Identifying and Understanding Regional Differences in Temperature and Precipitation in California Under the Influence of PDO

California, USA is currently in the midst of an extreme and unprecedented drought. Studies suggest that the current drought is likely the worst in at least the last 1200 years. Further, the three year accumulated precipitation deficit (2012 - 2015) for California is at least 500 mm, equivalent to a full years’ worth of precipitation. The 2016 El Niño influenced water-year however only ended with California receiving just slightly above average precipitation, meaning that the extreme drought conditions will persist into the future. The precipitation deficits California has experienced since 2012 are not out of the range of natural variability; however, California’s regional temperatures are, suggesting the possible role of co-occurring extremes as a primary driver in the current extreme and unprecedented drought conditions. However, understanding the statistics of climate related extremes and co-occurring extremes can be challenging as the assumption of stationarity of their respective distributions is likely violated, as long-term trends and natural variability are known to exert a detectable signature on California’s temperature and precipitation patterns. In order to control for one of the leading modes of climate variability in California, we consider the trivariate joint distribution of California temperature and precipitation, conditional on the phase of the Pacific Decadal Oscillation (PDO), a natural mode of large-scale, low-frequency climate variability. This methodology allows us to achieve stationarity in California’s joint temperature and precipitation distribution with respect to the phase of the PDO. Numerous previous studies have documented a statewide increase in precipitation during the warm (positive) phase of the PDO, however, using the conditional analysis methodology, we uncover important regional differences in how that additional precipitation is received. We find that both Northern and Southern California experience increases in mean precipitation during the warm phase of PDO, however, for very different reasons. Northern California experiences an increase in the mean due to a shift in the mode of the distribution while Southern California experiences an increase in the mean due to a disproportionate increase in the probability for experiencing extreme wintertime precipitation. This is important because the societal and environmental risks associated with an increase in the mean due to a uniform shift of all probabilities are not the same as the risks from an increase in the mean due to a disproportionate increase in the probability for experiencing extremes. We verify that this behavior is reproduced in the 20th Century Reanalysis dataset, and investigate the physical mechanisms and processes underlying and controlling the respective behavior of precipitation in Northern and Southern California under the influence of PDO. We explore decomposing precipitation, and its relationship with PDO, into fundamental statistical and dynamical characteristics of storms such as intensity, duration, frequency, updraft strength, and moisture content.