Since the start of the drought, a large-scale high-pressure ridge has dominated our weather at a longitude that almost never fluctuated far from the western edge of North America.
Except for a few months – for example, February 2014 – this condition hasn’t allowed the normal wintertime storm systems to progress southward into California.
Another area of high pressure about 3,700 miles away as the crow flies has also contributed.
The drought pattern across California is related to a strong and persistent area of high pressure in the upper atmosphere near Greenland. The “Greenland Block” is a blocking high-pressure ridge that has forced the polar jet stream south over the Midwest and East Coast and away form California.
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A large-scale ocean water temperature cycle in the North Pacific could help to move the Greenland Block by this winter.
This large-scale cycle is the Pacific Decadal Oscillation (PDO). It 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 through the end of 2013, the PDO has been in the cool phase.
However, this past January and February saw the PDO gradually change to a positive phase. Since March, the phase has shifted solidly to positive.
Historically, the warm phase of the PDO often moves the Greenland Block, allowing the jet stream along the West Coast to move farther southward and toward the Central Coast bringing precious precipitation. Rainfall data suggest that the PDO enhances the effects of El Niño and its sister, La Niña, which are also triggered by changing conditions in the Pacific Ocean.
Speaking of El Niño, the National Oceanic and Atmospheric Administration’s Climate Prediction Center recently decreased the chance of a weak or moderate El Niño developing this fall and early winter in the northern hemisphere to 65 percent. Last month the chances were 80 percent.
Weak and moderate El Niños generally don’t produce any reliable seasonal rainfall predictions. However, strong El Niños, usually produce above-normal rainfall — typically about 140 percent of what’s considered normal on the Central Coast as the storm track is shifted southward.
Another wildcard in this equation is climate change.
Dr. Jennifer Francis of Rutgers University hypothesize that Arctic Amplification (warming) is producing a more persistent and stronger ridges of high pressure and deeper and longer-lasting low pressure troughs.
Another interesting aspect is rainfall data from Cal Poly, home of climatology for San Luis Obispo. Since 1870, Cal Poly has never seen four straight years of drought. So what will this rain season bring? In my opinion, this multifaceted puzzle suggests a near-normal rainfall year.
Of course, these are long-range forecasts and should be taken with a grain of salt, only time will tell the story. Nevertheless, I do feel much more optimistic than in previous three rain seasons.
PG&E safety tip
Be sure smoke alarms are installed throughout your home. If the smoke alarm runs on batteries, or has battery back-up power, replace batteries at least once per year. If the low battery warning beeps, replace the battery immediately. All smoke alarms in your house should be tested every month using the alarm test button.