Pacific Decadal Oscillation and Multi-Decadal Variability

Core Idea

The Pacific Decadal Oscillation (PDO) is a climate pattern in the North Pacific with a dominant timescale of 20–30 years, characterized by anomalies in sea surface temperature, sea level pressure, and atmospheric circulation. PDO phases influence global weather patterns, precipitation in North America, salmon populations, and the intensity of ENSO events. Unlike ENSO, the PDO mechanisms are not fully understood, but both atmospheric forcing and ocean memory (via ocean gyres and mid-latitude currents) play roles.

How It's Best Learned

Compute the PDO index from North Pacific SST anomalies. Examine precipitation and temperature anomalies during positive and negative PDO phases and their impacts on regional climate.

Common Misconceptions

The PDO is not a single mode; principal component analysis of North Pacific SST reveals multiple modes with different timescales. Also, the PDO is not entirely predictable like ENSO; stochastic forcing and chaos limit predictability.

Explainer

From your understanding of ENSO, you know that the tropical Pacific undergoes irregular oscillations between El Niño (warm eastern Pacific) and La Niña (cool eastern Pacific) on timescales of 2–7 years, with global consequences for weather and climate. The Pacific Decadal Oscillation (PDO) is a related but distinct pattern that operates on much longer timescales — roughly 20–30 years per phase — and is centered in the *North* Pacific rather than the tropics. Think of it as the slow background rhythm over which ENSO's faster oscillations play out.

The PDO is defined by the leading pattern (first principal component) of monthly sea surface temperature anomalies in the North Pacific, poleward of 20°N. During a positive (warm) phase, the central North Pacific is cooler than normal while a horseshoe of warm water hugs the west coast of North America and the tropical Pacific. During a negative (cool) phase, the pattern reverses: the central North Pacific warms while coastal waters cool. These SST anomalies are accompanied by shifts in the Aleutian Low pressure system, the jet stream position, and storm tracks. The PDO was first identified in the 1990s by fisheries scientist Steven Hare, who noticed that Pacific salmon productivity in Alaska and the Pacific Northwest alternated in multi-decadal cycles that correlated with these SST patterns.

The impacts of PDO phase are wide-ranging. During positive PDO phases, the Pacific Northwest and Alaska tend to be warmer and drier, while the southwestern United States receives more precipitation. Negative PDO phases reverse these tendencies. The PDO also modulates ENSO's effects: El Niño events during a positive PDO phase tend to produce stronger impacts on North American weather than those occurring during a negative PDO phase, because the background SST pattern reinforces the tropical signal. Marine ecosystems respond dramatically — the "regime shifts" of 1976–77 (negative to positive) and the late 1990s (positive to negative) coincided with major reorganizations of fish populations, including the collapse of some salmon stocks and the boom of others.

Unlike ENSO, which has a well-understood mechanism rooted in tropical ocean-atmosphere coupling (the Bjerknes feedback), the PDO's driving mechanisms remain debated. It may not be a single dynamical mode at all, but rather the superposition of several processes operating on different timescales: tropical ENSO variability imprinting on the North Pacific through atmospheric teleconnections, ocean gyre circulation slowly advecting temperature anomalies around the North Pacific (the "ocean memory" component with ~20-year timescales matching gyre transit times), and stochastic atmospheric forcing exciting the ocean's natural response timescales. This mechanistic ambiguity means the PDO is harder to predict than ENSO. Nonetheless, recognizing which PDO phase the Pacific is in provides valuable context for seasonal and decadal climate outlooks, fisheries management, and interpreting whether observed temperature trends reflect long-term climate change or natural multi-decadal variability.