UCLA

Abstract: Atmospheric rivers (AR) are critically important to water resources management along the US west coast, driving variability in both droughts and floods across the region. Inter‐annual variability of ARs is well documented in the instrumental record back to the mid‐twentieth century, but long‐term variations in the frequency and landfall location of ARs along the US west coast are poorly understood due to limited records. This limitation impedes the ability to contextualize emerging trends and projections of AR activity. Here we use station‐based records of daily precipitation and tree‐ring records to present novel, spatially explicit estimates of daily AR occurrences in the first half of the twentieth century and annual AR counts over the last 600 years. First, we use neural networks and daily precipitation across Western North America to classify the daily occurrence of AR landfalls in three regions along the US west coast during the cold season back to 1916 CE. Then, we reconstruct the annual frequency of AR landfalls in those same regions back to 1400 CE using a gridded, tree‐ring based reconstruction of the standardized precipitation index and a Poisson regression framework. The skillful reconstruction of daily and annual AR occurrences provides previously unavailable estimates of AR landfall variability and highlights new peaks in AR activity and modes of low‐frequency variability prior to the instrumental record. Our reconstructions suggest that the average latitude of AR landfall has varied considerably on multi‐decadal scales over the last 600 years, but without any discernible trends beyond this quasi‐oscillatory behavior.