Conclusion

A major climate regime shift occurred in the North Pacific in 1999, in conjunction with the contemporaneous shift in the equatorial Pacific. The regime shift of 1999 was not as persuasive as the 1977 shift, and the climate has not simply returned to its pre-1977 state. The SST anomaly pattern established since 1999 was not close enough to the classical negative PDO phase (particularly in the western North Pacific), so that the shift could not be detected in the winter PDO index. However, the key elements of the negative PDO pattern, such as positive SST anomalies in the central North Pacific and negative anomalies along the west North American coast, are present in the new pattern. The PDO index is expected to decline until about 2017, with some slowdown in 2009 (due to a possible El Niño event) and in 2010-2012 (due to quasi-decadal variability).