Is the PDO a Single Mode of Climate Variability or the Result of Several? Implications for the Next Decade

There has been a great deal of attention recently on the so-called Pacific Decadal Oscillation. It is frequently characterized as a prominent mode of climate variability that explains a number of important climate fluctuations and can be a basis for extrapolations into the next decade. Its variability is usually quantified by an index of a characteristic signature of SST over the North Pacific. We argue that the variance of the unfiltered or low-pass filtered index (and the characteristic SST signature) has multiple physical sources. If this is the case, interpretations of climate fluctuations from the 1970s to the present and implications for the 2000s will be highly dependent on what portion of variance can be attributed to different phenomena.