Lawrence Berkeley National Laboratory

Snow anomalies in the western United States (U.S.) have been widely investigated by many researchers due to its impact on water availability. This study focuses on how anomalous atmospheric circulation affects snowpack accumulation in the western U.S. using observations and output from the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3). Our results indicate that the mid-latitude atmospheric circulation anomalies induced by the El Nino-Southern Oscillation (ENSO) tend to drive winter precipitation shifts, leading to an anomalous snowpack distribution in the western U.S. The warm phase of ENSO produces increased snowpack in the Southwest, while the cold phase of ENSO generates increased snowpack in the Northwest. Temperature has a secondary impact on the anomalous snowpack distribution during ENSO episodes. Additionally, the non-linear atmospheric dynamics-related Pacific-North American (PNA) pattern is found to strongly affect snow anomalies in the western U.S. independent from ENSO. The positive phase of the PNA pattern produces colder temperature and stronger precipitation due to the lower pressure in the region, leading to an above normal snowpack. Conversely, the negative phase of the PNA pattern generates warmer temperature and weaker precipitation resulting from the higher pressure, producing a below than normal snowpack in the western U.S. In general, the NCAR-CCM3 reproduces the observed processes. However, model biases are identified and reported. The information provided in this study strengthens our understanding of climate and water supply variability in the western U.S.