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Large parts of the western US are forecast to be battered by more rain and wind because of an “atmospheric river” phenomenon, after the Californian city of San Diego experienced one of the wettest days in its history this week.
The US’s National Weather Service has warned the west coast will experience an “impactful atmospheric river event” next week, bringing higher than normal levels of rain and wind, along with possible flooding.
Atmospheric rivers are often described as a powerful hose, as they are typically long and narrow and carry water vapour from the tropics over land, where it usually falls as very heavy rain or snow.
The average atmospheric river holds an amount of water roughly equivalent to the flow at the mouth of the Mississippi river, the US National Oceanic and Atmospheric Administration said. However, stronger atmospheric rivers can carry up to 15 times that amount.
The particular atmospheric river that carries moisture from the tropical Pacific around Hawaii to the western shores of Canada and the US is known as the “Pineapple Express”. Meteorologists working in the San Francisco Bay Area warned of a “potentially potent Pineapple Express” over the next week.
Torrential rain deluged California’s second-largest city on Monday, leaving thousands of people in San Diego without power and forcing public transportation services to be suspended. Mayor Todd Gloria declared a state of emergency because of the flooding.
Forecasters at the National Weather Service estimated there would be a 60 per cent to 80 per cent chance of “above normal” levels of rain and snow from southern Oregon to California, extending inland to the Great Basin region and the deserts of Arizona, in coming days.
The service also warned of “onshore high winds” and coastal erosion caused by persistently high waves.
A 2022 study by researchers found warming global temperatures led to greater rainfall from a 2017 atmospheric river that unleashed a deluge on California. Rain from the event caused the evacuation of nearly 200,000 people.
Broadly, scientists have warned warming sea surface temperatures lead to greater levels of precipitation.
The record warming of the oceans in 2023, also the hottest year on record on land, has surprised scientists as each consecutive year for the past five years has shown an increase.
The Earth’s oceans absorb more than 90 per cent of excess heat caused by greenhouse gases in the atmosphere.
The El Niño weather pattern, which results in the naturally occurring warming of sea surface temperatures in the Pacific, also remains an influence on weather patterns in the US.
However, scientists believe its effect is exacerbated by rising global temperatures created by human-caused climate change, and El Niño in isolation does not account for the changing patterns of weather.
In 2023, the world experienced its hottest year on record as the average global temperature reached almost 1.5C above pre-industrial levels, according to scientists at the European earth observation agency.
This is distinct from global temperature rise over the long-term, measured over several decades, which was estimated at 1.1C when the UN body of scientists issued its report in 2021.
The El Niño phenomenon is expected to last until at least April, the World Meteorological Organization has said. It has contributed to widespread flooding right across the southern hemisphere, from swaths of the east coast of Australia to southern and central Africa.
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