By Daniel Swain
1 April 2016
(The California Weather Blog) – Since early 2013, the state of California has been in the grip of an extraordinary multi-year drought. The accumulated precipitation deficit over the course of the ongoing drought is unprecedented in California’s century-long observational record, and when the additional drying effects of record-high temperatures are taken into account, the 2013-2016 event may in fact be the most severe in a millennium. The amount of water stored in the critically important Sierra Nevada snowpack reached its lowest level in over 500 years in 2015, and the loss of groundwater in the state’s aquifers has literally moved mountains. Drought-related impacts—including decreased agricultural and urban water availability, elevated wildfire risk, dramatically increased tree mortality, adverse effects upon riverine and marine ecosystems, and infrastructure damage to roads and pipelines —have been widespread.
Over the past several years, California weather watchers have become well acquainted with the now-infamous “Ridiculously Resilient Ridge” of atmospheric high pressure—the unusually persistent atmospheric anomaly responsible for redirecting winter storms over the Pacific and ultimately bringing record-breaking warmth and dryness to the Golden State. Like a boulder displacing a narrow stream of water, this sluggish atmospheric feature consistently deflected the storm track to the north of California during the typical “rainy season” months of October to May. As a result, much of the state was left high and dry—even during what is typically the wettest time of year. [more]
ABSTRACT: Recent evidence suggests that changes in atmospheric circulation have altered the probability of extreme climate events in the Northern Hemisphere. We investigate northeastern Pacific atmospheric circulation patterns that have historically (1949–2015) been associated with cool-season (October-May) precipitation and temperature extremes in California. We identify 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. Our analysis detects statistically significant changes in the occurrence of atmospheric patterns associated with seasonal precipitation and temperature extremes. We also find a robust increase in the magnitude and subseasonal persistence of the cool-season West Coast ridge, resulting in an amplification of the background state. 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. 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. Collectively, our empirical 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.