Atmospheric rivers deliver rain to California in 2017. NASA
Tom Corringham, University of California San Diego
Ask people to name the largest river in the world and most will likely guess it is the Amazon, the Nile, or the Mississippi. In fact, some of the world’s largest rivers are in the sky – and they can create strong storms, like the one that is now soaking California.
Atmospheric rivers are long, narrow bands of moisture in the atmosphere that extend from the tropics to higher latitudes. These rivers in the sky can carry 15 times the volume of the Mississippi. When this moisture reaches the coast and moves inland, it rises over the mountains, generating rain and snowfall, and sometimes causing extreme flooding.
Atmospheric rivers are an important source of water for the western United States. NOAA
In the past 20 years, as observation networks have improved, scientists have learned more about these important weather phenomena. Atmospheric rivers occur worldwide and affect the west coast of the world’s major landmasses, including Portugal, Western Europe, Chile, and South Africa. So-called “Pineapple Express” storms, which transport moisture from Hawaii to the west coast of the USA, are just one of their many flavors.
My research combines economics and atmospheric sciences to measure damage from severe weather events. Recently, I led a team of researchers from the Scripps Institution of Oceanography and the Army Corps of Engineers on the first systematic analysis of damage from atmospheric rivers due to extreme flooding. We found that while many of these events are harmless, the largest are causing most of the flood damage in the western United States. It is predicted that atmospheric rivers will become longer, wetter, and wider in a warming climate.
Rivers in the sky
On February 27, 2019, an atmospheric river drove a cloud of water vapor 350 miles wide and 1,600 miles long through the skies from the tropical North Pacific to the coast of Northern California.
North of San Francisco Bay, in Sonoma County’s famous wine country, the storm dropped over 21 inches of rain. The Russian river is 45.4 feet – 13.4 feet above the high tide level.
For the fifth time in four decades, the city of Guerneville was submerged under the murky brown flood of the lower Russian river. In Sonoma County alone, damage has been estimated at over $ 100 million.
Events like this have drawn attention in recent years, but atmospheric flows are not new. They have been meandering through the sky for millions of years, transporting water vapor from the equator to the poles.
In the 1960s, meteorologists coined the term “Pineapple Express” to describe storm tracks that formed near Hawaii and transported warm water vapor to the coast of North America. By the late 1990s, atmospheric researchers had found that over 90% of the world’s moisture from the tropics and subtropics was transported to higher latitudes by similar systems they referred to as “atmospheric rivers.”
In arid conditions, atmospheric rivers can replenish water supplies and extinguish dangerous forest fires. In wet conditions, they can cause harmful flooding and debris flows, and destroy the local economy.
Following an atmospheric river event that caused severe flooding in Chile, sediments washed from slopes into the Itata River can be seen flowing up to 50 kilometers from the coast. NASA Earth Observatory
Helpful and harmful
Researchers have known for some time that atmospheric river flooding can cost a lot of money, but until our study, no one had quantified the damage. We used a catalog of atmospheric river events compiled by the Scripps Institution of Oceanography’s Center for Western Weather and Water Extremes and compared it to 40 years of flood insurance records and 20 years of damage estimates from the National Weather Service.
We found that atmospheric rivers in the western United States caused an average of $ 1.1 billion in flood damage annually. More than 80% of all flood damage in the west in the years we studied was associated with atmospheric rivers. In some areas, such as the Northern California coast, these systems caused over 99% of the damage.
Our data showed that in an average year, about 40 atmospheric rivers landed along the Pacific coast somewhere between Baja California and British Columbia. Most of these events were harmless: about half caused no insured damage, and these storms replenished the area’s water supplies.
There were a number of exceptions, however. We used a recently developed atmospheric flux classification scale that ranks the storms from 1 to 5, similar to systems used to categorize hurricanes and tornadoes. There was a clear relationship between these categories and the damage observed.
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The storms of the Category 1 (AR1) and AR2 atmospheric rivers caused damage estimated to be less than $ 1 million. AR4 and AR5 storms caused moderate damage in the 10s and 100s million dollars. The most damaging AR4 and AR5 caused over $ 1 billion impact per storm. These billion-dollar storms occurred every three to four years.
A more humid atmosphere means worse storms
Our most important finding was an exponential relationship between the intensity of atmospheric rivers and the flood damage they caused. Each increase in the scale from 1 to 5 was associated with a 10-fold increase in damage.
Several recent studies have modeled how atmospheric fluxes will change in the coming decades. The mechanism is simple: greenhouse gases store heat in the atmosphere and warm the planet. As a result, more water evaporates from oceans and lakes, and increased humidity makes the storm systems stronger.
As with hurricanes, atmospheric rivers are likely to become longer, wider, and wetter in a warming climate. Our finding that damage increases exponentially with intensity suggests that even small increases in atmospheric river intensity could result in significantly larger economic impacts. https://www.youtube.com/embed/X3cP4egWRbk?wmode=transparent&start=0 Scientists have developed a scale to categorize atmospheric fluxes that reflect both their replenishment capacities and hazardous effects.
A better forecast is crucial
I believe that improving atmospheric forecasting systems should be a priority for adapting to a changing climate. A better understanding of the intensity, duration and landing locations of atmospheric rivers can provide residents and emergency services with valuable information.
It is also important to put a stop to new construction in risk areas and to help people move to safer places after major disasters rather than rebuilding them.
Finally, our study highlights the need to reduce global greenhouse gas emissions. These storms will keep coming and they will get stronger. In my view, stabilizing the global climate system is the only long-term way to minimize economic damage and risks to vulnerable communities.
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Tom Corringham, Postdoctoral Researcher in Climate, Atmospheric Sciences, and Physical Oceanography, University of California, San Diego
This article is republished by The Conversation under a Creative Commons license. Read the original article.
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