Wetter, Wilder Winters in Store for West Coast

More and stronger winter deluges will pummel the West Coast as the climate warms, projections show.

Researchers at the University of Washington looked into the size and frequency of future “atmospheric rivers” — those long, narrow swaths of moist air loaded with tropical or subtropical vapor that trigger many of the biggest flooding events on the West Coast. The stormy days that ride in on these “pineapple express” events will become more common, Michael Warner and colleagues found. Also, the extreme events of the future will be stronger than today’s extremes.

The study looked at the fastest and moistest 1 percent of days — that is, the most extreme atmospheric rivers. The researchers then examined water vapor, winds and precipitation on these extreme days from October through March at a number of West Coast points between Vancouver Island and Southern California. They compared end-of-century projections (2070 to 2099) under a high, business-as-usual emissions scenario to the historical baseline (1970 to 1999).

They found significant increases in water vapor transport on extreme atmospheric river days. Furthermore, those extreme days occurred up to four times more often than they do today. Winter mean precipitation increased by 11-18 percent, while precipitation on extreme days increased by as much as 39 percent along the West Coast.

This increase in water vapor transport — and, consequently, extreme precipitation — was largely due to an increase in the total amount of moisture in an atmospheric column because of warming and not to changes in near-surface winds, which changed very little.

What makes an atmospheric river so impactful (such as triggering large floods) is the enhanced rainfall when the moist air rises and passes over the mountains (“orographic enhancement”). Future work by the researchers will examine this local effect under climate change.

The researchers’ projected increases in cool season precipitation for the West Coast with 10 global climate models for 2070 to 2099, though not directly comparable to those that OCCRI/CIRC produced for the Northwest using the full set of available models, seem unusually high. In every season, some models in our analysis showed decreases in precipitation; results ranged from a decrease of about 10% to an increase of about 20 percent for the high-emissions scenario.

Citation: Warner, M.D., C.F. Mass, & Salathé Jr., E.P. (2015) Changes in Winter Atmospheric Rivers along the North American West Coast in CMIP5 Climate Models, Journal of Hydrometeorology, 14, 118-128. doi: 10.1175/JHM-D-14-0080.1
Photo Caption: An atmospheric river (thin yellow band) feeds torrential rain into northern California on Nov. 30. (Photo Credit: the National Oceanic and Atmospheric Administration.)

At OCCRI since 2011, Meghan Dalton works as CIRC’s project manager. A trained climate researcher with a BA in Mathematics from Linfield College and an MS in Atmospheric Science from Oregon State University, Meghan has worked closely with several Northwest communities working on Community Adaptation, including the water provider Seattle Public Utilities on the PUMA project. Meghan has worked as the lead on several regional climate assessments, including “Climate Change in the Northwest: Implications for Our Landscapes, Waters, and Communities” and “The Third Oregon Climate Assessment Report.”


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