An explanation of Sonoma County’s atmospheric river
Wet and weary Sonoma County residents may not appreciate it, but the giant system of wind and rain that blasted us this week was severe enough to capture headlines across the U.S., Europe and even China.
What just hit us?
It’s what meteorologists call an atmospheric river event, sometimes referred to as “pineapple express,” a recently discovered phenomenon that’s responsible for many of California’s wild swings in weather, from widespread flooding to record-setting drought.
California has long experienced weather extremes, but up until the mid-’90s scientists were not sure why. Most of California’s winter storms are born in the cold, far western Pacific near Russia. They ride the jet stream east toward North America, and spin out great arcing arms of moisture, hundreds of miles long.
If the storm bands reach California, they bring rain and snow. These cyclonic storms are regions of very low pressure in the atmosphere, and like a great spinning vacuum, they pull in air from far away.
Down along the Earth’s equator, conditions are very different. There, perpetual tropical sun and heated air each day evaporate vast amounts of water from the ocean.
If conditions are right, some of that warm, wet tropical air can be sucked north by the spinning deep low pressure storms off our coast.
When that happens, a “river” of warm, moisture-laden clouds carrying huge amounts of water vapor develops, barely 100 to 200 miles wide, propelled by very high winds and very close to the ocean’s surface.
Stretching up from the tropics, drawn by the spinning low pressure center, they take aim at the coast of California.
They can race across thousands of miles of ocean in a few days. And when these atmospheric rivers roar ashore and meet mountains like those on the Sonoma County coast, they’re pushed up, dropping some of their moisture in the form of dense rain as they cool. The longer they linger, the more rain they release before moving inland, up to 8 to 10 inches or more in a day.
The water these atmospheric rivers carry can be vast. Meteorologists compare them to giant fire hoses. A moderate one transports 7 to 15 times the amount of water discharged by the Mississippi River in a week, and they are unstoppable.
The resulting storms can produce widespread flooding. All the great California floods in recent decades have been caused by atmospheric rivers.
But not every atmospheric river is a flood-spawning monster. What’s important is that on average, these tropical atmospheric rivers deliver up to half of all the state’s precipitation each year. And when they don’t arrive, we experience drought.
Are there more of these rivers out there with California in the bull’s-eye? And what, if anything, can be done to reduce the unpredictability and impacts of atmospheric river extremes, drought and flood?
Those questions are on the top of the agenda for Sonoma County’s Water Agency, which is responsible for the managing the region’s water resources. According to Chief Engineer Jay Jasperse, that includes the water on the ground, and increasingly, water in the sky that has yet to fall to Earth.
Jasperse and other Water Agency staff are now spearheading advanced projects in partnership with an elite team of federal, state and university agencies, to handle atmospheric rivers and the wild variability in water resources they create.