A long-term fluctuation in sea surface temperatures in the extratropical north Pacific, waxing and waning every 20–30 years. The ‘cool’/‘negative’ phase is characterized by a wedge of unusually low ocean temperatures in the eastern Pacific and a warmer ‘horseshoe’ of unusually high ocean temperatures in the north, west, and southern Pacific. In the ‘warm’/‘positive’ phase, the western Pacific cools and the east warms. In western North America, positive phases of the PDO are associated with decreased winter precipitation, snowpack, and streamflow in the northwest, and higher precipitation in the southwest. Conditions reverse during negative PDO phases. Schneider and Cornuelle (2005) J. Climate 18, 21 argue that the north Pacific sea surface temperature anomaly and the PDO are ‘a response to changes of the north Pacific atmosphere resulting from its intrinsic variability, remote forcing by ENSO and other processes, and ocean wave processes associated with the ENSO and the adjustment of the north Pacific Ocean by Rossby waves’. See also Goodrich and Walker (2011) Phys. Geog. 32, 4, 295.