North-West USA & Canada December 2023 Atmospheric river likely influenced by both  human-driven climate change and natural variability

Press Summary (First published 2024/18/01)

• In the last two decades, Atmospheric rivers similar to that affecting North-West USA & Canada in early December 2023 are up  to 11 mm/day (0% to 15%) wetter over North Western USA and up to 11 mm/day (0% to 15%) drier over Western Canada.

• The North-West USA & Canada December 2023 Atmospheric river  was similar to several events in the past.

• We mostly ascribe the changes in precipitation to human driven climate change and natural climate variability likely played a modest role

Event Description

In early December 2023, regions in North Western USA and western Canada experienced severe flooding as a result of abnormally wet conditions caused by an atmospheric river. Atmospheric rivers (ARs) are distinct meteorological phenomena characterized by long, narrow, and shallow corridors of strong horizontal water vapor transport. Typically associated with a low-level jet stream ahead of the cold front of an extratropical cyclone, these atmospheric rivers can have significant impacts on weather patterns and precipitation. The atmospheric river responsible for this extreme weather event was part of a larger storm system that flooded the Pacific Northwest with heavy rains. Over 17 million people found themselves under flood alerts as several inches of rainfall were forecasted for Washington, Oregon, and northern Idaho. This particular type of atmospheric river, often referred to as a "Pineapple Express" due to moisture transported from  the tropical Pacific around Hawai'i, also brought warm air and thus high snow levels (the altitude at which precipitation falls as snow instead of rain), exacerbating the flooding potential through snowmelt and increased runoff.

Record daily rainfall amounts were documented across the Pacific Northwest, including at Seattle-Tacoma International Airport and Olympia Airport. Rivers, such as the Stillaguamish River at Arlington, set preliminary records for water levels, adding to the severity of the flooding. Furthermore, Seattle experienced a record daily high temperature (for that day-of-year) of 59°F during this period.

The storms resulted in the tragic loss of two lives and prompted numerous water rescues, road closures, and school closures. Amtrak had to cancel train services between Seattle and Portland for several days due to a landslide caused by the adverse weather conditions. 

The Surface Pressure Anomalies reveal a dipolar structure with a depression to the north and higher pressure to the south. This configuration advances warm air, leading to high Temperature Anomalies during the event. Precipitation data indicate that the majority of the area experiencing heavy precipitation is on the Pacific Coast, as the Rocky Mountains block moisture, with uplift of air by the mountains causing rain and snowfall on the western flank. The coastal area experienced extreme precipitation, reaching up to 80 mm/day. Windspeed data also indicate moderate winds, sustained mainly over the northwestern coasts of Canada.

Climate and Data Background for the Analysis

The IPCC AR6 Chapter 8 reports that it is likely that there was an increasing trend in the AR activity in the eastern North Pacific since the mid-20th century. However, there is low confidence in the magnitude of this trend and no formal attribution of the trend, although such an increase in activity is consistent with the expected and observed increase in precipitable water associated with human-induced global warming. A NASA-led study also reveals that climate change is likely to intensify atmospheric rivers globally by the end of the century. These rivers will become longer and wider, leading to more frequent heavy rain and strong winds. While this study projected about 10% fewer atmospheric rivers in the future, their increased size will result in a 50% rise in the global frequency of extreme conditions.

Our analysis approach rests on looking for weather situations similar to those of the event of interest having been observed in the past. For this event, we have high confidence in the robustness of our approach given the available climate data, as the event is very similar to other past events in the data record.

ClimaMeter Analysis

We analyze here (see Methodology for more details) how events similar to the low pressure system leading to North-West USA & Canada December 2023 atmospheric river have changed in the present (2001–2022) compared to what they would have looked like if they had occurred in the past (1979–2000) in the region [-150°E -115°E 30°N 60°N] (note that the dates are in GMT, and thus analysis started at 15:00 local time on 4th Dec). The Surface Pressure Changes indicate a weakening in the strength of the low-pressure system in the Pacific; however, the core of the depression has not significantly changed its intensity compared to the past, except in areas distant from the cyclone eye. Temperature Changes show a dipolar structure with cooler conditions for coastal areas of Western Canada and warmer conditions over Western USA. Precipitation Changes show that similar events now produce heavier (up to 11mm/day) precipitation in the present than in the past in some areas of the Western Part of the USA, while similar atmospheric rivers produce less precipitation (up to 11 mm/day) over the Northwestern coast of Canada. This can be connected to the previously explained temperature changes being cooler (resp. warmer) conditions yield to lower (resp. higher) precipitation amounts. Windspeed Changes indicate weaker winds (up to 10km/h) on the coastal areas analyzed. We also find that Similar Past Events occur with the same frequency per month as in the past. Considering the affected urban areas, Vancouver (Canada) shows no changes in precipitation, while Seattle and Portland (USA) experience a significant increase in precipitation, with up to 6mm/day more rain in the present than in the past in Portland.

Finally, we find that sources of natural climate variability, notably the Atlantic Multidecadal Oscillation, may have only partly influenced the event. This means that the changes we see in the event compared to the past may be mostly due to human driven climate change.

Conclusion

Based on the above, we conclude that events  similar to the North-West USA & Canada December 2023 atmospheric river are up  to 11 mm/day (0% to 15%) wetter over the NorthWestern USA and up to 11 mm/day (0% to 15%) drier over Western Canada. in the present than they have been in the past. We interpret the North-West USA & Canada December 2023 atmospheric river as an event whose extreme characteristics can mostly be ascribed to human driven climate change.

Contact Authors

-Davide Faranda, IPSL-CNRS, France  📨 davide.faranda@lsce.ipsl.fr   🗣️French, Italian, English

-Rachel White, University of British Columbia, Canada 📨 rwhite@eoas.ubc.ca 🗣️ English