A procession of storms is drenching Northern California this week, with rainfall already topping 2 inches in San Francisco and surpassing 8 inches in the Santa Cruz Mountains. More precipitation is on tap through the weekend, prompting concerns of widespread urban flooding and potential landslides.
The downpours are fueled by an atmospheric river, a band of moisture that can flow for thousands of miles, in this case from the tropical Pacific, near Hawaii, all the way to the West Coast. Even though you can’t see them, these rivers in the sky transport colossal amounts of water around the planet.
“An average atmospheric river instantaneously carries two to three times what the Amazon River typically carries,” said Alexander Gershunov, a research meteorologist at UC San Diego’s Scripps Institution of Oceanography.
The European Weather Model shows an approaching atmospheric river streaming into California on the last weekend of 2022. Gerry Díaz / The Chronicle
When that moisture falls as rain, it can have devastating consequences. Scientists believe that a series of atmospheric rivers caused the Great Flood of 1862, a megaflood that left 6,000 square miles of California’s Central Valley underwater. Atmospheric rivers cause about $1.1 billion in flood damage annually across the western United States.
But atmospheric rivers also provide important benefits.
“They're responsible for the majority of California’s water supply,” said Daniel Swain, a climate scientist at UCLA and the Nature Conservancy.
During the wet season, atmospheric rivers replenish agricultural and urban water supplies. Such storms also build up the snowpack that supplies the state with water beyond the end of the wet season. Climate change, however, is transforming the nature of atmospheric rivers — which makes California’s relationship with them more perilous than ever.
Beachgoers are seen on Ocean Beach under rain in San Francisco. Stephen Lam / The Chronicle
What is an atmospheric river?
Scientists generally distinguish atmospheric rivers from other storms based on the large amounts of water they transport through the atmosphere — also known as vertically integrated water vapor transport, or IVT.
“It's water moving fast above your head,” Swain said.
The higher the IVT, the more moisture being carried and the more precipitation on the way.
Atmospheric rivers also don’t look like typical storms. They stretch hundreds or even thousands of miles in length while spanning around 500 miles in width, on average. These corridors transport moisture relatively close to the Earth’s surface, within the lowest mile or two of the atmosphere. At any given moment, four or five atmospheric rivers, on average, are transiently ferrying moisture from tropical oceans to higher latitudes in each hemisphere.
Atmospheric rivers begin in the tropical Pacific, near the equator, where the sun’s rays heat up water particles, which evaporate and rise into the atmosphere.
This water vapor can get pulled toward Hawaii by the trade winds forming drawn-out bands of moisture.
The jet stream can stretch out moisture from Hawaii to the West Coast as it dips into California in the winter.
After crossing the Pacific Ocean, atmospheric rivers can encounter mountain ranges along the California coastline, like the Santa Cruz Mountains.
When this happens, moisture shoots upward and into colder layers of the atmosphere, becoming water vapor.
This water vapor condenses into water droplets, which fall back to the surface as rain.
Winds can continue to push rain from atmospheric rivers into the Central Valley, where falling rains can result in widespread flooding. Showers get trapped in the valley and can fall over cities like Sacramento for several hours or even days.
Stronger atmospheric rivers can continue east into the Sierra Nevada. Streams of heavy rain showers then shoot up into the summits of the Sierra, where chilly air causes rain to shift to snow. Squalls can bring several feet of fresh powder to the range.
In California, atmospheric rivers contribute up to 50% of the state’s annual precipitation, according to a 2011 study. The state’s unique topography along the coast shunts moisture upwards, where cooler temperatures transform water vapor to rain and snow.
“California's mountains are very effective at squeezing the moisture out of atmospheric rivers,” Gershunov said.
Atmospheric rivers can also end droughts: Between 33% and 40% of droughts from 1950 to 2010 in California ended because of landfalling atmospheric rivers, according to a 2013 study.
Categorizing atmospheric rivers and their impacts
A recently developed system, similar to ratings for hurricanes and tornadoes, categorizes atmospheric river events on a scale from 1 through 5, based on how much moisture they carry and how long they last. Events on the lower end of the scale are beneficial, up to a point.
“Beyond that point, that’s when we start to have problems with atmospheric rivers, when they’re very intense,” Swain said.
As rain falls at Powell and Market streets, a tour group from France tries to figure out a plan after finding out that cable cars were not running in San Francisco. Scott Strazzante / The Chronicle
A woman holds an umbrella while waiting for a ride along Lawton Street in San Francisco as rain falls on Election Day. Jessica Christian / The Chronicle
In February 2017, a category 5 atmospheric river drenched Northern California and filled Lake Oroville to the brim. Water cascaded over the emergency spillway and ate away at the ground holding up the structure. Fearing catastrophic failure, officials ordered evacuations of nearly 190,000 people downstream. The spillway held, but repairs cost over $1 billion.
