Why are we experiencing such a dry winter?

San Luis Obispo - The TribuneFebruary 2, 2013 

If you thought that January 2013 was dry in San Luis Obispo County, you’re right.

This January, the Paso Robles Airport recorded only 0.70 inches of rain, or about 33 percent of normal. Cal Poly, home of climatology for San Luis Obispo, recorded a meager 1.35 inches of rain.

Historically, Cal Poly has averaged about 5 inches of rainfall in January. The wettest January on record was 1969, when 24.64 inches of rain fell.

With a monthly total of just 0.49 inches, January 2013 will go down as the third driest in San Francisco since rainfall records began being kept there in 1850. The latest California snow survey total has dipped to 93 percent of normal to date. Only a month ago, the Sierra snowpack was at 134 percent of normal, according to Jan Null, former PG&E meteorologist and lead forecaster for the National Weather Service.

So why are we experiencing below-normal rainfall? From my point of view, the answer is probably hidden in the Pacific.

One of the key influences on our weather is El Niño and its sister, La Niña, which are triggered by changing conditions in the Pacific Ocean that produce warmer- or cooler-than-normal ocean temperatures. El Niños produce warmer seawater temperatures and usually bring average or above-normal rainfall, while La Niña periods yield below-average seawater temperatures that usually produce below-normal rainfall.

On the other hand, this year we have neither. The U.S. Climate Prediction Center is predicting that the current neutral conditions — the infamous El Nothing — will continue through summer. Neutral conditions generally don’t produce any reliable seasonal rainfall predictions along the Central Coast.

However, there is another ocean water temperature cycle that seems to be playing a larger role. It’s called the Pacific Decadal Oscillation, or PDO, and it probably has the same effect on our rainfall as El Niño and La Niña in San Luis Obispo County. The PDO was discovered by Steven Hare, a fisheries biologist who noticed it while studying salmon runs in 1997.

The phases of the PDO are called warm or cool phases. Unlike El Niño and La Niña events, which last only about a year or so, the PDO stays in one phase for a much longer period. In fact, the PDO waxes and wanes approximately every five to 30 years.

The cool phase is characterized by lower-than-normal sea-surface temperatures in the eastern equatorial Pacific. In the warm or positive phase, which lasted from 1977 through 1998 and again from 2002 through 2007, the Eastern Pacific became warmer. Since 2008, we have been in the cool phase.

Much like La Niña, the cool phase of the PDO phase usually steers the jet stream farther north and away from the Central Coast. This condition has historically decreased rainfall. With the cool phase of the PDO lurking in the northern Pacific Ocean, we may continue to experience below-average rainfall through spring. Remember, there is lot of winter left, and only time will tell the story.

Training courses offered

PG&E will offer training courses this year for recently discharged veterans that will provide a career path into the energy industry through the utility’s PowerPathway program. For more information, visit www.pge.com.

John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and a longtime local meteorologist. If you have a question, send him an email at pgeweather@pge.com.

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