Atmospheric rivers have been rocking California this month, resulting in heavy rainfall, snowfall, floods, mudslides, and power outages. In just a week, two atmospheric rivers struck the US state. On February 5, 2024, flooding conditions worsened, with some parts of Los Angeles County witnessing a foot of rain. It is expected that on-and-off showers will occur through February 6, and also expand to San Diego. There is high flood risk over Southern California, which is rare. 

Winds with a speed as high as 50 miles per hour struck regions around Sacramento, as a result of which trees fell. One large redwood tree which fell as a result of the high-speed winds killed a man in Yuba City, which is located 40 miles north of Sacramento, the Yuba City Police Department said in a statement. Another person who was killed as a result of the storm induced by the atmospheric rivers was a 45-year-old man from Santa Cruz County. He died after a tree fell into his home in Boulder Creek, a CNN report said, citing a spokesperson from the Santa Cruz County Sheriff’s office. 

Over 120 mudslides have occurred, and these have damaged about 25 structures. Homes in the Hollywood Hills area, and around the Santa Monica Mountains have been evacuated.

Since February 4, Los Angeles has been receiving incessant rain. The region has received over 6.65 inches of rain. This was the third-wettest two-day stretch seen by Los Angeles since January 26, 1956. While February is the wettest month of the year in Los Angeles, the average amount of rain received during this month is only 3.64 inches. 

About 900,000 people in California are suffering from power outages caused by strong winds and rain. Air travel has been disrupted, with over a 1,000 flights in and out of Los Angeles International Airport being delayed since February 4. 

There was a high risk of excessive rainfall for more than 16 million people across Southern California on February 5. 

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An avalanche has occurred in some regions of Nevada due to the atmospheric rivers. 

The National Weather Service’s (NWS’s) San Francisco Bay Area office has issued a rare  hurricane-force wind warning for Big Sur and nearby regions, Reuters reported. 

Flash flood warnings have been issued for several regions of Los Angeles, Santa Barbara, and San Luis Obispo counties.

According to a report by the Daily Mail, weather researcher Ryan Maue said that the bomb cyclone, which is a cyclone that rapidly strengthens over a 24-hour period, could result in over eight trillion gallons of rain over California. This phenomenon is referred to as bombogenesis.

The maximum amount of water that can be held by Lake Mead located in Nevada and Arizona is 9.3 trillion gallons, according to NASA. This means that the atmospheric rivers could result in rain equivalent to the amount of water that Lake Mead is capable of holding. 

The atmospheric rivers which battered California over the past week are also known as Pineapple Express systems. A Pineapple Express is a special type of atmospheric river. Let us understand in detail what an atmospheric river and a Pineapple Express are.

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What is an atmospheric river?

An atmospheric river is a long, narrow region in the atmosphere, similar to a river in the sky, that can carry most of the water vapour or moisture from the tropics to northern latitudes, according to the National Oceanic and Atmospheric Administration (NOAA). All atmospheric rivers are a part of the Earth’s ocean water cycle, and play a major role in driving water supply and flood risks, especially in the Western US. 

Atmospheric rivers can be of varying strengths and sizes. The amount of water vapour carried by an average atmospheric river is almost equivalent to the average flow of water at the mouth of the Mississippi River. If an atmospheric river is exceptionally strong, it can carry water vapour which is 7.5 to 15 times the volume of water at the mouth of the Mississippi River.

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The science behind atmospheric rivers

A decade of scientific studies based on observations from satellites, radar, and aircraft, and the latest numerical weather models have yielded a lot of information on atmospheric rivers. 

Atmospheric rivers release the condensed water vapour held by them when they make landfall. The water vapour is released in the form of rain or snow, especially in the Western US. 

Water vapour rises and cools to cause heavy precipitation when atmospheric rivers move inland and bump across mountains. 

Many atmospheric rivers are weak systems that simply provide beneficial rain or snow. 

Just a few atmospheric river events are responsible for 30 to 50 per cent of annual precipitation on the West Coast, which includes the US states of California, Oregon, and Washington, and sometimes includes Alaska and Hawaii. Atmospheric river events contribute to the water supply of these regions, and are often responsible for floods.

From December 10 to 22, 2010, strong winter storms rocked the US West Coast from Western Washington to Southern California. These storms, caused by atmospheric rivers, resulted in 11 to 25 inches of rain in certain regions. 

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Atmospheric rivers keep moving with the weather, and hence, are present at at least one place on the Earth at any given time. 

On average, atmospheric rivers are 402 to 604 kilometres wide. 

The atmospheric rivers that contain the largest amounts of water vapour and strongest winds can result in extreme rainfall and floods. The rainfall caused by such atmospheric rivers can stall over watersheds vulnerable to flooding, disrupting travel, inducing mudslides, and causing devastating damage to life and property. 

A Pineapple Express is one such type of atmospheric river. The two atmospheric rivers that rocked California over the past week are Pineapple Express systems.

What is a Pineapple Express?

A Pineapple Express is a strong atmospheric river which carries moisture from the tropics near Hawaii, and causes heavy rainfall and snow over the US West Coast and Canada’s West Coast.

A Pineapple Express is formed when winds cross over warm bands of water vapour over the tropical Pacific. The strong atmospheric river travels across the Pacific as part of the global conveyor belt, which refers to a constantly moving system in the ocean driven by temperature and salinity in the deep ocean, and wind-driven currents on the surface. 

When a Pineapple Express reaches the West Coast, it can result in up to five inches of rain over California in one day. 

A Pineapple Express is a type of atmospheric river that can affect the entire west coast of North America, resulting in extreme rainfall, excessive flooding, property damage, and disrupted travel.

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How the climate crisis is intensifying atmospheric rivers

Scientists believe that the climate crisis and El Niño are driving forces behind the atmospheric river and the resultant storms. The warming of the ocean surface in the central and eastern tropical Pacific Ocean is known as El Niño. The strongest atmospheric rivers are probably a part of “sequences”, according to a study published January 19, 2024, in the journal Science Advances. Sequences refer to periods of increased hydrologic hazards associated with temporally clustered atmospheric rivers. This means that when atmospheric rivers become clustered, they are more likely to cause heavy rainfall and devastating floods. Clusters of atmospheric rivers are four times more likely to cause damage than a single atmospheric river. 

Since ocean temperatures are increasing due to climate change, the amount of moisture over the Pacific is rising. As a result, atmospheric rivers are more likely to intensify. These, in turn, are likely to cause more severe floods over the West Coast.

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