[/?utm_medium=email&utm_source=]

SHOULD WE BE AFRAID OF ATMOSPHERIC RIVERS?  
[[link removed]]


 

Qian Cao
February 5, 2024
The Conversation
[[link removed]]


*
[[link removed]]
*
[[link removed]]
*
*
[[link removed]]

_ With flooding and mudslides in California, a hydrologist explains
the good and bad of atmospheric rivers, and how they are being
affected by global warming _

A satellite image shows a powerful atmospheric river hitting the U.S.
West Coast on Jan. 31, 2024, photo: NOAA GOES

 

_Millions of Californians were under flood alerts
[[link removed]]
and warnings of excessive rainfall
[[link removed]] on
Feb. 5, 2024, as a powerful atmospheric river
[[link removed]] sat
over Southern California. Los Angeles saw one of its wettest days on
record
[[link removed]]
with over 4 inches of rain on Feb. 4, communities reported widespread
flooding
[[link removed]],
and debris and mudslides shut down sections of highways
[[link removed]] and
roads into Malibu
[[link removed]]._

_It was the latest in a series
[[link removed]] of
atmospheric rivers to bring extreme rainfall to the West Coast. While
these storms are dreaded for the damage they can cause, they are also
essential to the region’s water supply, particularly in California,
as Qian Cao
[[link removed]], a
hydrologist at the University of California, San Diego, explains._

What are atmospheric rivers?

An atmospheric river is a narrow corridor or filament of concentrated
water vapor transported in the atmosphere. It’s like a river in the
sky that can be 1,000 miles long
[[link removed]]. On average, atmospheric
rivers have about twice the regular flow of the Amazon River
[[link removed]].

When atmospheric rivers run up against mountains or run into local
atmospheric dynamics and are forced to ascend, the moisture they carry
cools and condenses, so they can produce intense rainfall or snowfall.

A satellite view of atmospheric rivers.

Atmospheric rivers occur all over the world, most commonly in the
mid-latitudes. They form when large-scale weather patterns align to
create narrow channels, or filaments, of intense moisture transport.
These start over warm water, typically tropical oceans, and are guided
toward the coast by low-level jet streams ahead of cold fronts of
extratropical cyclones.

Along the U.S. West Coast, the Pacific Ocean serves as the reservoir
of moisture for the storm, and the mountain ranges act as barriers,
which is why the western sides of the coastal ranges and Sierra Nevada
see so much rain and snow.

Why are back-to-back atmospheric rivers a high flood risk?

Consecutive atmospheric rivers, known as AR families, can cause
significant flooding [[link removed]].

The first heavy downpours saturate the ground. As consecutive storms
arrive [[link removed]], their precipitation
falls on soil that can’t absorb more water. That contributes to more
runoff. Rivers and streams fill up. In the meantime, there may be
snowmelt due to warm temperatures, further adding to the runoff and
flood risk.

California experienced a historic run
[[link removed]]
of nine consecutive atmospheric rivers in the span of three weeks in
December 2022 and January 2023. The storms helped bring most
reservoirs back
[[link removed]]
to historical averages in 2023 after several drought years, but they
also produced damaging floods and debris flows
[[link removed]].

[An animation shows filaments of water heading toward the coast.]

Atmospheric rivers forming over the tropical Pacific Ocean head for
the U.S. West Coast. NOAA
[[link removed]]

The cause of AR families is an active area of research. Compared with
single atmospheric river events, AR families tend to be associated
with lower atmospheric pressure heights across the North Pacific,
higher pressure heights over the subtropics, a stronger and more
zonally elongated jet stream and warmer tropical air temperatures.

Large-scale weather patterns and climate phenomena such as the
Madden-Julian Oscillation
[[link removed]],
or MJO, also play an important role
[[link removed]] in the generation of AR
families. An active MJO shift occurred during the early 2023 events,
tilting the odds toward increased atmospheric river activity over
California.

