The NCEP-NCAR reanalysis together with the outgoing long wave radiation anomalies (OLRAs) and a gridded daily precipitation over the United States were used to analyze precipitation over California in the intraseasonal band. The intraseasonal (10-90 days) filtered OLRAs were subjected to singular spectral analysis, which identifies nonlinear oscillations in noisy time series. There are two dominant modes of oscillation associated with California rainfall with periods near 36-40 and 20-25 days.
The 40-day mode is moderated by the Madden -Julian oscillation (MJO) in the Tropics. Enhanced tropical convection propagates to the central Pacific from the western Pacific and rainfall starts in California 4 days later.
The 22-day mode is responsible for alternating wet and dry episodes over California with periods shorter than the time scales of the MJO. It is the leading mode in the 7-30 day band. Cloud bands in the eastern Pacific just north of the ITCZ propagate northward through California to the Pacific-Northwest. OLRAs along 10-20 N also propagate westward from the eastern Pacific to the western Pacific. The 200 hPa streamfunction anomaly composites keyed to the 22-day mode show a westward propagating wave train dominated by a zonal wave number 2. This mode also has the spatial structure similar to the traveling pattern described by Branstator (1987).