Pacific Decadal Oscillation

Pacific Decadal Oscillation
Name	Pacific Decadal Oscillation
Date discovered	1990s
Discoverer	Steven R. Hare
Related phases	ENSO, IPO

Pacific Decadal Oscillation. The Pacific Decadal Oscillation is a robust, long-lived pattern of Pacific Ocean climate variability centered over the mid-latitude Pacific basin. It is characterized by sea surface temperature and sea level pressure anomalies that shift phases on timescales of 20 to 30 years, influencing major climate and ecosystem regimes across the Pacific Rim. First identified in the late 1990s through analyses of salmon population data and North Pacific climate records, it represents a critical low-frequency background state modulating higher-frequency events like El Niño.

Definition and discovery

The Pacific Decadal Oscillation is formally defined as the leading empirical orthogonal function of monthly sea surface temperature anomalies in the North Pacific Ocean, north of 20°N. Its discovery is credited to fisheries scientist Steven R. Hare, who in 1996 noted a striking correlation between Alaskan sockeye salmon catches and Pacific climate indices, suggesting a regime shift around 1977. Subsequent work by researchers like Nathan J. Mantua and others at the University of Washington solidified the concept, linking it to broader atmospheric circulation patterns such as the Aleutian Low. The term itself was coined to describe the oscillation between distinct "warm" and "cool" phases, each persisting for decades, with pivotal transitions noted in 1925, 1947, and 1977.

Physical mechanisms

The physical drivers of the Pacific Decadal Oscillation involve complex couplings between the ocean and the atmosphere. A leading hypothesis involves oceanic Rossby wave dynamics and changes in the strength of the subtropical gyre and the subpolar gyre in the North Pacific. The Aleutian Low pressure system plays a key role, with its intensity and position influencing wind-driven ocean currents and heat fluxes. Processes like Ekman transport and oceanic thermocline adjustments help to reinforce and sustain the anomalies. The atmosphere–ocean interaction is thought to be a mix of stochastic atmospheric forcing and deterministic ocean memory, though the exact mechanisms remain an active area of study within institutions like the National Oceanic and Atmospheric Administration.

Climate impacts

The phase of the Pacific Decadal Oscillation exerts profound influences on regional climates, particularly around the Pacific basin. During its positive phase, the Pacific Northwest and Alaska often experience warmer, drier conditions, while the southwestern United States and northern Mexico may see increased precipitation. It significantly modulates hurricane activity in the Eastern Pacific and typhoon tracks in the Western Pacific. Terrestrial impacts include altered snowpack levels in the Sierra Nevada and Cascade Range, while marine impacts are seen in the productivity of ecosystems supporting salmon, sardine, and anchovy populations from California to Japan.

Detection and measurement

Detection of the Pacific Decadal Oscillation relies on statistical analysis of long-term instrumental records, primarily sea surface temperature and sea level pressure data. The standard index is calculated by the Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington. Paleoclimate proxies such as tree rings from Mongolia and the western United States, coral records from the Pacific, and ice core data from Mount Logan are used to reconstruct its behavior over centuries. Key monitoring tools include the network of TAO/TRITON buoys and satellite missions like NASA's Aqua.

Relation to other climate patterns

The Pacific Decadal Oscillation is intrinsically linked to other major modes of climate variability. It shares spatial features and may be a regional manifestation of the broader Interdecadal Pacific Oscillation, which spans both the North and South Pacific. It strongly modulates the amplitude and frequency of El Niño-Southern Oscillation events, with warm phases often associated with more frequent and intense El Niño events. Connections to the Arctic Oscillation and the North Pacific Gyre Oscillation have also been identified, suggesting it is part of a complex, interconnected climate system.

Research and future projections

Ongoing research spearheaded by organizations like the International Pacific Research Center and the National Center for Atmospheric Research focuses on improving climate model representations of the Pacific Decadal Oscillation. A major challenge is distinguishing its natural variability from the forced signal of anthropogenic climate change, particularly global warming. Future projections from models participating in the Coupled Model Intercomparison Project show considerable uncertainty, with some suggesting a potential weakening or spatial shift in its pattern. Understanding its evolution is critical for decadal climate prediction and managing resources in sensitive regions like the American West and East Asia.

Category:Climate patterns Category:Oceanography Category:Pacific Ocean