult for us to find models, or ways of looking at it, which we believe accurately represent what’s going on.”
Modelling by the UN’s Intergovernmental Panel on Climate Change favoured a shift towards more El Nino events, while some recent studies suggest a skew to La Nina.
In May, researchers from Australia’s CSIRO also published work suggesting both La Nina and El Nino events were becoming more frequently strong due to climate change.
Ultimately, Stone said more data and research was needed to test the latest findings.
“This paper is certainly throwing out a major hypothesis,” he said.
“Is it actually the final word? No. But it’s something for us to really delve into over coming years.”
Jamie Morton is a specialist in science and environmental reporting. He joined the Herald in 2011 and writes about everything from conservation and climate change to natural hazards and new technology.
