Sand Diego Lifeguards evacuate a Mission Valley hotel due to flooding in 2017. Courtesy San Diego Fire-Rescue

A study led by Scripps Institution of Oceanography suggests that a new pattern of wet and dry extremes is emerging in California with extreme precipitation caused by streams of moisture in the sky known as atmospheric rivers.

California already has the most volatile water resources in the country. Scripps scientists discovered that the state’s precipitation, as it becomes less frequent but occasionally stronger, will vacillate even more wildly between extremes of drought and flooding as a consequence of climate change.

The federal Bureau of Reclamation, the Southwest Climate Adaptation Science Center, NOAA, the U.S. Geological Survey, and NASA funded the study, “Precipitation regime change in Western North America: The role of Atmospheric Rivers,” which appears Tuesday in the journal Nature Scientific Reports.

The researchers examined simulations of historical and future climate scenarios from sixteen global climate models focusing on western North America. They found that all 16 models project that the heavy precipitation the West gets in the future will come more prominently from atmospheric rivers.

Overall, California is expected to get about the same or slightly more precipitation over the long term but in a progressively more dramatic fashion. The frequency of precipitation will diminish because storms not related to atmospheric rivers will occur less frequently, while the atmospheric rivers will be more potent in a warming climate.

“As Mediterranean climate regions around the world are becoming more subtropical, the dry season is expanding. California is no exception,” said lead author Alexander Gershunov. “What is exceptional about California is that the heavy precipitation is projected to become more extreme.

“Now we have identified the mechanism responsible for this bolstering of extremes, and that gives us a more nuanced understanding of what to expect from future hydroclimate and a clearer interpretation of ongoing changes,” Gershunov said.

Atmospheric rivers are warm storms with high snow levels. They are becoming wetter as they warm, causing even more precipitation to fall as rain and less as snow. It will fall in progressively less frequent but more extreme bursts requiring more adaptive reservoir management based on better forecasts.

“ARs are strengthening and becoming even bigger contributors to the annual [precipitation] total, and California’s topography is ideally aligned to extract increasingly heavy precipitation from strengthening ARs,” the study said. “Notwithstanding the challenges stemming from a more volatile precipitation regime, California can take solace in the fact that it is not projected to dry as are the other Mediterranean climate regions around the world.”

So far in the 21st century, only four years have been considered “wet” in California, with precipitation totals exceeding the historical average. One of those, 2017, was the wettest year on record for many parts of the state but had been preceded by five years of historic drought.

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Chris Jennewein

Chris Jennewein is Editor & Publisher of Times of San Diego.