The Pacific Decadal Oscillation, Revisited

Since its identification in the late 1990's, the Pacific decadal oscillation (PDO), the dominant pattern of North Pacific sea surface temperature (SST) variability, has been connected both to other parts of the climate system and to impacts on natural resources and marine and terrestrial ecosystems. Since then, however, studies have found that the PDO is not a single mode of climate variability but instead represents the combination of three groups of processes: (1) changes in ocean surface fluxes related to the Aleutian low, due to remote interannual to decadal tropical variability (largely El Nino) via the “atmospheric bridge” plus more rapidly varying, unpredictable weather “noise”; (2) oceanic memory processes, principally “re-emergence”, that act to integrate this forcing and thus generate added PDO variability on interdecadal time scales; and (3) SST anomalies along the subarctic front at about 40ºN in the western Pacific ocean, the surface manifestation of decadal changes in the Kuroshio-Oyashio current system forced by basin-wide oceanic processes. Thus, the PDO and its impacts represent the effects of different processes operating on different timescales, not all of which are driven by extratropical SST anomalies. This paper presents a synthesis of this current view of the PDO, and discusses corresponding implications for climate diagnosis, including of PDO climate impacts and predictability (both oceanographic and atmospheric); potential decadal "regime"-like behavior; simulations of the PDO in climate models; PDO relationships with "modes" in other ocean basins including the global trend; and the interpretation of paleoproxy multicentennial reconstructions of the PDO.