UC San Diego

California has extremely high interannual variability in precipitation, and this is projected to increase with climate change. During northern California’s wettest year on record (2017), successive atmospheric river (AR) events associated with extreme precipitation served both to alleviate drought conditions and contribute to the Oroville Dam crisis. This dissertation investigates the characteristics, synoptic patterns, and drivers of these successive ARs.

First, this dissertation introduces the concept of “AR families” using an objective method to identify successive AR events at a single location (Bodega Bay, CA). Compared to single AR events, AR families exhibit unique, identifiable and semi-stationary patterns across the North Pacific. The algorithm was then applied to all AR events impacting California over 39 years, 1981-2019. K-means clustering identified six distinct clusters of large-scale patterns associated with AR families. These clusters represent two types of predominant patterns: meridional and zonal, and two show strong relationships with the El Niño/Southern Oscillation.

Persistent anomalies (PAs) across the North Pacific are evaluated for their association with AR families and drought conditions. This analysis is sensitive to methodology including detrending and determining the appropriate magnitude for the PA, ultimately impacting the identification of PAs associated with drought conditions or AR families in California. Traditional methodologies lack a strong correlation between PA frequency and precipitation in California, therefore new methods are explored to meaningfully relate the frequency and location of PAs to anomalous precipitation statewide.

Two satellite sea surface temperature (SST) datasets are used to assess the relationship between wintertime SST variability and AR activity, showing low levels of spatial SST variability co-occur with low AR frequency. Within AR events, near shore buoy data shows that the air-sea temperature difference prior, during, and post AR passage is consistent with an approaching warm front, AR conditions and cold air advection, respectively. Case studies of AR families further investigate the variability between AR activity, SST, and precipitation.

These findings identify the importance of successive AR events to California’s climate and characterize their synoptic precursors and meteorological drivers that provide the foundation for increased situational awareness and predictability of their occurrence and impacts.