The influence of the Madden-Julian oscillation on precipitation in Oregon and Washington

The Madden-Julian oscillation (MJO) is the primary mode of large-scale intraseasonal variability in the Tropics. Previous work has explored the influences of the MJO on atmospheric circulation anomalies over the North Pacific Ocean and precipitation in California, among other effects. This study focuses on the relationship between the MJO and mean precipitation in the states of Oregon and Washington and that between the MJO and the occurrence of flooding in western Washington. The MJO is diagnosed using principal component analysis of 850-hPa zonal winds from the NCEP-NCAR reanalysis for 1979-2000. The dataset for precipitation is daily rain gauge data gridded on a scale of 50 km and covering 1979-94. The occurrence of flooding is based on streamflow records from the Sauk, Snoqualmie, and Chehalis Rivers for 1979-2000. The results indicate that the phase of the MJO has a substantial systematic effect on intraseasonal variability in precipitation in Oregon and Washington in both early winter (October-December) and late winter (January-March). The MJO is also associated with a statistically significant enhancement and modulation of floods in early winter. The phases of the MJO that promote enhanced precipitation in the mean and increased incidence of western Washington floods are substantially different during early winter than during late winter. It is suggested that this result is attributable to the difference in the atmospheric circulation of the North Pacific in early versus late winter.