Apr 1, 2016

Trends in atmospheric patterns conducive to seasonal precipitation and temperature extremes in California

Daniel L. Swain, Daniel E. Horton, Deepti Singh, and Noah S. Diffenbaugh
Science Advances
  • Investigates northeastern Pacific atmospheric circulation patterns that have historically (1949–2015) been associated with cool-season (October-May) precipitation and temperature extremes in California
  • Identifies changes in occurrence of atmospheric circulation patterns by measuring the similarity of the cool-season atmospheric configuration that occurred in each year of the 1949–2015 period with the configuration that occurred during each of the five driest, wettest, warmest, and coolest years
  • Detects statistically significant changes in the occurrence of atmospheric patterns associated with seasonal precipitation and temperature extremes
  • Finds a robust increase in the magnitude and subseasonal persistence of the cool-season West Coast ridge, resulting in an amplification of the background state
  • Finds that changes in both seasonal mean and extreme event configurations appear to be caused by a combination of spatially nonuniform thermal expansion of the atmosphere and reinforcing trends in the pattern of sea level pressure
  • Finds in particular, both thermal expansion and sea level pressure trends contribute to a notable increase in anomalous northeastern Pacific ridging patterns similar to that observed during the 2012–2015 California drought
  • Findings suggest that the frequency of atmospheric conditions like those during California’s most severely dry and hot years has increased in recent decades, but not necessarily at the expense of patterns associated with extremely wet years