NASA image of the Pacific Ocean in April 2008 showing La Nina and Pacific Decadal Anomalies.
The Pacific Decadal Oscillation (PDO) is a pattern of Pacificclimate variability that shifts phases on at least inter-decadal time scale, usually about 20 to 30 years. The PDO is detected as warm or cool surface waters in the Pacific Ocean, north of 20° N. During a "warm", or "positive", phase, the west Pacific becomes cool and part of the eastern ocean warms; during a "cool" or "negative" phase, the opposite pattern occurs.
The Pacific (inter-)decadal oscillation was named by Steven R. Hare, who noticed it while studying salmon production patterns. Simultaneously the PDO climate pattern was also found by Yuan Zhang. The two groups described the patterns in 1997.[1]
The mechanism by which the pattern lasts over several years has not been identified; one suggestion is that a thin layer of warm water during summer may shield deeper cold waters. A PDO signal has been reconstructed to 1661 through tree-ring chronologies in the Baja California area.[2]
The Interdecadal Pacific Oscillation (IPO or ID) display similar Sea Surface Temperature (SST) and Sea Level Pressure (SLP) patterns, with a cycle of 15–30 years, but affects both the north and south Pacific. In the tropical Pacific, maximum SST anomalies are found away from the equator. This is quite different from the quasi-decadal oscillation (QDO) with a period of 8-to-12 years and maximum SST anomalies straddling the equator, thus resembling the El Niño-Southern Oscillation (ENSO).
Although there are several patterns of behavior, the most significant one seems to be in regime shifts between "warm" and "cool" patterns which last 20 to 30 years.
1750: PDO displays an unusually strong oscillation.[2]
1905: After a strong swing, PDO changed to a "warm" phase.
1998: PDO index showed several years of "cool" values, but did not remain in that pattern.[4]
2008: The early stages of a cool phase of the basin-wide Pacific Decadal Oscillation.[5]
In all cases in the 1900s, PDO "regime shifts" were related to similar changes in the Tropical ocean.
Related patterns
ENSO tends to lead PDO/IPO cycling.
Shifts in the IPO change the location and strength of ENSO activity. The South Pacific Convergence Zone moves northeast during El Niño and southwest during La Niña events. The same movement takes place during positive IPO and negative IPO phases respectively. (Folland et al., 2002)
Interdecadal temperature variations in China are closely related to those of the NAO and the NPO.
The amplitudes of the NAO and NPO increased in the 1960s and interannual variation patterns changed from 3–4 years to 8–15 years.
The pattern of global warming during the past century changed to cooling during the 1946–1976 PDO "cool" regime.
Sea level rise is affected when large areas of water warm and expand, or cool and contract.
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