Tuesday, 11 February 2003
Impact of ENSO on Snowpack over the Western United States: A GCM Study
A 45-year output from the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3) and the reanalysis data from the National Centers for Environmental Prediction over the same period are used to study the relationship between the variations of tropical Pacific sea surface temperature (SST) and snowpack anomalies over the western U.S. Although the resulting snow cover patterns are similar for both simulations and observations over the northwestern U.S., the reasons are different. The observed anomalous snow patterns were caused by the winter precipitation variability which was associated with El Niņo-Southern Oscillation (ENSO), while the simulated snow patterns resulted from the temperature variations due to atmospheric model biases. These biases were caused by unrealistic surface heat flux transfer over the North Pacific ocean and the coarse convective scheme used in CCM3. Meanwhile, the positive snowpack anomalies over the southwestern U.S. that resulted from the stronger precipitation were associated with the warm phase of ENSO, which was consistent with the observed processes. However, the negative snowpack anomalies for both simulations and observations were attributed to exaggerated local precipitation in fall and appear to have no connection with SSTs.