by Canada and parts of the US were hit by heavy rainfall earlier in September, and now we may know why. The rainfall is believed to be the result of one of the most intense atmospheric rivers observed in the northeastern Pacific Ocean since 2000.
Atmospheric rivers are often described as “rivers in the sky”; they are narrow, flowing columns of condensed water vapor high in the atmosphere that can extend as long as 2,000 kilometers. When they reach land, that water vapor cools and is dumped down as rain or snow.
For the most part, they are quite weak and provide a water supply benefit with the precipitation they bring.
But when such a column passed through the Gulf of Alaska in late September, it was unusually powerful, battering coastal areas of Canada and southeastern Alaska with heavy rains for several days.
Satellite images of the atmospheric river as it made landfall.
Image credit: NASA Earth Observatory
The heaviest was observed in the Coast and Hazelton Mountains and Glacier Bay National Park, but in a small town in British Columbia, between 56.2 and 99.3 millimeters (2.2 to 3.9 inches) fell every day for four consecutive days. inches) of rain. For reference, the same city received a total of 118.7 millimeters (4.7 in) of rain during the entire month of September in 2023.
When it made landfall, this particular atmospheric river was actually a category 4 or 5 – 5 being the highest category in existence. Category 5 categories are also known as ‘exceptional’ and are considered dangerous rather than useful – meaning they can go hand in hand with flood risks.
This relatively new categorization system – introduced in 2019 – is somewhat similar to the Saffir-Simpson scale used for hurricanes, and is based on the maximum intensity and duration of the atmospheric river.
One of the defining calculations on the scale is that of integrated water vapor transport (IVT), which combines wind speed and humidity to give an indication of the intensity of the atmospheric river.
As NASA Earth Observatory reported, when scientists at the Center for Western Weather and Water Extremes at the University of California, San Diego initially calculated the IVT for the recent atmospheric river event, the result was an intensity that was particularly high compared to others observed in the region over the past 23 years.
Atmospheric scientist Bin Guan told NASA Earth Observatory that this intensity was “remarkable.” But what caused it?
According to Guan, it could have been changes in a climate pattern known as the Arctic Oscillation that normally rarely occur in September: “This could be one of the conditions that may have contributed to this exceptionally strong atmospheric river event.”
