In Retrospect

Battling rivers in the sky

As the Earth’s climate continues to warm at a historic pace, atmospheric rivers will only become more frequent and intense, causing non-stop rain similar to what is currently being experienced in the Los Angeles basin in the State of California

Battling rivers in the sky

Days of record-breaking heavy rainfall in California have caused flooding and mudslides, leaving infrastructure like roads and homes damaged, leading the governor to declare a state of emergency in eight counties.

Los Angeles officials have tallied 475 landslides and dozens of damaged structures with over 104,000 people left without power and at least five lives being claimed by the storm since reports last came in.

Glimpses from several of LA’s most upmarket neighbourhoods show cars trapped in mud, streets blanketed by water and homes covered in debris.

The city rushed in additional resources, including search-and-rescue crews and helicopter pilots, adding to the 1,000 firefighters already on duty, the Los Angeles Fire Department said.

“The ground is extremely saturated — supersaturated,” said Ariel Cohen, chief National Weather Service (NSW) forecaster in LA.

“It’s not able to hold any additional water before sliding. It’s not going to take much rain for additional landslides, mudslides, rockslides and other debris flows to occur.”

A state of emergency has been declared in eight California counties covering Los Angeles, Orange, Riverside, San Bernardino, San Diego, San Luis Obispo, Santa Barbara and Ventura with President Joe Biden calling Los Angeles Mayor Karen Bass and offering federal assistance to the city.

Much like last year, when record rainfall lashed the state, the storms are attributed to atmospheric rivers (ARs), systems that have long played a role in California’s precipitation levels — both for good and for bad.

The National Oceanic and Atmospheric Administration refers to these systems as “rivers in the sky” for good reason. Characterised by long streams of moisture in the atmosphere that span between 250 and 375 miles wide on average, the ARs that affect the US West are supercharged by water vapour that evaporates off the Pacific Ocean and they are carried by other weather systems from the tropics or the subtropics.

The average atmospheric river carries an amount of water vapour that rivals the flow at the mouth of the mighty Mississippi River — and strong ones can hold more than 15 times that amount. That moisture is released as rain or snow when ARs make landfall and, depending on the size, timing and intensity, the storms that result can be highly destructive or extremely beneficial.

ARs are important contributors to California’s water supply, providing up to half of the rainfall and snow that the dry state relies on throughout the year. But the big ones can also overload rivers and reservoirs, causing damaging floods. The systems also tend to come equipped with strong winds that tear down trees and power lines, adding to their destructive tendencies.

The storms hitting California this week are part of a “Pineapple Express” system, which is just one kind of atmospheric river. Named for their origin around the Hawaiian Islands, they are often strong systems and have been known to unleash torrents of precipitation when they reach the west coast of the US and Canada. According to the National Ocean Service, they can dump as much as 5 inches of rain on California in a single day.

This week’s storms have put officials and residents on edge, in part because of their timing. The back-to-back systems are packing a stronger punch together and carry greater potential to wreak havoc when systems are already inundated and soils are saturated.

The second storm in the set is also a harrowing one, and forecasters have urged residents across the state to prepare for potentially life-threatening and destructive weather. Equipped with overlapping hazards — thrashing winds, heavy downpours, surging surf along the coast and whiteout conditions in the mountains — the enormous system is positioned to cause quite a lot of damage across the state.

However, while Californians will welcome the break from torrential rains, a University of Miami-based research institute scientist has been quoted as saying that as the Earth’s climate continues to warm at a historic pace, atmospheric rivers will only become more frequent and intense.

“The science is undeniable — warmer air holds more moisture; and as a result, the concentration of water vapour increases,” said Breanna Zavadoff, an assistant scientist at the Cooperative Institute for Marine and Atmospheric Studies (CIMAS). “Our planet’s atmosphere has warmed by about 1.3 degrees Celsius since pre-industrial times; and if we continue at that rate, climate models show that atmospheric river frequency and strength across the globe could increase by as much as 50 per cent and 25 per cent, respectively.”

Like rivers in the sky, atmospheric rivers are long, narrow ribbons in the atmosphere that transport moist air from the tropics to higher latitudes. Combined with high wind speeds, the moist air produces heavy rain and snow, leading to flash flooding, mudslides, and infrastructure damage.

These intense atmospheric moisture bands are invisible to the naked eye, but satellite instruments can help us see these “rivers in the sky” by using infrared sensors, which is how scientists visualise them.

California saw a similar pattern last year when a series of brutal atmospheric rivers drenched the state, unleashing historic levels of rainfall that caused urban flooding, landslides and road closures. The dangerous conditions caused 22 confirmed fatalities and researchers at Stanford University tallied more than USD 3 billion in losses.

While these storms were incredibly destructive and dangerous and, in some cases, set new records, scientists have warned that they are just a taste of what’s to come. Fuelled by the climate crisis, the risks of a catastrophic megaflood — one that could displace millions and generate over USD 1 trillion in losses — have doubled in California.

California’s climate has long vacillated dramatically from wet to dry, but models show these shifts will occur with increasing intensity as the world warms. ARs are also becoming more likely to arrive in sets, which can cause up to four times more economic damage than each would have individually, according to a study published in Science Advances.

Supercharged by more moisture coming off the Pacific as ocean temperatures rise, scientists expect that ARs could also grow more severe, adding more risks for floods across California and the West. As temperatures rise, precipitation is more likely to fall as rain rather than snow, which could pose problems for the state’s water supply.

For example, in California, up to 50 per cent of total annual precipitation and streamflow can come from a few intense atmospheric rivers during fall and winter. Although they provide essential water resources and relieve regional drought, strong atmospheric rivers often pose serious risks, including floods, landslides and levee breaks in California, as is evident this winter.

With rising sea surface temperatures and increased vertically integrated horizontal vapour transport (IVT), ARs are projected to increase in intensity, a process which appears to have already started. Warming-boosted ARs encountering western topography are projected to produce more frequent and stronger precipitation extremes even while the frequency of light and moderate precipitation decreases. The probability density function of daily precipitation is projected to shift in favour of extreme events due to thermodynamically increased AR intensity, while the frequency of low and medium-intensity precipitation decreases due to non-AR storms being pushed poleward by the expanding subtropical belt.

“Flood damages associated with ARs have been found to increase exponentially with AR intensity, so even a modest shift towards higher-intensity ARs could have significant impacts on flood damages. In their projections of AR-related flood damages Rhoades et al. consider the impacts of climate change under three Community Earth System Model (CESM) stabilised warming scenarios. Under a 0.85 degrees Celsius warming scenario they predict that damages by the end of the century would remain in the historical USD 1b per year range but under a 3 degrees Celsius warming scenario they predict that average damages would increase above USD 3b per year. Their study considers a set of outputs from a single global climate model (GCM) and models flood damages based on AR loss statistics aggregated over the entire western United States,” a Nature article mentions.

It also goes on to say that while observed flood damages in the western US have remained relatively constant over recent decades, flood damages across the entire US have increased.

In several studies, the impacts of projected changes in AR frequency and intensity over the 21st century have been estimated under the simplifying assumption that exposure and vulnerability will remain at historical levels. However, population and wealth in the western United States are projected to increase in the coming decades while vulnerability will likely decrease with continued investments in adaptation and flood prevention — factors that will alter the AR-related flood damage futures to a considerable extent.

Views expressed are personal

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