Monthly Pacific Decadal Oscillation Anomalies

The University of Washington is the lead institution on the study of the PDO, having discovered the phenomena while asking the question "Where are the salmon" back in 1996.
From http://jisao.washington.edu/pdo/PDO.latest :

PDO INDEX

Updated standardized values for the PDO index, derived as the
leading PC of monthly SST anomalies in the North Pacific Ocean,
poleward of 20N. The monthly mean global average SST anomalies
are removed to separate this pattern of variability from any
"global warming" signal that may be present in the data.


For more details, see:

Zhang, Y., J.M. Wallace, D.S. Battisti, 1997:
  ENSO-like interdecadal variability: 1900-93. J. Climate, 10, 1004-1020.

Mantua, N.J. and S.R. Hare, Y. Zhang, J.M. Wallace, and R.C. Francis,1997:
  A Pacific interdecadal climate oscillation with impacts on salmon
  production. Bulletin of the American Meteorological Society, 78,
  pp. 1069-1079.
  (available via the internet at url:
      http://www.atmos.washington.edu/~mantua/abst.PDO.html)

Data sources for this index are:
UKMO Historical SST data set for 1900-81;
Reynold's Optimally Interpolated SST (V1) for January 1982-Dec 2001)
*** OI SST Version 2 (V2) beginning January 2002 - ...  

It's been ten straight months with a minus sign. Here's the Jan. 1900-May 2008 table.