THE VARIABILITY OF WINTERTIME PRECIPITATION IN THE REGION OF CALIFORNIA

Rotated principal component (RPC) analysis, subject to the varimax cri terion and including area weighting, is applied to a 58-yr record (193 1-88) of monthly-and seasonal-mean Climatic Division precipitation ano malies for the contiguous United States to document wintertime precipi tation variability in the region of California. Rotated principal comp onents (time series) derived from this analysis are related to anomali es of seasonal-mean global sea surface temperature, and monthly mean N orthern Hemisphere 500-hPa geopotential height and sea level pressure (SLP). Wintertime seasonal-mean precipitation in California is capture d by two RPCs. The first RPC documents coherent precipitation anomalie s centered in northern California, Oregon, southern Idaho, and eastern Washington, and explains the largest portion of area-averaged varianc e of any of the patterns in the decomposition. A second RPC captures c oherent precipitation variability in the south coast and southeast des ert regions of California, southern Nevada, southern Utah, and norther n Arizona. Fluctuations in the first RPC correlate poorly with Pacific Ocean SST anomalies. However, wet winters in the region of the second RPC correlate modestly with simultaneous cool western subtropical Pac ific Ocean SST anomalies and weakly with warm SST anomalies over a bro ad region of the central and eastern tropical Pacific. The spatial sca le of the tropical SST correlations and the prominent multidecadal tim escale signal of the RPC are consistent with ENSO fluctuations on this timescale influencing southern California precipitation. Consistent w ith the results of earlier studies, significant correlations are found between California wintertime monthly mean precipitation variability and regional 500-hPa geopotential height and SLP anomalies. Linear reg ression analysis is used to construct estimates of the total 500-hPa g eopotential height and SLP fields (climatology plus anomaly) that are representative of the extreme wet and dry California winter months; th ese are then compared with the observed conditions in the individual e xtreme months. Several different Bow patterns appear capable of produc ing anomalously large monthly precipitation totals in California.