| Advances in the understanding of the nature and causes of multi-decadal precipitation and temperature variability in the western United States could assist stakeholder groups in their management and distribution of resources and personnel. The three studies undertaken in this dissertation address this issue of multi-decadal climate variability and its potential implications for user groups. In the first study, the relationship between ENSO conditions and winter precipitation in the Western U.S. is examined within the context of decadal-scale variability, as represented by phasing of the Pacific Decadal Oscillation (PDO). Spatial inconsistencies in the ENSO-precipitation relationship, commensurate with PDO phase shifts, are identified, taking the form of a 'dipole' signature across the Western U.S. This finding has implications for the knowledge of uncertainty of ENSO teleconnections, and may prove meaningful for users of climate information throughout the region. In the second study, the reliability of fall season ENSO conditions as a predictor of Western U.S. winter circulation anomalies is shown to vary depending on the phase of the PDO pattern. During the PDO cold phase of 1948--1976, fall season El Nino events tended not to precede the expected winter troughing pattern over the West, while during the PDO warm phase of 1977--1998, fall season El Nino conditions were a more reliable predictor of winter circulation anomalies over the Western U.S. Fall season La Nina conditions during both the cold and warm phases of the PDO generally correlated well with the occurrence of wintertime high-pressure ridging centered off the Pacific coast. These results highlight uncertainty on multi-decadal time scales surrounding the use of ENSO conditions as a seasonal climate forecast tool. In the third study, the spatio-temporal variability of spring season minimum temperatures in the western United States is examined as a function of multi-decadal Pacific climate variability for the period 1925--1994. Variations in minimum temperature patterns, as determined by a principal components-based regionalization analysis, indicate a significant statistical relationship between March and April minimum temperatures and an index of the PDO. These results have implications for an improved understanding of multi-decadal climate dynamics across the West, including growing season length and intensity. |
