Communities up and down the West Coast of the United States can expect the potent storms known as atmospheric rivers to evolve as the climate warms. But residents in Southern California will see much different changes than residents in more northerly locations like Seattle.

New research, led by scientists at the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR), found that warming conditions will increase evaporation of ocean waters and significantly alter atmospheric rivers to the south. Farther north, however, atmospheric rivers will be most influenced by rising temperatures in the ocean and atmosphere.

While such differing processes may seem arcane, they will have real-world consequences for millions of people. By 2100, for example, atmospheric rivers that strike the Pacific Northwest could increase flooding risks by temporarily raising the heights of ocean waters as much as three times more than current storms if society continues to emit greenhouse gases at a high rate. Residents of Southern California will also experience higher storm-driven waters, but not to the extent of those living farther north.

“On the ground, people will see a different response between the Southern California coast and the Pacific Northwest,” said NSF NCAR scientist Christine Shields, a co-lead author of the new study. “It’s not a one-size-fits-all situation. You have these regional responses that can be quite different.”

The study, in Nature Communications Earth & Environment, was by a research team that included co-lead author NSF NCAR scientist Hui Li and additional co-authors from NSF NCAR, Texas A&M University, and Pennsylvania State University. The work was funded by the U.S. Department of Energy and NSF.

Rivers in the sky

Atmospheric rivers are long and narrow “rivers in the sky” that meander from lower to higher latitudes, transporting enormous amounts of water. The most powerful atmospheric rivers can unleash torrential rains and cause widespread flooding, especially when they stall over vulnerable watersheds. But they’re also a vital feature of Earth’s hydrological cycle, often delivering needed moisture and replenishing snowpacks.

Past research has shown that climate change is likely to make atmospheric rivers larger and stronger, potentially wreaking more havoc on vulnerable communities. As is the case with other types of storms, a warmer climate means the atmosphere can hold more moisture, thereby increasing the amount of precipitation that a storm can release.

In the new study, Shields, Li, and their co-authors focused on the impacts of Pacific atmospheric rivers on the upper region of the ocean. They wanted to gain insights into how future storms are likely to affect oceanic variables such as the surface temperature and height and the mixing of waters at different depths. Such variables have implications for fisheries and coastal communities.

To simulate future atmospheric rivers, the research team used a high-resolution version of the NSF NCAR–based Community Climate System Model. This enabled them to get an unusually detailed view of atmospheric processes every 25 kilometers (about 16 miles) and an even more detailed view of ocean processes every 10 kilometers (6 miles) — a simulation so fine that they could recreate individual ocean eddies. The simulations required large amounts of computing, and the scientists ran some of them on the Derecho supercomputer at the NSF NCAR-Wyoming Supercomputing Center.

To the surprise of the scientists, the simulations revealed that a warming climate will have greatly varying impacts on atmospheric rivers in different locations along the West Coast — and, potentially, other regions around the globe.

Atmospheric rivers currently are driven in part by the evaporation of ocean waters, which temporarily cools the atmosphere and propels the storms forward. In a warmer world, that process will be amplified for a type of atmospheric river, known as a “Pineapple Express,” that plows into Southern California.

But atmospheric rivers that crash into Northern California and the Pacific Northwest will begin to be affected instead by warmer temperatures in both the atmosphere and ocean. These temperatures will fuel more powerful storms and lead to a dramatic increase in sea level height for several days as the atmospheric river approaches the coast and makes landfall.

Although more research is needed to tease out the reason for these differing effects, Shields noted that atmospheric rivers are affected by varying processes even in the current climate. The Pineapple Express, for example, moves along a subtropical jet of very moist air from Hawaii to Southern California. In contrast, atmospheric rivers to the north are often connected with a less moist polar jet of air. They tend to be windier and more meandering than the Pineapple Express.

Regardless of the processes, vulnerable communities on the West Coast can expect the storms to change and become more powerful.

“Atmospheric rivers are like tropical cyclones in that they pack powerful winds and carry enormous amounts of water that can devastate local infrastructure,” Shields said. “Communities need to understand how they will change in the future so they can adapt and plan.”

The modeling also showed that the complex downstream and upstream effects of atmospheric rivers on the upper region of the ocean will evolve as the climate warms. 

“The strong winds and precipitation linked to atmospheric rivers can significantly impact the upper ocean, potentially impacting ocean dynamics and ecosystems over larger spatial scales and longer timescales than the present,” Li said. “It is important to understand the influence of atmospheric rivers under the current climate and how they may change in the future.”
 
 

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