Climate Change Effects on the December 2022 - January 2023 High Impact Series of 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 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 (e.g., AR family) often result in multiple distinct pulses of precipitation over one area that vary in intensity, orientation, and temperature between different families and individual ARs within the family, leading to a variety of onshore impacts post-landfall and creating additional 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 aim to understand how landfalling ARs and their associated impacts may evolve under future climate conditions, with a focus on their intensity, duration, and the timing of successive events.

Here, we examine the 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). 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. Our present-day MPAS-A simulation replicated seven distinct ARs making landfall in Northern California with a mean AR scale of ~2, a total of ~240 hours of AR conditions, and a mean maximum IVT of ~730 kg m^–1 s^–1 across all pulses. In the future simulation, only six ARs made landfall in Northern California, yet the mean AR scale in the future simulation was ~4, a total of ~270 hours of AR conditions occurred, and the mean maximum IVT was more than 1100 kg m^–1 s^–1 across all pulses. Furthermore, a future climate intensified the precipitation over several watersheds and shortened the timing between events, effectively reducing the recovery time between onshore impacts. We estimate that the effects of climate change to-date are not as drastic, but still contribute to increases in AR intensity, duration, and landfalling precipitation