Climate Change Effects on the December 2022 - January 2023 High Impact Series of Landfalling Atmospheric Rivers Along the U.S. West Coast

Current modeling studies suggest regional decreases in the number of precipitation days, but an increase in extreme precipitation events, especially in regions such as California. This impact on California’s precipitation is evident through atmospheric rivers (ARs) which are long, narrow bands of water vapor that are commonly associated with a low-level jet (LLJ) ahead of a cold front of an extratropical cyclone. The beneficial properties or severity of ARs vary between each individual event, ranging from weaker/short lived ARs that are primarily beneficial and strong/long-lived ARs that are primarily hazardous. Multiple ARs that occur in succession of one another within a certain aggregation period (e.g., an AR family) often result in multiple distinct pulses of precipitation over one area with varying intensities, leading to a variety of onshore impacts post landfall, and creating compounding challenges for water resource management. Recent work has suggested that not all individual ARs, and consequently, not all AR families, will respond the same to a warming climate.

Here, we examine a recent sequence of landfalling ARs from 26 December 2022 through 19 January 2023 in past, present, and future environments using the Model for Prediction Across Scales – Atmosphere (MPAS-A) version 8.0. During this period, a family of nine ARs made landfall throughout California, producing about half of California’s annual precipitation. Heavy rain and snow over this three-week period were beneficial in building the snowpack across the mountainous regions and alleviating drought conditions in several areas; however, flooding, mudslides, and power outages were also common occurrences, underscoring the beneficial vs. hazardous nature of ARs. We hypothesize that climate change intensifies each individual AR event with the warmer, more meridional ARs experiencing a larger degree of strengthening in terms of both vertically integrated water vapor transport (IVT) and resultant precipitation. Positionally, consistent with previous work, we expect these ARs to shift poleward in accordance with the rise of sea surface temperatures due to climate change. We also expect climate change to shorten the timing between events, effectively reducing the recovery time between onshore impacts.