Atmospheric rivers, which are long, narrow pools of water vapor, are becoming denser and more frequent with climate change. A new study shows that a recently developed measure of the atmospheric intensity of rivers (similar to the hurricane scale) can be used to classify atmospheric rivers and identify hotspots of the most intense atmospheric rivers not only along the US West Coast but also around the world. .
Atmospheric rivers usually form when warm temperatures create moist packets of air, which are then carried by strong winds across the ocean; Some make landfall. The density scale ranks these atmospheric rivers from AR-1 to AR-5 (with AR-5 being the densest) based on how long they last and how much moisture they transport.
In part because some West Coast weather outlets use the intensity scale, “atmospheric river” is no longer an obscure meteorological term, but sharply brings to mind endless dangerous rainfall and flooding. The series of Rivers of Atmosphere that battered California in December and January, for example, occasionally reached the AR-4. Earlier in 2022, the atmospheric river that contributed to catastrophic floods in Pakistan was AR-5, which is the ranking of most harmful and most atmospheric rivers.
The scale helps communities know whether an atmospheric river will bring benefit or cause chaos: Storms can provide much-needed rain or snow, but if they’re too intense, they can cause floods, landslides, and power outages, as happened in my home state. California and Pakistan. The most severe atmospheric rivers can cause hundreds of millions of dollars in damage in days in the western United States; Damage in other areas has not been comprehensively assessed.
“Atmospheric rivers are West Coast hurricanes when it comes to public awareness of conditions,” said F. Martin Ralph, an atmospheric scientist at the Scripps Institution of Oceanography and co-author of the new study. He said people need to know when they’re coming, have a sense of how severe the storm will be, and know how to prepare. “This scale is designed to help answer all of these questions.”
Ralph and colleagues originally developed the scale for the west coast of the United States. The new study shows that atmospheric river events can be directly compared globally using an intensity scale, which is how the researchers determined where the most intense (AR-5) events form and fade, and how many events make landfall.
The researchers used climate data and their previously developed algorithms to identify and track atmospheric rivers to build a database of density-classified atmospheric river events around the world over a 40-year period (1979/1980 to 2019/2020). The study has been published in Journal of Geophysical Research: Atmosphere, Available here. which publishes research that advances understanding of the Earth’s atmosphere and its interaction with other components of the Earth system.
“This study is the first step toward making the Atmospheric Navometer a globally useful tool for meteorologists and city planners,” said Ben Guan, an atmospheric scientist at the Joint Institute for Regional Earth System Science and Engineering, a UCLA collaboration. Los Angeles and NASA’s Jet Propulsion Laboratory, who led the study. “By globally mapping the footprints of each atmospheric river, we can begin to better understand the societal impacts of these events in many different regions.”
The authors also found that more intense atmospheric rivers (AR-4 and AR-5) are less common than weaker events, with AR-5 events occurring only once every two to three years when calculating the global average. The densest atmospheric rivers are also less likely to fall, and when they do, they are less likely to maintain their strength for long and penetrate farther inland. “They tend to dissipate soon after landfall, leaving their effects more felt in coastal areas,” said Guan.
The study found four “centers,” or hotspots, where AR-5s tend to die, in the North Pacific and Atlantic Oceans, the Southeast Pacific Oceans, and the Southeast Atlantic Oceans. Cities on the coasts within these hotspots, such as San Francisco and Lisbon, are likely to see a heavy landing of AR-5s. Mid-latitudes are generally the regions most likely to have atmospheric rivers of any order.
Strong El Niño years are likely to have more rivers in the atmosphere, and stronger rivers at that, which is noteworthy because the National Oceanic and Atmospheric Administration (NOAA) recently projected that an El Niño was likely to develop by the end of summer this year.
Ralph said that while local meteorologists, news outlets, and other Western frontiers may have incorporated Atmospheric River and densitometry into their lives, adoption has been slower elsewhere. He hopes to see, in five years or so, meteorologists on television around the world incorporate a river’s atmospheric density scale into their forecasts, telling people whether an atmospheric river would be useful or if they need to prepare for a serious storm.
