Jan 13, 2014

California drought: What’s causing it?

California
USA
by
Paul Rogers
,
The Mercury News

eteorologists have fixed their attention on the scientific phenomenon they say is to blame for the emerging drought: a vast zone of high pressure in the atmosphere off the West Coast, nearly four miles high and 2,000 miles long, so stubborn that one researcher has dubbed it the “Ridiculously Resilient Ridge.”

Like a brick wall, the mass of high pressure air has been blocking Pacific winter storms from coming ashore in California, deflecting them up into Alaska and British Columbia, even delivering rain and cold weather to the East Coast. Similar high-pressure zones pop up all the time during most winters, but they usually break down, allowing rain to get through to California. This one, ominously, has anchored itself for 13 months, since December 2012, making it unprecedented in modern weather records and leaving researchers scratching their heads.

“It’s like the Sierra — a mountain range just sitting off the West Coast — only bigger,” said Bob Benjamin, a forecaster with the National Weather Service in Monterey. “This ridge is sort of a mountain in the atmosphere. In most years, it comes and goes. This year it came and didn’t go.”

The current high-pressure ridge is even stronger and more persistent than a similar ridge that parked over the Pacific Ocean during the 1976-77 drought, one of the driest in the 20th century.

Scientists know that changes in temperature cause high- and low-pressure zones around the world. In many ways, air works like water. The deeper you swim in the ocean, the stronger the water pressure, because the weight of the water above is pressing down on the water below. Air in the atmosphere also has weight, and as temperatures of the ocean and land fluctuate, the atmospheric pressure also changes, helping drive much of our weather.

What researchers don’t know, however, is why the current high-pressure ridge is so persistent, or when it is going away, allowing California to enjoy some much-needed rain. A few scientists say that it may be related to climate change, but nobody knows for sure.