Pacific ocean conditions that often portend a dry winter sure haven’t so far.
Scientists like to joke that “climate is what you expect, weather is what you get.” The relatively soggy winter so far is a classic example of that.
A closely-watched oscillation in the Pacific is in the La Niña phase this winter, creating colder-than-normal surface temperatures and distorting weather patterns. Usually a La Niña means drier-than-normal conditions for Southern California in particular and often for northern parts of the state as well. Not this year–at least not so far. The rain set multiple records over the weekend. Los Angeles has had a third of its average annual rainfall in a week. So what’s going on?
“In La Niña conditions, which is what we have now, the main storms that come into North America come barreling into Washington, Oregon and British Columbia more,” Trenberth told me in a phone interview.
But lately a persistent region of high pressure in the north Pacific is diverting storms south, into California. Trenberth says: “There’s a crapshoot or a random component to it, if you like, in the more northern latitudes, that’s adding some extra flavor to what’s going on, I think.
But that doesn’t mean it’ll keep raining. The tap could be shut off at any time and Trenberth, for one, still thinks it’ll happen. He says this is considered a “strong” La Niña and is still likely to wield influence over the winter as a whole. One clue is ocean temperatures in the central-to-eastern Pacific, which are running 2 degrees C (3.5 F) below normal. “That only occurs—probably less than 10% of the time, so it’s a relatively rare event and certainly stronger than anything we’ve seen in recent years,” said Trenberth.
That’s more the case for the Southland. As you move north, La Niña’s influence on precipitation reverses itself and the Pacific Northwest gets doused. Trenberth says the transition line is right about 40 degrees north latitude. Around that line, La Niña’s effects become murky and things can go either way. San Francisco is at 37 degrees north and Reno is at 39.