Pacific Decadal Oscillation
The Pacific Decadal Oscillation (PDO) is an El Niño-like pattern of Pacific climate variability, oscillating between its warm and cool phase about every 20–30 years as defined by ocean temperature anomalies in the northeast and tropical Pacific Ocean. When sea surface temperatures are cooler than average in the interior North Pacific and warm along the Pacific Coast, and when sea level pressures are below average over the North Pacific, the PDO has a positive value and is classified as a “warm phase.” The opposite also holds true (Figure 1).
The oscillating pattern correlates with relatively wetter or drier periods in the western portion of North America. Shifts in the PDO regime occurred in 1925, 1947, and 1977 (Figure 2). Some climatologists believe that another shift began around 1995, although more observations are needed to discern the phase change.
Analysis of the instrumental record suggests the decadal-scale pattern of the PDO is equally influenced by three climatic factors:
- fluctuations in El Niño and La Niña events that have a four to seven year cycle
- changes in the atmospheric low-pressure pattern known as the Aleutian Low
- changes in the Kuroshio-Oyashio current that swirls through the northern Pacific Ocean
|PDO Phase||North Pacific Sea Surface Pressure||North Pacific Sea Surface Temperatu||Influence on El Niño Conditions||Influence on La Niña Conditions|
Researchers from the Climate Impacts Group at the University of Washington suggest that the PDO phases can combine with El Niño and La Niña conditions in certain ways to affect precipitation in the West, particularly in winter. The positive phase of the PDO, with warm sea surface temperatures in the equatorial Pacific, can enhance an El Niño episode, amplifying the effects this warm pool of water has on the western United States. This tends to lead to wetter-than-average winters during El Niño events and relatively average winters during La Niña events. This same phase of the PDO would weaken any La Niña episodes that would occur during this time period. Similarly, during a negative phase of the PDO, La Niña events would be enhanced, and El Niño events would be weakened.
Decreases in winter precipitation associated with negative PDO conditions and intensified La Niña conditions could, for example, put stress on the urban water systems of the Southwest. Even moderately drier-than-normal conditions could have serious effects in some sectors. For example, CLIMAS researchers have documented that ranching operations in the Southwest are highly sensitive to climatic conditions. Forest fire management is another area crucially influenced by climatic conditions and trends.