The study, “Precipitation regime change in Western North America: The role of Atmospheric Rivers,” was published Tuesday in the journal Nature Scientific Reports. It projects that California will face greater extremes of wet and dry seasons, with rainy periods dominated by atmospheric rivers — powerful plumes of airborne moisture that drench the West Coast.
“This study basically suggests or shows that in the future, more and more of our water resources will come from these atmospheric rivers that generate precipitation differently from other storms,” said lead author Alexander Gershunov, a climate scientist at Scripps.
The study analyzed data from about 16 different climate models for the period from 1950 to 2100, he said, and concluded that as time goes on, California will see heavier precipitation in the winter, with less rainfall at other times of year.
“As time goes on, we will be losing storms in the fall and spring,” Gershunov said. “Atmospheric rivers will be more intense, but more focused on January, February and March.”
Atmospheric rivers function as high-altitude pipelines that channel water vapor onshore. Those jets of moist air originate in the Pacific, carrying twice as much water as the Amazon River, according to Scripps Oceanography.
“They are ephemeral, so they come and go,” Gershunov said. “They transport tropical moisture to higher latitudes so they’re very important to the transport of water and energy in the global atmosphere.”
The warm, wet stream rises when it collides with mountains, such as California’s coastal ranges or the Sierra Nevada, and then dumps its payload on the state.
“When they make landfall, they produce extreme precipitation,” Gershunov said. “They contribute 40 percent to 50 percent of rainfall in California, from just a few events. They’re warmer, so the snow levels associated with them are much higher.”
Typical storms bring cold weather and drop snow at levels down to 2,500 feet, he said, while atmospheric rivers produce heavy rains, with snowfall levels as high as 9,000 feet.
This past winter, the state saw cold storms alternating with atmospheric rivers, causing warm rain to sluice over the snowpack and accelerate runoff. California’s water system and reservoirs are engineered to capture snowmelt rather than rainfall, so that scenario can be hard to handle.
“It means more rain at the expense of snow, and more challenges in how you manage it, because of the flood risk,” Gershunov said.
For instance, he said, rainfall from atmospheric rivers extinguished the 2017 Thomas Fire in Santa Barbara and Ventura Counties, but also caused subsequent flooding and mudslides.
Warm air holds more water than cooler air, so as the climate warms, atmospheric rivers are likely to become even wetter, with more potential for big storms on shore, he said. Moreover, the wet season is projected to occur in a collapsed time frame, so officials will have to prepare for that deluge.
They’ll need a more diversified portfolio of tools for managing water resources, Gershunov said, such as increased use of groundwater basins, along with reservoirs, to capture and store water. And they will need better forecasting abilities to contain the potential damage.
“In the future, they are going to be our most extreme events and in the future they will become even more extreme,” said Julie Kalansky, program manager at Scripps and one of the study’s authors. “So it’s important to improve our forecasting in the short term and the long term to give water managers more time to prepare for emergency events.”
The study also suggests that California may not face the same persistent drought as other Mediterranean climates, which are expected to get continually drier as the climate heats up. Instead, it will face an amplified rainfall pattern, with Southern California likely to see less rain, and Northern California possibly getting more.
“The dry parts of the state will probably become drier, and the wet parts will probably become a little wetter on average,” Gershunov said.
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