On the forcing of the Pacific Decadal Oscillation

The Pacific Decadal Oscillation is a widely used index for decadal variability of the North Pacific, yet its dynamics have remained opaque. Recent works point to forcing from the tropics by ENSO, warm pool and south Pacific convergence zones, and from the anomalies of the Aleutian Low.

We present a statistical analysis of sea surface temperature and sea level pressure of the NCEP/NCAR reanalysis. The linkage of the PDO and atmospheric forcing via an autoregressive process indicates that tropical forcing by ENSO, by sea level pressure in the North Pacific, and by geostrophic circulation anomalies in the Kuroshio Extension capture up to 70 % of the PDO variance. Of note is that the tropical forcing is limited to the well known influences of El Nino as captured by NINO34 temperature anomalies.

Analysis of SST clarifies the regional distribution of the forcing. The decorrelation time of SST is typically about 1 year, and notably longer in the Kuroshio off Japan. The influence of ENSO is largest in the eastern North Pacific and along the coast of North America. North Pacific sea level pressure independent of ENSO are largely co-located with ENSO, but show a stronger delayed influence, pointing to the reemergence mechanism. Geostrophic circulation indices, constructed to be independent of contemporaneous NPI and ENSO, are largest in the frontal regions of the Kuroshio extension at the date line.

These results are used to determine if the leading the empirical orthogonal function of SST - the PDO - is a stable statistical descriptor of North Pacific variability.