Understanding the Recent Enhancement in Precipitation Variability over California

In recent decades, cycles of dry-to-wet extremes have been increasing over the US West Coast, especially over California. For example, the 2015 snowpack reached a 500-year low due to below average precipitation and above average temperatures, while the 2017 snowpack was above average due to record-high precipitation totals. The question we are asking here is whether or not these dry-to-wet extreme patterns are anomalous, or if they have occurred in the past? We analyzed gridded January-March 1901-2017 CRU precipitation data and found that Central and Southern California have a significant positive trend in winter precipitation variability. Furthermore, the separation of the time series into two periods, 1901-1959 and 1960-2017, shows that the variability is statistically higher in the more recent period compared to the former. Additionally, the occurrence of anomalous dry events triples in frequency and dry and wet events have become significantly drier and wetter in magnitude after 1960. We can place these findings into context by using previously published precipitation reconstructions that span from 1585-2015 over the region in question. These reconstructions show that the recent increase in precipitation variability is unprecedented over the past 430 years. Our results are consistent with current literature that suggests an increasingly volatile future. Water resources managers will need to incorporate this increased volatility into their planning for public and agricultural water supply, while being aware of the higher risks for wildfires and floods.