Pacific decadal oscillation

• 1. Introduction

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  The Pacific decadal oscillation, or PDO, is a vast, recurring pattern of ocean and atmosphere variability that stretches across the mid-latitude Pacific basin. It was named by Steven R. Hare, who noticed it while studying salmon production patterns in 1997. That detail alone is worth pausing on: a climate signal powerful enough to reshape fish populations across an entire ocean basin was formally identified not by a climatologist chasing temperature records, but by a researcher puzzling over why salmon catches kept swinging in long, irregular cycles.

  The PDO shows itself as alternating bands of warm and cool surface water in the Pacific Ocean north of 20 degrees north latitude. It flips between a positive phase, when the western Pacific cools and parts of the eastern ocean warm, and a negative phase, when those conditions reverse. What makes it strange and difficult to pin down is its rhythm. Unlike the relatively predictable two-to-seven-year drumbeat of El Nino, the PDO lurches between phases over periods ranging from a few years to multiple decades.

  The questions the rest of this documentary will pursue are: What actually drives those lurches? What happens to weather, wildlife, and water when the PDO shifts? And how far back can scientists trace this slow-moving giant through the geological record?

• 2. Salmon and the Naming Moment

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  Steven Hare's observation in 1997 was not a laboratory discovery. It emerged from a practical frustration: North Pacific salmon populations were rising and falling in patterns that decades of fisheries data could not easily explain. When Hare laid the sea surface temperature records alongside the salmon production numbers, a coherent oscillating signal appeared.

  The connection proved real. Two of the PDO's documented polarity reversals, the ones occurring around 1947 and 1977, corresponded with dramatic shifts in salmon production regimes across the North Pacific Ocean. Fishers and fishery managers had been watching the consequences of these shifts for generations without knowing the underlying cause.

  The 1997-1998 transition added another chapter. After that shift, sea surface temperatures declined along the United States west coast, and substantial changes were observed in the populations of salmon, anchovy, and sardine. Scientists described that phase as a cool anchovy phase, a label that captures how tightly marine ecosystems track these slow ocean fluctuations. The spatial pattern of that particular shift was unusual: rather than matching the classic PDO structure, it showed a meridional sea surface temperature seesaw in the central and western Pacific that more closely resembled a strong shift in the North Pacific Gyre Oscillation.

• 3. A Signal Built from Several Sources

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  Unlike El Nino-Southern Oscillation, the PDO is not a single physical mode of ocean variability. Several studies have shown that the PDO index can be reconstructed as the superposition of tropical forcing and extra-tropical processes, meaning it is the sum of several mechanisms with different dynamic origins.

  At inter-annual time scales, the PDO index reflects random variability and El Nino-induced changes in the Aleutian Low. At decadal timescales, three contributors share the work roughly equally: ENSO teleconnections, stochastic atmospheric forcing, and changes in the North Pacific oceanic gyre circulation.

  El Nino reaches into the North Pacific through what scientists call the atmospheric bridge. During El Nino events, deep convection over anomalously warm sea surfaces generates Rossby waves that propagate poleward and eastward and are then refracted back toward the tropics. The teleconnection pattern establishes itself within two to six weeks. The atmospheric bridge is most effective during boreal winter, when the deepened Aleutian Low drives stronger, colder northwesterly winds over the central Pacific and warm, humid southerly winds along the North American west coast.

  Random atmospheric forcing adds a separate layer. Frankignoul and Hasselmann proposed the stochastic climate model paradigm, in which storms passing through the atmosphere alter ocean mixed layer temperatures via surface energy fluxes and Ekman currents. Modeling studies suggest this process contributes as much as one third of the PDO variability at decadal timescales. It generates sea surface temperature anomalies at long timescales but without the sharp spectral peaks that a purely periodic signal would show.

• 4. Memory in the Ocean

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  North Pacific sea surface temperature anomalies have a peculiar form of seasonal memory called the reemergence mechanism, named by Alexander and Deser. The mixed layer over the North Pacific runs typically 100-200 meters deep in winter. When winter anomalies form and penetrate to the base of that deep layer, they get trapped beneath the shallow summer mixed layer when it reforms in late spring, insulated from air-sea heat exchange.

  When autumn arrives and the mixed layer deepens again, those stored anomalies can resurface and influence temperatures all over again. The process is especially pronounced in the western North Pacific, where the winter mixed layer is deeper and the seasonal cycle is more pronounced. The effect lets a single winter's temperature anomaly echo into the following winter, extending the apparent persistence of the PDO signal.

