Residents along the West Coast of the United States should prepare for changes in the powerful storms known as atmospheric rivers due to climate warming. However, the shifts will vary greatly between Southern California and northern areas like Seattle.
Recent research conducted by scientists at the U.S. National Science Foundation National Center for Atmospheric Research (NSF NCAR) indicates that as temperatures rise, the evaporation of ocean water will increase, leading to substantial changes in atmospheric rivers, particularly in Southern California. In contrast, those farther north will see atmospheric rivers more affected by warming ocean and atmospheric conditions.
These variations may sound complex, but they carry significant implications for millions. By the year 2100, for instance, atmospheric rivers impacting the Pacific Northwest may elevate the flood risk by temporarily raising ocean levels up to three times more than they do today if greenhouse gas emissions continue at high levels. Southern California will also experience increases in storm-driven water levels, though not to the same severity as in the north.
“On the ground, people will notice different reactions between the Southern California coast and the Pacific Northwest,” stated NSF NCAR scientist Christine Shields, co-lead author of the study. “It’s not a uniform scenario. Different regions respond in notably different ways.”
The research, published in Nature Communications Earth & Environment, involved contributions from co-lead author Hui Li and other scientists from NSF NCAR, Texas A&M University, and Pennsylvania State University. Funding was provided by the U.S. Department of Energy and NSF.
Sky Rivers
Atmospheric rivers are elongated, narrow streams of moisture in the atmosphere that flow from lower to higher latitudes, carrying vast amounts of water. The strongest atmospheric rivers can bring heavy rain and lead to extensive flooding, especially when they linger over areas that are prone to such disasters. However, they also play a crucial role in the Earth’s water cycle, often providing essential moisture and replenishing snowpacks.
Previous studies have indicated that climate change may cause atmospheric rivers to become larger and more intense, possibly causing greater destruction in susceptible areas. A warmer climate holds more moisture in the atmosphere, which results in increased precipitation during storms.
In this latest study, Shields, Li, and their colleagues examined how Pacific atmospheric rivers influence the upper ocean. Their goal was to understand the impending effects of future storms on ocean temperatures, sea levels, and water mixing at various depths, all of which are vital for fisheries and coastal populations.
To forecast future atmospheric rivers, the research group utilized a high-resolution version of the NSF NCAR-based Community Climate System Model, allowing for intricate assessments of atmospheric dynamics every 25 kilometers (about 16 miles) and even finer details regarding ocean behavior every 10 kilometers (6 miles). This detailed modeling enabled the recreation of specific ocean currents and required significant computing power, some of which was provided by the Derecho supercomputer at the NSF NCAR-Wyoming Supercomputing Center.
The findings surprised the scientists, revealing that a warming climate will lead to varying impacts on atmospheric rivers at different locations along the West Coast—and potentially across other global regions.
Currently, atmospheric rivers are partially driven by the evaporation of ocean water, which temporarily cools the atmosphere and drives the storms ahead. In a warmer climate, this mechanism intensifies for a specific type of atmospheric river known as a “Pineapple Express,” which impacts Southern California.
Conversely, atmospheric rivers that hit Northern California and the Pacific Northwest will start to experience the effects of warmer atmospheric and ocean temperatures, resulting in stronger storms and a significant rise in sea levels for several days before they reach the land.
Although further research is needed to clarify the reasons behind these different outcomes, Shields observes that atmospheric rivers are influenced by distinct processes, even in the current climate. For instance, the Pineapple Express flows along a moist subtropical jet from Hawaii to Southern California, while northern atmospheric rivers are often associated with a drier, polar jet stream. They tend to be windier and less linear than the Pineapple Express.
No matter the underlying mechanisms, vulnerable communities along the West Coast should prepare for stronger storms and notably different weather patterns.
“Atmospheric rivers function similarly to tropical cyclones, as they generate fierce winds and transport massive amounts of water that can severely damage local infrastructure,” Shields said. “It’s vital for communities to comprehend how these storms will evolve in the future to adapt and plan appropriately.”
The modeling also illustrated that the complex interactions of atmospheric rivers with the upper ocean will change as the climate shifts.
“The powerful winds and rainfall associated with atmospheric rivers could significantly affect the upper ocean, potentially altering ocean dynamics and ecosystems over broader spatial ranges and extended timelines than what we currently observe,” said Li. “Understanding how atmospheric rivers influence the ocean today and how they might shift in the future is essential.”
