An El Niño developed in the Pacific Ocean in February, and it’s probably too weak to have much global impact, the U.S. Climate Prediction Center said.

Sea-surface temperatures in critical areas of the ocean warmed to El Niño thresholds, and corresponding reactions in the atmosphere, also needed to declare an event, were detected, prompting the center to issue an advisory. There is a 50-60 percent chance the criteria will persist into summer.

“One of the reasons we are declaring is that the central Pacific sea-surface temperatures continue to be warm; in fact, they seem to be increasing,” said Michelle L’Heureux, a meteorologist with the center in College Park, Md. “The other main reason, unlike the end of 2014, is that we see some pulse forming in the atmosphere.”

El Niño can change weather patterns around the world to bring more rain across the U.S. South during the winter, disrupt hurricane formation in the tropical Atlantic, and create drier conditions across the western Pacific and northern Australia, as well as parts of Brazil.

“Due to the weak strength, widespread or significant global impacts are not anticipated,” the center said in its advisory.

Forecasts of possible El Niño development persisted through 2014 with neither the U.S. nor the Australian Bureau of Meteorology declaring it had occurred. The Japan Meteorological Agency said El Niño-like conditions had been reached in regards to sea surface temperatures though the corresponding reaction from the atmosphere hadn’t taken place.

Australia declared last month that the Pacific had returned to neutral conditions, then on Tuesday raised its odds that an El Niño would occur later this year.

L’Heureux said the range of opinion is in part due to the various agencies having different criteria for what an El Niño is and giving weight to different parts of the tropical Pacific Ocean and atmosphere.

The decision to declare comes at a time of year when forecast models don’t work as well and when many El Niños start to fade. That the signals pointing to an El Niño were able to emerge in those circumstances helped prompt the U.S. to declare the event had begun.

In addition, a downwelling Kelvin wave has been noted in the ocean.

“That is really elevating the heat,” L’Heureux said. “It’s the supply of fuel for El Niño formation, though it doesn’t guarantee it.”

A year ago, a similar wave was detected in the Pacific and researchers thought it might provide the spark for an El Niño. It didn’t.

L’Heureux said the difference between this year and last was that the sea-surface temperatures in 2014 were cooler, so the wave had to make up a deficit to drive readings higher than normal. This year, the ocean is starting out warm.

There is a chance the El Niño will fade.

“There is an element of risk here; we are taking a monthly average and saying something about a phenomenon that is seasonal,” L’Heureux said. “I cannot sit here and rule out that in a month or two this all fades away.”

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