Climate warning over increase in La Nina events

BISHOP, CA - OCTOBER 28: Water cascades around fallen leaves stuck on a branch as the fall color season comes to an end with a rare third year of La Niña expectedly ushering in a low snowfall winter and a fourth year of extreme drought on October 28, 2022 near Bishop, California. La Niña, or the girl, is an upwelling of unusually cold water in the Pacific Ocean off the western coast of South America, creating warmer, drier winters in California. Its brother, El Niño, pushes tropical moisture northward, causing heavier winter precipitation. The Sierra Nevada mountain snowfall is the source of more than 75 percent of the drinking water consumed by millions of Californians yet only a fraction of the normal amount of snow fell last winter. The current three years of extreme drought, from 2020 to 2022, is the driest period on record going back to 1896. (Photo by David McNew/Getty Images)
Water cascades around fallen leaves stuck on a branch in California in a third year of La Nina last year. (Getty Images) (David McNew via Getty Images)

Long ‘La Nina’ events in the Pacific Ocean have become far more common over the last 100 years, a new study has shown – and it could have alarming consequences.

El Nino and La Nina, the warm and cool phases of a recurring climate pattern across the tropical Pacific, affect weather and ocean conditions.

Researchers at the University of Hawai'i (UH) at Manoa found that five out of the six La Nina events since 1998 have lasted more than one year – and one lasted an unprecedented three years.

Bin Wang, emeritus professor of atmospheric sciences in the UH Manoa School of Ocean and Earth Science and Technology, said, "The clustering of multi-year La Nina events is phenomenal given that only 10 such events have occurred since 1920.”

Long-lasting La Ninas could cause persistent climate extremes and devastating weather events, the researchers warn.

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The researchers examined 20 La Nina events from 1920-2022 to investigate the fundamental reasons behind the historic change of the multiyear La Nina.

Some long-lasting La Ninas occurred after a super El Nino, which the researchers expected due to the massive discharge of heat from the upper-ocean following an El Nino.

But three recent multiyear La Nina episodes (2007–08, 2010–11, and 2020–22) did not follow this pattern.

They discovered these events are fueled by warming in the western Pacific Ocean and steep gradients in sea surface temperature from the western to central Pacific.

Wang said, ‘"Warming in the western Pacific triggers the rapid onset and persistence of these events.

“Additionally, our study revealed that multiyear La Nina are distinguished from single-year La Nina by a conspicuous onset rate, which foretells its accumulative intensity and climate impacts."

What is La Nina – and can it affect the UK?

La Nina refers to the cooling of ocean surface temperatures coupled with winds and rainfall in the Pacific, but it can have knock-on effects on weather around the world.

La Nina often has the opposite impact on weather and global climate as the better-known El Nino, which is the warm phase of the so-called El Nino Southern Oscillation (ENSO).

The Met Office says that during La Nina strong trade winds blow warm water towards the west Pacific causing an upwelling of cool water from the ocean depths in the east Pacific.

This leads to variations in global weather – and the Met Office says it can influence the Atlantic jet stream and our weather here in the UK.

Prof. Adam Scaife, head of long range prediction at the Met Office said in 2020, “La Nina has a profound effect on weather across the globe with us even seeing impacts that extend across the UK.

“In late autumn and early winter it historically promotes high pressure in the mid-Atlantic, which stops Atlantic weather systems from delivering mild air to the UK, and therefore can allow cold conditions to intensify.

“However, in late winter La Nina can drive a shift of the jet stream towards the Poles increasing storminess and heavy rainfall, while bringing milder conditions."