Winter precipitation intensity and ENSO/PDO variability in the Willamette Valley of Oregon

Abstract

AbstractThere is growing concern about the effects of inter‐annual climatic variability, such as the El‐Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), on regional hydrology and water resources. We analysed patterns of wintertime precipitation intensity, using both simple intensity and number of heavy precipitation days per year, for eight stations in northwestern Oregon's Willamette Valley for the period 1972–2006, and examined the separate and combined influence of ENSO and PDO on precipitation intensity. The analysis was accomplished using Kendall's tau and analysis of variance (ANOVA) to determine differences in precipitation intensity among combinations of positive and negative ENSO and PDO phases. The results suggest that the relation between ENSO phase and precipitation intensity is negative in November and positive in April, while the relation between PDO and intensity is negative and strongest in January and March. These varying seasonal associations with ENSO/PDO phase may be related to the Willamette Valley's location in the transitional zone between positive and negative El Niño response and to the moderating effects of out‐of‐phase ENSO/PDO. This seasonal variability of heavy rainfall has implications for the quantity and quality of regional water resources. Copyright © 2009 Royal Meteorological Society