Assessing the Connection between Atmospheric River Events and Exploring their Relation to Extratropical and Tropical Large-Scale Drivers

Longer duration atmospheric rivers (ARs) have been correlated with greater hydrologic impacts. This study uses coastal Atmospheric River Observatory measurements at Bodega Bay (BBY) in Northern California and reanalysis from water years (WY) 2005 - 2017 to examine 228 landfalling AR events. During the winter of 2016-2017, 34 ARs hit California’s Russian River basin, many in quick succession. These AR events that occur in close temporal sequence without allowing the watershed time for recovery were also associated with higher impacts. These factors provided motivation to develop a definition of “atmospheric river families,” describe their characteristics, and begin to assess their predictability.

An AR family is identified when at least two AR events occur within a 120-hour period, and families often include more than two AR events. Using this definition, a catalog of AR families was created for BBY. Out of the 228 AR events observed, 109 (i.e., almost 50%) initiated an AR family. Compared to single AR events, AR families exhibit a zonally extended Aleutian low pressure area, a steering subtropical high, and an enhanced zonal North Pacific jet. A case study on WY 2017 AR families, with emphasis on the Oroville Dam crisis in Northern California, illustrates the persistent Rossby wave train pattern of high and low pressure centers and the strong positive anomaly of integrated water vapor close to the California coast. These two factors are hypothesized to lead to the record breaking WY in Northern California.

This study further explores the initiation of AR families to determine whether extra-topical or tropical mechanisms such as North Pacific jet dynamics or tropical Pacific sea surface temperatures drive the occurrences of AR families. Understanding the frequency in which these events occur, their associated impacts and the cause of their genesis is beneficial for weather forecasters, water managers and risk management experts.