[A truck drives through muddy streets that fill a large section of
town. People stand on one small patch of pavement not flooded.]

An aerial view shows a flooded neighborhood in the community of Pajaro
in central California on March 11, 2023, after a series of atmospheric
rivers. Josh Edelson/AFP via Getty Images
[[link removed]]

A recent study by scientists at Stanford and the University of Florida
found that storms within AR families cause three to four times more
economic damage [[link removed]]
when the storms arrive back to back than they would have caused by
themselves.

How important are atmospheric rivers to the West Coast’s water
supply?

I’m a research hydrologist, so I focus on hydrological impacts of
atmospheric rivers. Although they can lead to flood hazards,
atmospheric rivers are also essential to the Western water supply.
Atmospheric rivers have been responsible for ending more than a third
[[link removed]] of the region’s major
droughts, including the severe California drought of 2012-16.

Atmospheric rivers provide an average of 30% to 50% of the West
Coast’s annual precipitation
[[link removed]].

They also contribute to the snowpack, which provides a significant
portion of California’s year-round water supply.

In an average year, one to two extreme atmospheric rivers with snow
will be the dominant contributors to the snowpack in the Sierra
Nevada. Together, atmospheric rivers will contribute about 30% to 40%
[[link removed]] of an average season’s total
snow accumulation there.

[A dam spillway with a full reservoir behind it.]

After several winter storms brought record snowfall to California’s
Sierra Nevada in early 2023, Lake Oroville, California’s
second-largest reservoir, was at 100% capacity. The previous year,
much of the state had faced water restrictions. Justin Sullivan/Getty
Images
[[link removed]]

That’s why my colleagues at the Center for Western Weather and Water
Extremes [[link removed]] at the Scripps Institution of
Oceanography, part of the University of California, San Diego, work on
improving atmospheric river forecasts and predictions
[[link removed]]. Water managers need to be
able to regulate reservoirs and figure out how much water they can
save for the dry season while still leaving room in the reservoirs to
manage flood risk from future storms.

How is global warming affecting atmospheric rivers?

Warmer air can hold more moisture
[[link removed]].
As global temperatures rise in the future, we can expect more intense
atmospheric rivers, leading to an increase in heavy and extreme
precipitation events [[link removed]].

My research also shows that more atmospheric rivers are likely to
occur concurrently during already wet conditions
[[link removed]]. So, the chance of extreme
flooding also increases. Another study, by scientists from the
University of Washington, suggests that there will be a seasonal shift
[[link removed]] to more atmospheric rivers
earlier in the rainy season.

There will likely also be more year-to-year variability
[[link removed]] in the total annual
precipitation, particularly in California, as a study by my colleagues
at the Center for Western Weather and Water Extremes projects.

_This article was update Feb. 5, 2024, with flooding and mudslides in
California._[The Conversation]

Qian Cao [[link removed]],
Hydrologist, Center for Western Weather and Water Extremes,
_University of California, San Diego
[[link removed]]_

This article is republished from The Conversation
[[link removed]] under a Creative Commons license. Read
the original article
[[link removed]].

* Climate Change
[[link removed]]
* Global warming
[[link removed]]
* atmospheric rivers
[[link removed]]
* Science
[[link removed]]
* meteorology
[[link removed]]

*
[[link removed]]
*
[[link removed]]
*
*
[[link removed]]

 

 

 

INTERPRET THE WORLD AND CHANGE IT

 

 

Submit via web
[/contact/submit_to_xxxxxx?utm_medium=email&utm_source=]
Submit via email
Frequently asked questions [/faq?utm_medium=email&utm_source=]
Manage subscription [/subscribe?utm_medium=email&utm_source=]
Visit xxxxxx.org [/?utm_medium=email&utm_source=]

Twitter [[link removed]]

Facebook [[link removed]]

 




[link removed]

To unsubscribe, click the following link:
[link removed]