(a1) Sample crosscorrelation function (CCF) between the pre-whitened  Pacific Decadal Oscillation (PDO) series denoted as X and the filtered PDSI serie denoted as Y for the training period.  (a2) The sample CCF between the PDO series and the  PDSI series. (b1) CCF pre-whitened  Niño3.4 and the filtered PDSI. (b2) The sample CCF between the Niño3.4 serie and the  PDSI serie.