The Pacific Decadal Oscillation, Revisited

The Pacific decadal oscillation (PDO), the dominant year-round pattern of monthly North Pacific sea surface temperature (SST) variability, is an important target of ongoing research within the meteorological and climate dynamics communities, and is central to the work of many geologists, ecologists, natural resource managers, and social scientists. Research over the last fifteen years has led to an emerging consensus: the PDO is not a single phenomenon, but is instead the result of a combination of different physical processes, including both remote tropical forcing and local North Pacific atmosphere/ocean interactions, which operate on different timescales to drive similar PDO-like SST anomaly patterns. How these processes combine to generate observed PDO evolution, including apparent regime shifts, is shown using simple autoregressive models of increasing spatial complexity. Simulations of recent climate in coupled GCMs are able to capture many aspects of the PDO, but do so based on a balance of processes often more independent of the Tropics than is observed. Finally, it is suggested that assessment of PDO-related regional climate impacts, reconstruction of PDO-related variability into the past with proxy records, and diagnosis of Pacific variability within coupled GCMs should all account for the effects of these different processes, which only partly represent direct forcing of the atmosphere by North Pacific Ocean SSTs.