  The Kuroshio-Oyashio Extension region, where the powerful Kuroshio current meets the cooler Oyashio, shows the strongest sea surface temperature variability in the North Pacific. Changes in the axis and strength of that current system generate decadal and longer timescale variance in temperatures. Saravanan and McWilliams showed that a mechanism called advective resonance, the interaction between spatially coherent atmospheric forcing and an advective ocean, can generate decadal sea surface temperature variability in the eastern North Pacific. Dynamic gyre adjustments driven by westward-propagating oceanic Rossby waves are essential to generating those decadal peaks. At the latitude of the Kuroshio Extension, the Rossby wave speed is 2.5 centimeters per second, setting a gyre adjustment timescale of roughly five to ten years depending on where the wave was initiated.

• 5. Weather Across Continents

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  During a positive PDO phase, the wintertime Aleutian Low deepens and shifts southward. Warm and humid air moves along the North American west coast, pushing temperatures above normal from the Pacific Northwest to Alaska. Mexico and the southeastern United States experience below-normal temperatures during the same period.

  Precipitation patterns shift in parallel. The Alaska coast range sees above-average winter precipitation during the positive phase, as does Mexico and the southwestern United States. Canada, eastern Siberia, and Australia see reduced precipitation. Research by McCabe and colleagues showed that the PDO, combined with the Atlantic Multidecadal Oscillation, strongly influences multidecadal drought patterns across the United States. Drought frequency rises over much of the northern United States during the positive PDO phase when that phase is associated with a positive AMO; the negative PDO phase increases drought risk over the southwestern United States under the same AMO condition.

  The reach of the PDO extends well beyond North America. During the negative phase, the Indian subcontinent sees increased rainfall and decreased summer temperatures, meaning the Asian Monsoon responds to sea surface temperature shifts in a basin on the other side of the Pacific. The Great Drought of 1968 in Chile has been traced to an overlap of La Nina and a cold period of the PDO that ran from 1965 to 1976.

• 6. Regime Shifts Through History

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  MacDonald and Case reconstructed the PDO back to the year 993 using tree rings from California and Alberta. That reconstruction reveals a 50-70 year periodicity in the index, though the PDO operates as a strong mode of variability only after 1800. A persistent negative phase dominated the medieval period from 993 to 1300, consistent with La Nina conditions reconstructed in the tropical Pacific and with multi-century droughts documented in the southwestern United States.

  The 20th century record shows a sequence of sharper transitions. A regime shift in 1924-1925 pushed the PDO into a warm phase. The 1945-1946 shift reversed it to a cool phase; that transition showed its maximum amplitude in the subarctic and subtropical front, with a stronger signature near Japan than the shift that followed. The 1976-1977 flip back to a warm phase was more pronounced near the American west coast, and its effects on salmon production were among the clearest signals that prompted Hare's original inquiry.

  A separate change in 1988-1989 weakened the Aleutian Low with associated sea surface temperature shifts, but researchers noted it appeared driven by extratropical oscillations in both the North Pacific and North Atlantic rather than the tropical processes behind other regime shifts. The 2014 flip from a cool to a warm phase contributed to record-breaking surface temperatures across the planet that year. A PDO signal has been reconstructed as far back as 1661 through tree-ring chronologies in the Baja California area, anchoring the instrumental record within a longer context of natural variability.

• 7. Forecasting a Slow Giant

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  The NOAA Earth System Research Laboratory produces experimental statistical forecasts of the PDO using a linear inverse modeling method, abbreviated as LIM. LIM treats the PDO as separable into a linear deterministic component and a non-linear component represented by random fluctuations.

  Much of what makes LIM useful for PDO forecasting comes from ENSO dynamics and the global temperature trend rather than from extra-tropical processes. That dependence limits predictive skill to roughly four seasons ahead. The mechanism behind this window is the seasonal footprinting process, in which an optimal sea surface temperature structure evolves into the ENSO mature phase six to ten months later, which then influences North Pacific sea surface temperatures through the atmospheric bridge.

  Improving long-range forecasts of the PDO's decadal variability likely requires accounting for both externally forced Pacific variability and internally generated variability. The related interdecadal Pacific oscillation, a similar but less localized phenomenon covering the range from 50 degrees south to 50 degrees north, shows how shifts in this family of patterns can also reposition and resize El Nino activity itself. Shifts in the interdecadal Pacific oscillation move the South Pacific convergence zone northeast during positive phases and southwest during negative phases, a reach that hints at how much of the planet's climate is coupled to slow oscillations that no single season reveals.