The moisture that led to the 2017 storms emerged near Hawaii, but atmospheric rivers that make landfall on the West Coast can begin throughout the Pacific Ocean — even as far as Japan and Indonesia.
“‘Pineapple Express’ just refers to one type, or sub-flavor, of atmospheric rivers,” said Katerina Gonzales, an climate researcher with the University of Minnesota Climate Adaptation Partnership.
Gonzales led a recent study that reported that “Pineapple Express”-type atmospheric rivers that begin near Hawaii tend to carry more moisture and are slightly warmer than their counterparts, during the winter months.
Whether an atmospheric river produces rain or snow upon landfall depends directly on temperature, Gonzales said. The warmer an atmospheric river, the higher it needs to travel before precipitation shifts from rain to snow.
In California, snowpack provides around a third of the state’s water supply. When drought isn’t raging, this critical resource steadily feeds reservoirs during warmer, drier months. Rain’s impact on streams, aquifers and reservoirs can be harder to predict and more ephemeral.
Having a clearer picture of how much rain or snow an incoming atmospheric river will bring may help water managers and adaptation planners prepare for deluges. Reconnaissance flights and data collected by instruments parachuted through atmospheric rivers are improving forecasts and contributing to research studies.
The ‘other Big One’
Strong atmospheric rivers in quick succession could produce a megaflood on the scale of the 1862 Great Flood, when San Francisco tallied nearly 3 feet of rainfall and Sacramento was completely flooded, forcing Gov. Leland Stanford to row to his inauguration.
“It would be a very, very bad month for California,” Swain said. “It would be bad for virtually everybody.”
Swain and colleague Xingying Huang of the National Center for Atmospheric Research performed climate simulations and identified month-long sequences that could produce widespread flooding across the state.
Given today’s climate, a series of atmospheric rivers could produce staggering amounts of precipitation in just 30 days: 40 to 55 inches in the Sierra Nevada, for example. This rain wasn’t constant over the scenario period. Instead, there were bursts of high precipitation intensities. Such a deluge would produce catastrophic flooding.
People react as they were doused by a wave at Fort Point in San Francisco. Stephen Lam / The Chronicle
People walk down an alley during rain in San Francisco. Jungho Kim / Special To The Chronicle
A megaflood could be even more likely in the coming decades if greenhouse gas emissions continue to rise, the researchers found. In simulations of a warmer future, the precipitation was even more extreme, with the Sierra Nevada seeing an astounding 55 to 70 inches in 30 days. Across the entire state, the scenario packed 71% of the precipitation of an average water year into a 30-day period.
“That (scenario) would just blow anything we've seen out of the water — maybe even 1862,” Swain said.
On top of the heavier precipitation, in a warmer world, snow would be limited to higher, cooler elevations. That means a shift from water storage in snowpack to rain and potential runoff — a “double whammy” that increases flood risk even more, as described in Huang’s earlier work.
Atmospheric rivers in a warming world
As the climate warms, atmospheric rivers and storms are expected to become more intense and unleash more precipitation. But that doesn’t mean that droughts will be a thing of the past. California’s climate is also expected to swing more dramatically between dry and wet periods
“When it does rain, it's more likely to pour,” Swain said.
This future will present additional challenges for reservoir managers trying to balance water supply needs with flood management.
One of the reasons for more extreme weather? Changes to the paths of jet streams — winds that circle the globe from west to east about 5 to 9 miles above the Earth’s surface and steer weather systems, like storms and atmospheric rivers. With global warming, these winds are expected to slow, causing heavy rains and heat waves to linger.
Scientists also expect the polar jet stream to move northward due to global warming. That would mean atmospheric rivers that make landfall in California today could just pass by to the north. Warming temperatures could also bring a northward shift of the subtropical jet, which would bring more atmospheric rivers to Southern California, according to a 2016 study.
But exactly to what degree atmospheric rivers will be redirected in the future is an emerging area of research.
“For California, it’s actually sort of uncertain what’s going to happen,” said Christine Shields, a climate scientist at the National Center for Atmospheric Research and co-author of the 2016 study.
Harnessing the benefits and mitigating the damage of atmospheric rivers in the face of climate change will require human actions, like reining in greenhouse gas emissions and investing in resilient water infrastructure, Gonzales wrote in an opinion essay last year.
“Atmospheric rivers may be painted as the bad guy, when they’re neither the villain nor the savior of us,” Gonzales said.
As rain falls, BART riders walk through Halladie Plaza in San Francisco. Scott Strazzante / The Chronicle
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