SAN DIEGO — In a La Niña year, usually associated with dry and warm weather, California has been displaying elements of El Niño: big waves, snowcapped mountains and flooded coastal streets.
Some earth scientists have been searching for an answer to explain why the usually opposite-behaving weather phenomena appear to have switched roles, with global warming emerging as a possible factor.
Jin-Yi Yu, a University of California, Irvine, atmospheric scientist, said Tuesday that climate change may have an impact on how long each phenomenon lasts, which in the last 25 years has often been for back-to-back years. That, in turn, might affect how they shape the weather.
"The research community has noticed that the duration of El Niño and La Niña events seems to be elongated so far in the 21st century, which may be a result of global warming," Yu said by email.
La Niña, defined by cooler-than-normal sea surface temperatures in the equatorial Pacific, is in its third consecutive year, although federal researchers believe the phenomenon is ending and we're headed for a "neutral" midwinter and spring, in which neither La Niña nor El Niño rules the Eastern Pacific.
El Niño is marked by warmer-than-normal sea surface temperatures in the equatorial Pacific that help encourage the tropically supercharged atmospheric rivers that bring serious precipitation to California, which describes what’s been happening since Sunday.
Yu, who has been watching it all unfold for years, documented the disappearance of traditional El Niño impacts after its spectacular showing in 1997, when snow fell in urban Southern California and waves were described as historic.
La Niña this year looks like the El Niño he used to know.
"This storm pattern in Southern California is not what we typically expect for a La Niña year," Yu said. "It is more like a winter rainfall pattern we would expect in Southern California during an El Niño year."
Yu, who is conducting National Science Foundation-funded research aimed at explaining the changing phenomena, said there may be another culprit: the so-called North Pacific marine heat wave.
"The above-normal sea surface temperatures have been lasting for seasons over the North Pacific, which can alter the pathway of the winter storms and bring more rainfall into Southern California," he said.
There's one other possibility: La Niña will show up in traditional form soon. That is what happened last winter in California, when a wet December showed promise for a drought-ending season but quickly succumbed to La Niña's warmth.
National Weather Service meteorologist Ryan Kittell noted that downtown Los Angeles has gotten nearly 9 inches of rain since the water year started Oct. 1. But if the rain stops before that period ends Sept. 30, as it did last year, that's below average for a season that typically has an average of 14 inches. That's La Niña.