Wednesday, February 25, 2015

For the West, a Winter that Has Felt More Like Spring

Temp extremes have been remarkable this month across the US. 10-13° warmer than normal across much of the West! (Credit: NWS Sacramento) Click to Enlarge.
“California's really feeling the effect of these incredibly warm temperatures,” Daniel Swain, an atmospheric science PhD student at Stanford University, said in an email.  That, combined with “very large multi-year precipitation” shortfalls is “resulting in the continuation of severe drought conditions,” he said.

Return of the Ridiculously Resilient Ridge

Record warm temperatures have been piling up across the West this year like snow in Boston.  The town of Sandberg, Calif., outside Los Angeles, had nine straight days with highs above 70°F this month, besting the previous record of seven such days set last year.  Salt Lake City had 43 straight days of above-average temperatures.  Portions of western Oregon had a record warm January.

California, for the second year in a row, saw it’s warmest December-January, with a monthly average temperature 5.1°F higher than its 20th century average.  With that heat having continued into February, it’s almost certain to be the warmest winter on record in California, surpassing the previous record set just last year.

“In the 121-year period of record, never have there been back-to-back Dec-Jan periods that were record warm for California, making this unprecedented in the state's history,” Jake Crouch, a NOAA climatologist, said in an email.

The reason for the warmth out West is an area of high pressure off the coast that Swain dubbed the Ridiculously Resilient Ridge back in 2013 for its incredible persistence as a feature in the atmosphere — week after week, month after month.

What’s keeping the ridge around for so long is something scientists, including Swain, are actively looking into.  So far, there are three main theories, Swain said:  one is that it’s just random chance in a chaotic atmosphere, though Swain says the longer the ridge holds on, the less likely that is.  Another is the idea that the amplified warming across the Arctic is lessening the temperature difference between pole and equator that fuels the jet stream, causing that river of air to meander more wildly and become stuck in certain patterns.

The most favored explanation for now, though, seems to be the extremely warm waters across the Pacific Ocean, particularly just off the West Coast, which can give rise to high pressure systems — and hold them in place.  “At least part of the answer lies in the Pacific,” Swain said.

Understanding why the waters in the Pacific are so anomalously hot requires further research, but they’re expected to retain the additional warmth for some time.

“Ocean temps change very slowly, so the record and near-record warm [sea surface temperatures] off California really help explain the continuation of the warmth out West,” Crouch said.

Read more at For the West, a Winter that Has Felt More Like Spring

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