Multi-century Evaluation of Sierra Nevada Snowpack

Abstract:

California (CA) is currently experiencing a record-setting drought that started in 2012. Snowpack conditions in the Sierra Nevada (SN) mountains present an ominous sign of the severity of this drought: the April 1, 2015 snow water equivalent (SWE) was at only 5% of its historical average. However, no long-term historical context is available for the recent SN snowpack decline. We present an annually-resolved reconstruction of SN-wide April 1 SWE conditions for the past 500 years (Fig. 1). We combined an extensive compilation of blue oak tree-ring series that reflects large-scale CA winter precipitation anomalies with a CA February-March temperature reconstruction in a reconstruction that explains 63% of the SN SWE variance over the instrumental period. Our reconstruction reveals that the 2015 low is unprecedented in the context of the last 500 years. Our error estimation indicates that there is a possibility that a few - primarily16^thcentury - years exceeded the 2015 low, but the estimated return interval for the 2015 SWE value - as calculated based on a generalized extreme value (GEV) distribution - is 3,100 years and confirms the exceptional character of the 2015 SWE low. We further find that the 2015 SWE value is strongly exceptional - exceeding the 95 % CI for a 1000-yr return period - at low elevation SN sites where winter temperature has a strong control on SWE, but less so at high elevation sites, where it exceeds the 95 % CI for only a 95-yr return period.

The 2015 record low snowpack coincides with record high CA January-March temperatures and highlights the modulating role of temperature extremes in CA drought severity. Snowpack lows – amongst other drought metrics- are driven by the co-occurrence of precipitation deficits and high temperature extremes and we find that the exacerbating effect of warm winter temperatures is stronger at low than at high SN elevations. Anthropogenic warming is projected to further increase the probability of severe drought events, advance the timing of spring snowmelt, and increase rain-to-snow ratios. The ongoing and projected role of temperature in the amount and duration of CAs primary natural water storage system thus foreshadows major future impacts on CA water supplies.