Pacific Decadal Oscillation

The Pacific Decadal Oscillation (abbreviated PDO) is the dominant multidecadal mode of coupled ocean–atmosphere variability across the extratropical North Pacific Ocean, operating on characteristic regime timescales of 20 to 30 years (a full quasi-cycle of 40 to 60 years). Coined and defined in 1996–1997 by fisheries scientist Steven R. Hare and climatologists Nathan J. Mantua, Yuan Zhang, John M. Wallace, and Robert C. Francis while investigating 20th-century boom-and-bust cycles in Alaska versus Pacific Northwest salmon catches, the PDO is mathematically defined as the leading principal component (Empirical Orthogonal Function 1, EOF 1) of monthly sea surface temperature (SST) anomalies in the North Pacific basin poleward of 20°N (after removing the global mean SST warming trend). When extended across the entire Pacific basin from 60°S to 60°N, the corresponding basin-wide symmetric pattern is termed the Interdecadal Pacific Oscillation (IPO).

Spatially, the surface temperature and atmospheric pressure fingerprint of the PDO resembles a low-frequency, extratropically amplified counterpart of the interannual El Niño–Southern Oscillation (ENSO)—so much so that whereas ENSO peaks in the equatorial eastern Pacific with secondary teleconnections in the North Pacific, the PDO exhibits its strongest amplitude in the central and western North Pacific (along the Kuroshio–Oyashio Extension at 35°N–45°N) and along the North American continental margin:

In 2016, Matthew Newman and colleagues synthesized two decades of observational and modeling research to demonstrate that the PDO is not a single physical oscillation with one spectral peak; rather, the PDO index represents the red-noise integration of four distinct physical processes operating across timescales from months to decades:

1. Stochastic Atmospheric Forcing ("The Ocean as a Red-Noise Integrator"): Following Klaus Hasselmann's (1976) stochastic climate model, high-frequency weather fluctuations in the winter Aleutian Low (driven by internal extratropical atmospheric variability) force surface turbulent heat fluxes and Ekman surface currents across the deep winter mixed layer of the North Pacific (100–200 m deep). Because the ocean mixed layer possesses a large thermal heat capacity, it integrates white-noise atmospheric weather forcing into low-frequency, red-noise SST anomalies (dT/dt = −λT + F_atm(t)) with a thermal damping timescale λ⁻¹ of ~6 to 18 months.
2. Thermal Re-Emergence Mechanism: First identified by Michael Alexander and Clara Deser (1995), deep North Pacific temperature anomalies created across the 150-m winter mixed layer in February–March become capped and insulated beneath a shallow, solar-heated seasonal thermocline (20–30 m deep) during summer. When autumn and early-winter storms deepen the mixed layer again in November–December, the sequestered subsurface thermal anomaly is entrained back into the surface layer (re-emergence), carrying the PDO anomaly from one winter to the next.
3. Tropical ENSO Teleconnections ("The Atmospheric Bridge"): During El Niño events, anomalous deep convection in the central/eastern equatorial Pacific excites a Pacific–North American (PNA) stationary Rossby wave train that deepens the winter Aleutian Low, forcing a positive PDO+ SST pattern across the North Pacific. Conversely, La Niña events weaken the Aleutian Low and project onto the negative PDO− phase. Thus, low-frequency decadal clustering of El Niño versus La Niña events drives roughly half of the variance in the observed PDO index.
4. Oceanic Rossby Wave Propagation and Kuroshio–Oyashio Extension (KOE) Dynamics: Decadal fluctuations in wind-stress curl (∇ × τ) across the central North Pacific excite westward-propagating baroclinic oceanic Rossby waves at latitudes 30°N–45°N. Because mid-latitude baroclinic Rossby waves travel slowly (c_R ≈ 1–3 cm s⁻¹), they require 3 to 7 years to cross the Pacific basin and reach the western boundary off Japan. Upon arrival, these Rossby waves displace the latitude and jet strength of the Kuroshio–Oyashio Extension (KOE) front—where warm subtropical waters meet cold subarctic waters—generating persistent decadal SST anomalies that feed back onto the overlying storm track.

Instrumental SST and sea-level pressure records since 1900 document several abrupt climate regime shifts in which the North Pacific transitioned between opposite PDO phases:

• 1925 Shift (PDO− → PDO+): Transition to a warm phase that lasted until 1946.
• 1947 Shift (PDO+ → PDO−): Transition to a cool phase spanning nearly three decades (1947–1976), marked by frequent La Niña events, heavy Cascade Mountain snowpacks, and booming Oregon/Washington coho and chinook salmon runs.
• The Great 1976–1977 Pacific Climate Shift (PDO− → PDO+): During the winter of 1976–1977, the Aleutian Low abruptly deepened by ~4–6 hPa, coastal Gulf of Alaska SSTs jumped by +1.0 to +1.5 °C, and the tropical Pacific shifted toward frequent, strong El Niño events (including the extreme 1982–1983 and 1997–1998 El Niños).
• 1998–1999 Shift (PDO+ → PDO−): Following the collapse of the 1997–1998 El Niño, the Pacific entered a prolonged cool PDO− regime (1999–2013, briefly interrupted by the 2014–2016 Northeast Pacific marine heatwave "Blob" and the 2015–2016 El Niño before returning to deeply negative PDO− values in 2020–2024).

