The weeks of heavy rain that triggered historic flooding in northeast Australia this month have been blamed on what climatologists are now calling one of the strongest La Nina events in the Pacific Ocean since record-keeping began a half-century ago.
La Nina's global influence is also being blamed for heavy rains in Indonesia and Brazil.
But unusually persistent cold weather this winter in Maryland, and in much of the eastern United States, heavy December rains in Southern California, and snow across the Deep South are the work of separate weather patterns in the Arctic, scientists say.
Called the Arctic, or North Atlantic, Oscillation, these air and ocean patterns have been unusually persistent for the second winter in a row, overwhelming weather "signals" from the tropical Pacific.
"That has really impacted what's going on, more than [events] out in the Pacific," said Mike Halpert, deputy director of National Oceanic and Atmospheric Administration's Climate Prediction Center in Camp Springs, Prince George's County.
La Nina is the cool "negative" phase of cyclical warming and cooling of surface waters in the tropical Pacific. The warm "positive" phase is called El Nino. Climate scientists track them by measuring air and water temperatures with data buoys and satellites.
The warm El Nino event that contributed storms and moisture to fuel last winter's heavy snowstorms in the Mid-Atlantic weakened quickly and by May yielded to a cool La Nina.
Measured by atmospheric temperatures in the region, this La Nina "might be amongst the strongest" on record, Halpert said. If you use ocean temperatures as a guide, it is "maybe the sixth-strongest" of the 22 on record since 1949.
But while a La Nina event cools the tropical Pacific in the east, it generates stronger trade winds that push the ocean's heat and increased rainfall to the west. That has helped fuel the heavy rains in Australia, Indonesia and the Philippines. Pacific cooling has even enhanced rainfall in northern Brazil, Halpert said. "They are global phenomena."
In the United States, forecasters expected the La Nina winter to bring storms and cold weather to the Pacific Northwest, the northern Plains and the Ohio Valley. It should have meant relatively dry, mild weather from the Southwest to the Southeast.
Instead, a persistent "negative" phase of the Arctic Oscillation pattern has sent the northern jet stream into what NASA climatologist Bill Patzert calls "wild meanders" around the Northern Hemisphere.
Instead of a more west-to-east flow, he said, the jet stream has looped southward and opened the door to intrusions of Arctic air.
"Starting in December, there were frigid outbreaks on the East Coast of the United States. It was also extremely cold and snowy in Europe, as well as … in western Japan," he said.
And it has persisted. Normally, Patzert said, "the Arctic Oscillation has a time scale more like a few weeks. But it's been in a negative phase that's … almost historic, lasting for a couple of months, almost."
It's the explanation for the storms that swept out of the Gulf of Alaska last month and brought heavy rains to Southern California; deadly tornado outbreaks in Arkansas and Missouri on New Year's Eve; persistent cold weather in Maryland and the rest of the eastern half of the nation; and snow across the Deep South this week.
In the 43 days since Dec. 1, temperatures at BWI Marshall Airport have been below long-term averages on 32 days.
"My story is that it's really the Arctic Oscillation" that lies behind the rough weather, Patzert said. It trashed his forecast for a dry "La Nina winter" in the Southwest and dumped 18 inches of rain last month on his home outside Los Angeles.
And while La Nina could last until summer, or even become a multiyear event, "the Arctic Oscillation can't remain this strong, or this negative, for the rest of the winter," he said.
"I came to this dance with La Nina, and I'm sticking with her."
Maryland weather blog: Frank Roylance on meteorology
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