4. Comparison with the Transfer Function Modeling approach

El Niño Southern Oscillation (ENSO) has an important influence on the rainfall regime in California and most of the US with most dramatic impacts during the winter (Ropelewski and Halpert, 1986). The Pacific Decadal  Oscillation (PDO) encompasses the ocean temperatures in the northern part of the ocean including variability modes at different temporal scales. The PDO has a warm phase and a cool phase. Each phase can last anywhere from a few years to decades. The cool phase is linked to dry conditions in Southern California and neighboring states (Patzert, 2015)Forget El Niño, the 'PDO' could be the real drought buster... 
ENSO times series and PDO time series were available from the NOAA website.   A plot of all ENSO time series jointly with the PDSI time series is shown in Figure \ref{157229}.