The discovery of the PDO revolutionized fisheries oceanography by explaining why salmon catches in Alaska and those in the Pacific Northwest (Washington, Oregon, California) oscillate 180° out of phase on multidecadal timescales:

• During Warm PDO+ Regimes (e.g., 1977–1998):
  • Gulf of Alaska Boom: Cyclonic winds around a deepened Aleutian Low enhance coastal convergence and water-column stability in the light-limited Gulf of Alaska, while transporting warm, iron- and nutrient-bearing coastal currents northward. Subarctic zooplankton biomass doubles, and Alaskan sockeye (Oncorhynchus nerka) and pink salmon (O. gorbuscha) harvests rise to historic highs.
  • California Current Collapse: Simultaneously, off Oregon, Washington, and California, the weakened North Pacific High reduces southward upwelling-favorable winds, deepens the nutricline, and replaces large, lipid-rich subarctic copepods with small, lipid-poor subtropical species—causing coho (O. kisutch) and chinook (O. tshawytscha) smolt survival to plummet.
• During Cool PDO− Regimes (e.g., 1947–1976, 1999–2013): Strong northerly winds along the US West Coast invigorate coastal upwelling of cold, nutrient-rich water, restoring lipid-rich northern copepod communities and boosting Columbia River and Oregon coastal salmon survival while cooling and destratifying the Gulf of Alaska.
• Anchovy vs. Sardine Regime Alternations: Across the California, Humboldt (Peru–Chile), and Kuroshio currents, Francisco Chavez and colleagues (2003) demonstrated a basin-wide multidecadal alternation between sardine regimes (Sardinops sagax, which thrive during warm PDO+ decades in offshore, weaker-upwelling waters) and anchovy regimes (Engraulis mordax and Engraulis ringens, which dominate during cool PDO− decades characterized by intense coastal upwelling and diatom blooms).

In 2020, observational and coupled model evaluations of the North Pacific showed that following the 2014–2016 Northeast Pacific marine heatwave ("The Blob"), non-stationary changes in North Pacific atmospheric variance weakened the historical covariance between the Pacific Decadal Oscillation index and Alaskan versus California Current salmon recruitment (Litzow et al., 2020).

In 2021, a comprehensive synthesis of tropical and extratropical Pacific decadal variability established that internal stochastic atmospheric forcing of the Aleutian Low and tropical El Niño–Southern Oscillation teleconnections jointly govern PDO phase transitions while modulating multi-year marine heatwave risks across the Kuroshio–Oyashio Extension and eastern North Pacific (Capotondi et al., 2023; Power et al., 2021).

In 2023, during the protracted 2020–2023 triple-dip La Niña and the subsequent onset of the 2023–2024 El Niño, the Pacific Decadal Oscillation index remained locked in a strongly negative phase (reaching below −2.4 standard deviations) due to record warm sea surface temperature anomalies across the western and central North Pacific gyre that persisted despite tropical eastern Pacific warming (Capotondi et al., 2023).

1. Mantua, N. J., Hare, S. R., Zhang, Y., Wallace, J. M., & Francis, R. C. (1997). A Pacific interdecadal climate oscillation with impacts on salmon production078<1069:APICOW>2.0.CO;2). Bulletin of the American Meteorological Society, 78(6), 1069–1079.
2. Newman, M., Alexander, M. A., Ault, T. R., Cobb, K. M., Deser, C., Di Lorenzo, E., et al. (2016). The Pacific Decadal Oscillation, revisited. Journal of Climate, 29(12), 4399–4427.
3. Chavez, F. P., Ryan, J., Lluch-Cota, S. E., & Ñiquen, C. M. (2003). From anchovies to sardines and back: Multidecadal change in the Pacific Ocean. Science, 299(5604), 217–221.
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6. Power, S., Lengaigne, M., Capotondi, A., Khodri, M., Vialard, J., Jebri, B., et al. (2021). Decadal climate variability in the tropical Pacific: Characteristics, causes, predictability, and prospects. Science, 374(6563), eaay9165.
7. Capotondi, A., McGregor, S., McPhaden, M. J., Cravatte, S., Holbrook, N. J., Imada, Y., et al. (2023). Mechanisms of tropical Pacific decadal variability. Nature Reviews Earth & Environment, 4(11), 754–769.