TROPICAL INFLUENCES ON CALIFORNIA PRECIPITATION

Atmospheric circulation anomalies and hydrologic processes associated with California wet and dry events were examined during Northern Hemis phere winter. The precipitation anomaly over the west coast of North A merica shows a north-south three-cell pattern. Heavy precipitation in California is accompanied by dry conditions over Washington, British C olumbia, and along the southeastern coast of Alaska and reduced precip itation over the subtropical eastern Pacific. The inverse relationship between California and the Pacific Northwest is supported by the tran sport of moisture flux. During wet events, the southern branch of mois ture flux transport strengthens and brings moisture from the North Pac ific to California, hence enhanced rainfall. Strengthened moisture flu x transport northward to the area north of Washington is consistent wi th suppressed rainfall in California. The local precipitation anomaly pattern in the eastern tropical Pacific just north of the equator has a large influence on precipitation events in California. The enhanced precipitation generates strong rising motion. The associated sinking m otion is located over California. Strong sinking motion and strong upp er-level convergence favor dry conditions in California. Conversely, s uppressed rainfall in the eastern Pacific is associated with above-nor mal precipitation in California. Precipitation in California is likely below normal during cold ENSO events. When convection in the central Pacific is enhanced, California has heavy precipitation if rainfall in the subtropical eastern Pacific is suppressed. In addition to ENSO, p recipitation in California is also modulated by the tropical intraseas onal oscillation. Wet (dry) events are favored during the phase of the oscillation associated with enhanced convection near 150 degrees E (1 20 degrees E) in the tropical Pacific.