Precipitation in the Colorado River Basin and its low frequency associations with PDO and ENSO signals

The spatial and temporal distribution of point precipitation quantiles representing abnormal moisture conditions over the Colorado River Basin (CRB) is analyzed by means of the Standardized Precipitation Index (SPI), calculated in annual and seasonal aggregations. From a cell-by-cell analysis, the area covered by abnormally wet and dry conditions during the last century shows an inverse relationship with their frequency of occurrence, with frequent events (occurring 80% of the time) in which abnormal conditions cover less than 10% of the basin and infrequent events (occurring 5% of the time) in which abnormal conditions cover around 50% of the basin. During El Nino years, both extremely wet and dry conditions are likely to occur, while only extremely dry conditions are probable during La Nin~a years. Regions of homogeneous SPI realizations were delimited using principal components analysis (PCA) to highlight major variation modes distinguishable in the basin, and a frequency analysis was performed over the reconstructed values of SPI to identify their multiannual oscillation modes. Common multiannual oscillations between the SPI index, the Pacific Decadal Oscillation (PDO) and the Bivariate El Nin~o-Southern Oscillation (BEST) time series were explored using multichannel singular spectrum analysis (M-SSA). The coupled impact of PDO-ENSO indicates the presence of a trend and two significant oscillations around 5 and 15 years on the SPI time series. The occurrence of extreme SPI values associated with ENSO and PDO was also evaluated as a common product of these indices that highlights moisture conditions affected by common enhancement phases of ENSO and PDO.