A long-term pattern of Pacific Ocean climate variability, with events lasting 20 to 30 years and oscillating between warm and cool regimes. Unlike the El Niño-Southern Oscillation (ENSO), the North Pacific and North America are most strongly affected, with secondary signatures seen in the tropics. The PDO has been linked to major changes in northeastern Pacific marine ecosystems-during warm events, coastal productivity is enhanced in Alaska and inhibited off the U. S. west coast; the opposite pattern is seen during cool events.