Subseasonal Clustering of Atmospheric Rivers Over the Western United States.

The serial occurrence of atmospheric rivers (ARs) along the US West Coast can lead to prolonged and exacerbated hydrologic impacts, threatening flood‐control and water‐supply infrastructure due to soil saturation and diminished recovery time between storms. Here a statistical approach for quantifying subseasonal temporal clustering among extreme events is applied to a 41‐year (1979–2019) wintertime AR catalog across the western United States (US). Observed AR occurrence, compared against a randomly distributed AR timeseries with the same average event density, reveals temporal clustering at a greater‐than‐random rate across the western US with a distinct geographical pattern. Compared to the Pacific Northwest, significant AR clusters over the northern Coastal Range of California and Sierra Nevada are more frequent and occur over longer time periods. Clusters along the California Coastal Range typically persist for 2 weeks, are composed of 4–5 ARs per cluster, and account for over 85% of total AR occurrence. Across the northwest Coast‐Cascade Ranges, clusters account for ∼50% of total AR occurrence, typically last 8–10 days, and contain 3–4 individual AR events. Based on precipitation data from a high‐resolution dynamical downscaling of reanalysis, the fractions of total and extreme hourly precipitation attributable to AR clusters are largest along the northern California coast and in the Sierra Nevada. Interannual variability among clusters highlights their importance for determining whether a particular water year is anomalously wet or dry. The mechanisms behind this unusual clustering are unclear and require further research. Plain Language Summary: Atmospheric rivers, or long narrow regions of enhanced water vapor transport, are an important component of the midlatitude water cycle, responsible for producing precipitation and associated extremes. Atmospheric rivers occurring close in time or one after another can lead to exaggerated impacts due to the decreased time between periods of precipitation. Based on the historical record, we identify the timescales at which wintertime atmospheric rivers cluster at a rate different from what would be expected by random chance over the western United States. Compared to the Pacific Northwest, atmospheric river clusters over the mountainous regions of California occur more frequently, over longer periods of time, and account for a larger proportion of total atmospheric river occurrence. Atmospheric rivers in this region are also responsible for a large fraction of total precipitation and are commonly associated with precipitation extremes. The year‐to‐year occurrence of clusters is important for determining wet versus dry years. Future work will identify the atmospheric patterns behind atmospheric river clustering over different geographies. Key Points: Atmospheric rivers (ARs) temporally cluster at a greater‐than‐random rate across the western United States (US) with a distinct geographical patternCompared to the Northwest, Southwest clusters are longer‐lived, consist of more ARs and account for a larger fraction of total AR occurrenceA considerable fraction of western US total and extreme hourly precipitation is attributable to AR clusters [ABSTRACT FROM AUTHOR]