An atmospheric river drenches California with heavy rain in 2019.
Atmospheric rivers, like the one pictured over California in 2019, play a role in chronic coastal flooding. Credit: NOAA satellites
AGU Fall Meeting 2021

Atmospheric rivers are narrow bands of moisture that travel across the lower troposphere, generally at the leading edges of massive low-pressure systems. At their peak, they can carry as much water through the sky as the Amazon River does on land. They unleash intense winds and heavy rain as they surge across the Pacific Ocean, eventually making landfall on the U.S. West Coast, contributing to many high-tide flooding events. However, the detailed relationship between atmospheric rivers and coastal high-tide flooding has not been well described.

In new research that will be presented on 17 December at AGU’s Fall Meeting 2021, scientists revealed how atmospheric rivers affected many high-tide flooding events on the U.S. Pacific coast over the past 4 decades. The team determined that depending on the location, anywhere from 10% to 63% of coastal high tide flood events observed from 1980 to 2016 occurred in confluence with atmospheric river events. Pacific Northwest sites experienced more high-tide floods and atmospheric river events overall, with the exception of more secluded Puget Sound sites.

“If we want to provide the most helpful information to people assessing future coastal risk, we need to know more than just sea level rise.”

But storm surges caused by atmospheric rivers were rarely enough to cause high-tide floods without help from other factors, such as peak tides and seasonally high sea levels, the authors said. This work can help paint a fuller picture of how interactions among atmospheric rivers and other factors lead to chronic high-tide flooding, which can help with risk assessment in the region.

“If we want to provide the most helpful information to people assessing future coastal risk, we need to know more than just sea level rise,” said team leader Chris Piecuch, a physical oceanographer at the Woods Hole Oceanographic Institution and first author of the study.

A Recipe for Chronic Flooding

When an atmospheric river makes landfall, it can fuel a storm surge that combines with seasonally high water levels and a high tide, causing a high-tide flood. In these recurring minor flood events, seawater comes up through storm drains, disrupts sewage infrastructure, and engulfs streets in coastal towns.

These low-level floods may seem minor, but they interfere with transportation, damage homes and businesses, and threaten the public safety and long-term health of coastal communities over time.

“Understanding both minor and extreme flooding events is important because they inevitably are changing,” said Katy Serafin, an expert in coastal hazards at the University of Florida who was not involved in the study. “Rising sea level is increasing the frequency of these events.”

For more insight into flooding events on the West Coast, Piecuch and his team scrutinized 4 decades of sea level data from tidal gauge records at 24 locations from San Diego to the Puget Sound. They compared these data to a catalog of where and when atmospheric rivers made landfall, then tracked how many floods and atmospheric rivers each location experienced to calculate how often these factors occurred simultaneously.

The researchers found that the recipe for chronic flooding involves several factors coming together, including storm surges, high tides caused by full moons, higher background sea levels during El Niño years, and atmospheric river events. They also found that high-tide floods have become more frequent in the past 40 years as climate change raises baseline sea levels and churns up more powerful storms.

During the studied period, more than half of the high-tide flooding events in the San Francisco Bay were accompanied by an atmospheric river. This correlation, as well as identifying other factors that contribute to chronic high-tide floods, provides new insights into what causes floods on the West Coast and how to prepare for them.

—Guananí Gómez-Van Cortright, Science Writer

Citation: Gómez-Van Cortright, G. (2021), Atmospheric rivers spur high-tide floods on U.S. West Coast, Eos, 102, https://doi.org/10.1029/2021EO210669. Published on 17 December 2021.