Large-Scale Drivers of Connected Atmospheric Rivers along the U.S. West Coast

Long 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 - 2019 to examine landfalling AR events. During WY 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 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, yet families often include more than two AR events. Using this definition, a catalog of AR families was created for BBY. Out of the 266 AR events observed, 130 (i.e., 57%) initiated an AR family. Compared to single AR events, the synoptic environment of AR families is characterized by lower geopotential heights throughout the mid-latitude North Pacific, an enhanced subtropical high, and a stronger zonal North Pacific jet. Analysis of water year 2017 demonstrated a persistent geopotential height dipole throughout the North Pacific and a positive anomaly of integrated water vapor extending toward California. These two factors are hypothesized to have contributed to the record breaking WY in Northern California.

We expand the detection of AR families to the entire US West Coast using the Guan and Waliser (2015) AR detection algorithm on reanalysis data to compare WY 2017 and 2019 AR families. Widening the region of detection allows for further exploration of the initiation of AR families to address hypotheses on what extratropical or tropical mechanisms (e.g. North Pacific jet dynamics or outgoing longwave radiation anomalies) may drive the occurrence of AR families. Understanding the frequency at which these events occur, their associated impacts and the cause of their genesis supports improved weather forecasting and flood risk and water management.