Orthogonal PDO and ENSO Indices

A framework for interpreting the Pacific decadal oscillation (PDO) and ENSO indices is presented. The two leading principal components (PCs) of sea surface temperature [SST; strictly speaking, the departure from globally averaged SST (SST*)] over the entire Pacific basin comprise a two-dimensional phase space. A linear combination of these pan-Pacific PCs corresponding to a +45 degrees rotation (designated by P) is nearly identical to the PDO, the leading PC of Pacific SST* poleward of 20 degrees N. Both P and the PDO index exhibit apparent "regime shifts'' on the interdecadal time scale. The orthogonal axis (rotated by -45 degrees and designated by T') is highly correlated with conventional ENSO indices, but its spatial regression pattern is more equatorially focused. SST variability along these two rotated axes exhibits sharply contrasting power spectra, the former (i.e., P) suggestive of "red noise'' on time scales longer than a decade and the latter (i.e., T') exhibiting a prominent spectral peak around 3-5 years. Hence, orthogonal indices representative of the ENSO cycle and ENSO-like decadal variability can be generated without resorting to filtering in the time domain. The methodology used here is the same as that used by Takahashi et al. to quantify the diversity of equatorial SST patterns in ENSO; they rotated the two leading EOFs of tropical Pacific SST, whereas the two leading EOFs of pan-Pacific SST* are rotated here.