When it rains, it pours: Atmospheric rivers explained

On January 25, Environment Canada issued a weather statement warning the Metro Vancouver area, Sunshine Coast, and parts of Vancouver Island for a potential of flooding due to an atmospheric river.

Atmospheric rivers are a regular occurrence in BC— but climate change might be impacting their frequency and severity.

An atmospheric river is a band of warm moist air spanning hundreds of kilometers long next to a cyclonic low-pressure system, moving inland from the Pacific Ocean.

While Metro Vancouver was spared from heavy rains, Pemberton declared a state of emergency.

Alpine areas, as detailed by Avalanche Canada, are particularly prone to flooding due to snowmelt which includes the regions of Squamish, Whistler, Pemberton, and Garibaldi Provincial Park.

In BC, they bring rainfall to the Rocky Mountains during the winter, and 33 per cent of annual precipitation comes from atmospheric rivers in coastal British Columbia and Southeast Alaska, mostly during September and October.

They’re also especially difficult to predict, according to Dr. John Richardson, a professor in the department of Forest and Conservation Sciences.

“With these atmospheric rivers, [meteorologists] can see them forming out in the Pacific …So they have a measure of how much precipitation there is.” said Richardson. “But where exactly they hit along the coast, one can’t predict.”.

For instance, he mentioned how California was “smacked really hard” with mudslides and flooding last week, as reported by BBC.

This year, atmospheric rivers have largely been affected by El Niño weather conditions, which have returned after 7 years. El Niño conditions warm the ocean surface and cause above average sea surface temperature in the central and eastern tropical Pacific Ocean. As more moisture is absorbed through warmer temperatures, atmospheric rivers are exacerbated.

Climate change is also a factor, in particular increasing the flooding risks of atmospheric rivers. With drier conditions over the last few summers contributing to more forest fires, “less vegetation lets the rain hit the ground directly instead of being intercepted by the tree canopy,” Richardson said.

“The soils in burned areas also become hydrophobic...meaning more [water] will run off to nearby streams,” he said.

Atmospheric rivers are known to replenish freshwater resources, and while their intensity determines flooding and grim conditions, they are a contributor to BC’s seasonal cycles. As the climate changes though, it’s important to prepare for times of both flooding and drought.