"Flooding in southern California in recent years has been attributed to El Niño, a climate condition based on sea surface temperatures of the equatorial Pacific Ocean and on trade wind velocities. During El Niño conditions, trade winds diminish, and upwelling of colder water in the ocean is inhibited along the Pacific Coast of the United States. Precipitation is enhanced by increased evaporation from the warmer surface water. The result is an increase in the number and intensity of storms (Harmon, 1999). The storms that occurred in the Cuyama River Basin (fig. 1) in January and February 1998 were influenced by a very strong El Niño condition."
U.S. Geological Survey: http://www.usgs.gov/