Stubborn La Niña may stick around for a rare third year

A persistent La Niña climate pattern in the tropical Pacific is likely to persist through the summer and into 2023, forecasters say.

La Niña is implicated not only in the unrelenting drought in the US Southwest, but also in droughts and floods in various parts of the world, including ongoing droughts and famines in the Horn of Africa.

If La Niña continues into fall and winter, it would be only the third time since 1950 that the climate pattern in the Northern Hemisphere has continued for three consecutive winters, the United Nations World Meteorological Organization said last week.

La Niña is the cooler brother of El Niño, which together with a neutral phase forms the El Niño/Southern Oscillation (ENSO). La Niña is an annual or perennial phenomenon characterized by cool sea surface temperatures in the equatorial central and eastern Pacific Ocean associated with altered global atmospheric circulation.

The WMO’s update reflected a forecast from the US National Oceanic and Atmospheric Administration last week that favors a continuation of La Niña through the summer and into winter. However, in its July-September forecast, NOAA found that persistence of La Niña had a fairly small advantage — 52% to 46% — over a return to neutral ENSO territory. But it also said there was about a 59% chance of returning to La Niña by early winter.

Image of cool sea surface temperatures in the equatorial Pacific.

Image showing cooler sea surface temperatures in the equatorial Pacific.

(Paul Duginski/Los Angeles Times)

In addition to the historic drought in the US and the deadly drought in the Horn of Africa, La Niña is also credited with drought in southern South America and above-average rainfall in Southeast Asia and Australia, New Zealand and surrounding islands.

In the US, La Niña is often associated with wetter conditions in the Northwest and drier conditions in the Southwest, points out Marty Ralph of UC San Diego’s Scripps Institution of Oceanography. A strong and highly unusual June atmospheric flow, a mechanism that normally transports tropical moisture during the cooler rainy season, drenched Seattle and the Pacific Northwest a few days ago. Flooding and historic flooding hit the region around Yellowstone National Park this week. The Associated Press reported that there was 2.5 inches of rain Saturday through Monday in Yellowstone and up to 4 inches in the Beartooth Mountains northeast of the park. The US Geological Survey reported a record flow of one and a half times the previous 100-year record peak at the Clarks Fork tributary of the Yellowstone River in Montana.

Map graphic showing what happens in the Pacific during a La Niña.

Stronger trade winds push warm surface waters into the western Pacific.

(Paul Duginski/Los Angeles Times)

Meanwhile, hot, dry conditions in the arid U.S. Southwest have fueled extreme wildfires, including the pipeline fire near Flagstaff, Ariz. Climate scientist Daniel Swain tweeted that the fire “visually resembles an erupting volcano.”

The unusually strong atmospheric flow in the Pacific Northwest and the southwest heatwave are definitely anomalies for mid-June, said Alex Tardy, a weather forecaster with the National Weather Service in San Diego. As atmospheric flow weakened and slumped southward, it also brought thunderstorms to northern California. But he warns that he’s not sure if this can be blamed on a weak La Niña or climate change.

“I think it has to do with that [an] general east pacific pattern we’ve been stuck in for three years in northern California and two in southern California,” he said. For the second half of June, he forecasts an ongoing battle between unusually cool and wet weather in the Pacific Northwest along an active jet stream and expanding high pressure in Texas and the Southwest. There is potential for heat recovery, particularly in the deserts, he said. “This could be a very warm, well above average desert and mountain scenario, similar to what we saw in 2021,” he said.

La Niñas usually lead to fewer hurricanes in the eastern Pacific and more of them in the tropical Atlantic.

Typically, during La Niñas, there are fewer eastern Pacific hurricanes due to stronger vertical wind shear and more Atlantic hurricanes due to weaker vertical wind shear, trade winds, and lower atmospheric stability.

(Paul Duginski/Los Angeles Times)

La Niña also affects the Atlantic and Pacific hurricane seasons. NOAA forecasts there is a 65% chance that the Atlantic hurricane season will be above average with more and stronger storms. At the same time, according to NOAA, there is a 60% chance that the Eastern Pacific season will be subpar.

This is due to atmospheric shear – the difference between wind speed and/or direction near the Earth’s surface and higher in the atmosphere. A large difference between low and high winds disrupts hurricane formation. This is happening in the eastern Pacific because of La Niña, but in the main developing region of the tropical Atlantic — a swath stretching from about Mauritania and Senegal to the Caribbean, where surface winds are mainly from the east — upper westerly winds are weaker and hurricanes can occur form with less threat from vertical wind shear.

The U.S. Census estimates that at least 60 million Americans live in hurricane-prone areas, not counting residents who live farther inland and may be at risk of flooding due to heavy rains.

“Those are the effects of La Niña, but they’re being exaggerated by climate change,” said climatologist Bill Patzert, formerly of NASA’s Jet Propulsion Laboratory, referring to droughts, floods and hurricanes around the world. “There’s more going on here than your regular La Niña.”

https://www.latimes.com/california/story/2022-06-15/stubborn-la-nina-may-stick-around-for-a-rare-third-year Stubborn La Niña may stick around for a rare third year

Alley Einstein

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