It’s called the PDO, the long-term weather pattern sucking the Golden State dry.
Think of it as the Pretty Dry Oscillation – the Pacific Decadal Oscillation. The official name means Pacific (a Pacific Ocean pattern) Decadal (it lasts more than 10 years) Oscillation (it goes back and forth). It is why California is stuck in a drought.
It’s as simple as hot and cold.
Hot air holds more moisture than dry air. Just think of how steamy and sticky the air can feel during a heat wave.
The Pacific Ocean provides moisture for California and the West. When the waters off the West Coast are warm, they heat and moisten air overhead. The prevailing westerly winds blow the ocean air inland and it rains and snows over the cooler lands, particularly over the mountains. When the ocean is cool, the overhead air is cooler and drier.
(Left) The Positive PDO kept the waters off the West Coast warm. (Right) Since 1999, the Negative PDO usually keeps the waters cooler. Source: Browning Newsletter
There is a 50-to-60-year weather pattern, the PDO that determines how hot or cold the West Coast water is. This Pacific Decadal Oscillation used to be positive, keeping the waters relatively warm from 1976 to 1999. Since then, the waters have usually been cooler. This means the ocean airs bring very little moisture inland.
Since 2012, the situation has gotten very bad. Over 80% of California is in drought and there is almost no snow on Southwestern mountains. Nearly 70% of the water California uses comes from melted snow.
No wonder the Governor of California declared a drought emergency. He is listening to weather experts warning him that the next few months may be as dry as this January!
Compare how much snow there was last winter compared to now! Source: NASA
The negative PDO is a trend that usually lasts for 25 to 30 years. Even when the drought ends, the state will be drier than it used to be. Californians are going to have to conserve their water.
There is some good news. Pacific experts are warning that there is a good chance this year will see the arrival of an El Niño by autumn. El Niños usually bring glorious rain and snow to the West. Hang in there, California!
Evelyn Browning Garriss, historical climatologist, is a longtime writer for The Old Farmer's Almanac. She is also editor of The Browning World Climate Bulletin and has advised farmers, businesses, and investors worldwide on upcoming climate events and their economic and social impact for the past 21 years.