Atmospheric rivers boost high tide flooding on US West Coast

AGU Fall Meeting 2021

Atmospheric rivers are narrow bands of moisture that cross the lower troposphere, usually at the leading edges of massive low-pressure systems. At their peak, they can carry as much water in the sky as the Amazon River on land. They unleash intense winds and heavy rain as they cross the Pacific Ocean, eventually making landfall on the west coast of the United States, contributing to widespread flooding at high tide. However, the detailed relationship between atmospheric rivers and high tide coastal flooding has not been well described.

In new research to be presented Dec. 17 at the 2021 AGU Fall Meeting, scientists have revealed how atmospheric rivers have affected many high-tide flooding events on the U.S. Pacific Coast during of the last 4 decades. The team determined that depending on location, between 10% and 63% of coastal high-tide flooding observed from 1980 to 2016 occurred in confluence with atmospheric river events. Sites in the Pacific Northwest experienced more high tide flooding and atmospheric river events overall, except for the more isolated sites in Puget Sound.

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

But storm surges from atmospheric rivers were rarely sufficient to cause flooding at high tide without the help of other factors, such as peak tides and seasonally high sea levels, the authors said. This work can help paint a more complete picture of how interactions between atmospheric rivers and other factors lead to chronic flooding at high tide, which can aid in risk assessment in the region.

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

A recipe for chronic flooding

When an atmospheric river makes landfall, it can feed a storm surge which combines with seasonally high water levels and high tide, causing high tide flooding. During these recurring minor floods, seawater backs up through storm drains, disrupting sewer infrastructure and engulfing the streets of coastal towns.

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

“Understanding minor and extreme floods is important because they inevitably change,” said Katy Serafin, a coastal risk expert at the University of Florida, who was not involved in the study. “Rising sea levels increase the frequency of these events.”

For more insight into flooding events on the West Coast, Piecuch and his team looked at 4 decades of sea level data from tide gauge records at 24 locations from San Diego to Puget Sound. They compared this data to a catalog of where and when atmospheric rivers made landfall, then tracked the number of 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 the conjunction of several factors, including storm surges, high tides caused by full moons, higher sea floor levels during El Niño years, and river events. atmospheric. They also found that high tide flooding has become more frequent over the past 40 years as climate change raises sea base levels and causes more powerful storms.

During the period studied, more than half of the high tide floods in San Francisco Bay were accompanied by an atmospheric river. This correlation, along with the identification of other factors that contribute to chronic flooding at high tide, provides new insights into what causes West Coast flooding and how to prepare for it.

—Guananí Gómez-Van Cortright, science writer

Quote: Gómez-Van Cortright, G. (2021), Atmospheric Rivers Boost High-Tide Flooding on US West Coast, Eos, 102, Published on December 17, 2021.
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