17 January 2018

California's Extreme Swing in Extreme Climate Events

California from drought to deluge, to fire


In the winter of 2016-2017, an enhanced low-pressure trough appeared in the same location, initiating and directing sequential atmospheric river events toward California and causing high precipitation.  Evidence indicates that the dramatic appearance of this enhanced trough in 2016-2017 is “the other side of the coin” of the persistent high-pressure ridge stationed off the West Coast causing the prolonged drought.


The strikingly opposite pattern of the atmospheric circulation between the winters of 2013-2014 and 2016-2017 suggests that we are observing intensified water cycle extremes in California.  The alternation of extreme wet/dry seasons every few years prompts the growth of vegetation thereby increasing “fuel” for wildfires and fire danger.


There is always a component of internal variability in the atmosphere that underlies the persistence of the North Pacific circulation anomalies and unique regional processes that increase event extremeness.  Meanwhile, a large body of research has shown evidence of remote teleconnections to North Pacific circulation stagnation through pronounced atmospheric Rossby waves of tropical origin.  Coupled model studies have connected the strenthening of the relationship between the dipole and California’s precipitation to relevant climate oscillations with resulting projections of an increase of both drought and deluge, and these are also linked to the eastern U.S.'s cold spells while the western states experience dry-warm weather.  Despite the debate concerning the causes of the drought-producing ridge in the Northeast Pacific Ocean, the various hypotheses about natural variability did suggest one thing in common: the flip-flop nature of the anomalous circulation.  All things considered, the amplified ridge in one winter can turn into a deepened trough in another and this tendency appears to have intensified.

Simon Wang
Utah State University
Wang, S. "California From Drought to Deluge." Nature Climate Change 7, 465-468 (2017). [10.1038/nclimate3330].