If we're in a La Nina, why so much rain?
Friday, January 7, 2011 at 9:48 a.m.
La Niña, Southern California's diva of drought, has been strangely silent.San Diegans, used to doing rain dances when La Niña is forecast, have instead been swamped with 8.37 inches of rain so far this season — more than twice the normal amount. Even if it doesn't rain another drop for the next five months, this year will end up one of the wettest La Niñas the city has had in the last 60 years.
What gives?
"We'd like to know, too," said Dan Cayan, a researcher for the U.S. Geological Survey and the Scripps Institution of Oceanography in La Jolla, part of the University of California San Diego. "This is one of those cases that make us look foolish."
But Cayan and other humbled long-range forecasters, who said last fall that the odds greatly favor a dry winter, think the diva is warming up her vocal chords. The prognosticators pad their predictions with caveats, but they say the next few months, when Southern California usually gets the bulk of its annual rainfall, are likely to be drier than normal.
La Niña is the dry cousin of the far wetter El Niño. When ocean waters in the Central Pacific turn abnormally cool for several months, La Niña is in the house. Those cool waters tend to force the jet stream and storm track north, leaving the Pacific Northwest wet and Southern California dry.
The current La Niña has been recognized for months as one of the strongest of the last half century. Since 1949, there have been 22 La Niña episodes, not counting this year. San Diego recorded below-normal rainfall every one of those years but one, and that one was just barely wetter. That preponderance of arid years was the basis for the dry prediction.
But instead of the diva dominating the stage, something turned La Niña into a shrinking violet, at least in California.
"La Niña and El Niño are just one aspect of the forcing of the storm track," said Marty Ralph, chief of the water cycle branch of the Earth System Research Lab in Boulder, Colo. "There are more factors."
The week before Christmas, an atmospheric river of subtropical moisture pummeled the state, flooding Mission Valley and bringing more than half of San Diego's rain so far this season. What caused that river to slam into us is open to debate.
"What we saw in December was pretty remarkable – for any time, and certainly remarkable for La Niña," Cayan said.
One theory is that another global-scale fluctuation called the Arctic Oscillation took over the show. In the positive phase of the AO, cold air in the northern latitudes stays in a relatively tight circle fairly close to the North Pole.
But in the negative phase of the AO, outbreaks of cold air from the polar region reach far into the lower latitudes. Since mid November, the AO has been strongly negative.
Some climatologists say that negative phase is what messed with the storm track and brought rain to areas that would normally be dry during a La Niña. Early last year, a similar negative phase of the AO brought copious rain to Southern California.
<SNIP>View entire article here: http://www.signonsandiego.com/news/2011/jan/07/why-so-much-rain-during-la-nina/?sciquest
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