Dynamical mechanisms of the 1995 California floods

The link between El Nino and the California wintertime rainfall has been re ported in various studies. During the winter of 1994/95, warm sea surface t emperature anomalies (SSTAs) were observed in the central Pacific, and wide spread significant flooding occurred in California during January 1995 and March 1995, However, the El Nino-Southern Oscillation alone cannot explain the flooding. In March 1995 California suffered flooding after the warm SST A over the central Pacific had weakened considerably. During November and D ecember, in spite of El Nino conditions, California was not flooded, and mo re than two standard deviations above normal SSTA in the North Pacific were observed. A possible link between midlatitude warm SSTA and the timing of the onset of flooding is suspected within the seasonal forecasting communit y. The climate condition during the northern winter of 1994/95 is described us ing the National Centers for Environmental Prediction-National Center for A tmospheric Research reanalysis data. Diagnostics show the typical El Nino p attern in the seasonal mean and the link between the position of the jet ex it and the flooding over California on the intraseasonal timescale. The relationship among California floods, the Pacific jet, tropical rainfal l, and SSTA is inferred from results of general circulation model (GCM) exp eriments with various SSTAs. The results show that the rainfall over Califo rnia is associated with an eastward extension of the Pacific jet, which its elf is associated with enhanced tropical convection over the warm SSTA in t he central Pacific. The GCM experiments also show that rainfall over the In dian Ocean can contribute to the weakening of the Pacific jet and to drynes s over California. The GCM experiments did not shaw significant impact of N orth Pacific SSTA, either upon the Pacific jet or upon rainfall over Califo rnia. The agreement with diagnostics results is discussed. GCM experiments suggest the link between the tropical intraseasonal oscillation (TIO) and t he flooding in March in California, since there is a strong TIO component i n rainfall over the Indian Ocean.