This year is shaping up to be the driest in downtown Los Angeles since 1877.
Only 3.60 inches have fallen at the National Weather Service station at USC since Jan. 1, about half an inch less than was recorded in 1953 and 1947, which until now had tied for the lowest rainfall.
In the cycle's negative phase, the surface waters of the western Pacific warm while the eastern Pacific cools, rather like a big La Niña that pushes the jet stream and the storms it carries to the north of California.
The reverse ocean temperature pattern prevails when the oscillation is in the positive phase, producing wetter, El Niño-like conditions.
For more than a decade, the oscillation has tended toward the negative. "Since 1997-98 more or less, we've been in a dry pattern" in the West, Patzert said.
A glance at the weather service records backs that up. Of the 10 driest years recorded in downtown L.A., two — 2013 and 2007 — have occurred in the last decade.
But meteorologists look at rainfall years, which extend from July 1 through June 30. Three of the 10 driest rainfall years were recorded since 2001 — though at 5.85 inches, 2012-13 falls to the sixth-driest in downtown L.A. The driest rainfall year was 2006-07, when a mere 3.21 inches fell.
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