On the Recent State of the North Pacific Climate System

The Pacific Decadal Oscillation (PDO) is the leading pattern of North Pacific climate variability. It underwent a major transition from a persistently negative to positive state in 1976/77; considerable recent discussion has centered on whether a return to a negative phase in the PDO occurred during the winter of 1998/99. The spatial patterns in sea level pressure anomaly (SLPA) and sea surface temperature anomaly (SSTA) during the winters of 1999-2002 for the North Pacific actually bear little resemblance to those of the PDO. In essence, the southeastern (northern) portion of the North Pacific was subject to atmospheric forcing characteristic of that before (after) the regime shift of 1976/77. Recent major changes in the ecosystems off the west coast of the United States (increased productivity and the return of sub-Arctic species) and continued conditions similar to those after the 1976/77 shift in the Gulf of Alaska and Bering Sea are consistent with these SLPA and SSTA patterns. The lack of a consistent sense to the PDO since 1998 can be attributed, at least in part, to the concurrent lack of a systematic sense to El Nino/Southern Oscillation (ENSO). The PDO was significantly positive during the winter of 2003; it is uncertain whether this is just a temporary condition due to the moderate El Nino of 2003 or the beginning of a sustained positive PDO state. Our results illustrate that a single indicator such as the PDO is incomplete in characterizing the climate variability of the North Pacific.