Pacific Decadal Oscillation

The Pacific Decadal Oscillation (PDO) is a long-lived pattern of climate variability in the North Pacific Ocean, characterized by changes in sea surface temperature that persist for 20 to 30 years — roughly an order of magnitude longer than the interannual cycle of ENSO. In its positive phase, the eastern North Pacific is cool and the western North Pacific is warm. In its negative phase, the pattern reverses. The PDO is not a single oscillation with a fixed period but rather a regime-like shift in the background state of the North Pacific.

The PDO's mechanism is less well understood than ENSO's. Unlike ENSO, which is driven by equatorial ocean-atmosphere coupling and the Bjerknes feedback, the PDO appears to involve mid-latitude ocean dynamics, including the adjustment of the subtropical gyre to wind stress anomalies and the communication between the tropics and extratropics through teleconnections. Some research suggests that the PDO is partly forced by ENSO itself — the cumulative effect of tropical variability propagating into the North Pacific through atmospheric bridges — and partly generated by independent mid-latitude processes.

The PDO has profound impacts on North American climate. The positive phase is associated with increased winter precipitation in the southern United States and decreased precipitation in the Pacific Northwest, while the negative phase produces the opposite pattern. It also affects salmon populations, with warm PDO phases reducing salmon survival in the Pacific Northwest and cool phases enhancing it.

The PDO illustrates a fundamental principle of climate dynamics: the Earth system contains oscillations on multiple timescales, from the intraseasonal Madden-Julian Oscillation to the interannual ENSO to the decadal PDO, and these oscillations are coupled in ways that are only beginning to be understood.