The Pacific Decadal Oscillation or PDO is similar to the El Nino Southern Oscillation in that it is characterized by changes in sea surface temperature, sea level pressure and wind patterns.

The PDO is described as being in one of two phases, a warm phase and a cool phase.

The PDO index value is positive for warm phase conditions and negative for cool phase conditions.

The images below show what the Pacific sea surface temperature anomalies are typically like during the warm and cool phases of the PDO.

The second image shows the latest northern Pacific sea surface temperature anomalies, and you can see that current conditions certainly more reflect the cool phase of the PDO. Sea surface temperatures are well below normal off the BC and Pacific Northwest coast.

According to the University of Washington, the latest PDO index number from September 2011 was -1.79, which is the lowest it has been since January of 2000.

Image courtesy of the University of Washington.

One significant difference between the PDO and ENSO is the fact that the PDO can remain in one phase for 20-30 years, while the ENSO alternates much more frequently.

The PDO was only identified back in 1996, so there is still a clear lack of understanding about how the PDO works and its impacts on climate.

Recently, the PDO was in an extended warm phase from 1977 to at least 1998, then it began to shift toward negative in 1999, but surprisingly it went back to positive in the mid 2000's before going back to negative in 2008.

Was 2008 the start of a long term (20-30 years) cool phase of the PDO? Time will tell.

What are some known impacts of the cool phase of the PDO across Canada and the Pacific Northwest?

1. Enhanced snowpack Pacific Northwest and Rockies. 2. Generally cooler and wetter for the Pacific Northwest. 3. Cooler James Bay in the summer. 4. Greater productivity of Chinook and Coho Salmon in Oregon Rivers. 5. Precipitation and Temperature patterns across BC are more exacerbated (more extremes) during years when the PDO and the ENSO are in the same phase. Right now, the PDO and ENSO (La Nina) are both in cool phases. 6. Often, cool phase PDO's intensify La Nina events, while moderating El Nino events.

How about climate change?

I saw this excerpt from a story in JamesBay.org from 2008 in regards to the PDO....... If lower sea surface temperatures (SST) off the coast of North America, which characterize the cool mode of the PDO, do indeed translate into somewhat cooler, albeit muted, local and regional temperatures (in BC), the longer-term climate change signal becomes masked — more difficult to detect and quantify (Figure 3). Climate scientist and oceanographer Josh Willis of JPL concludes that: “These natural climate phenomena can sometimes hide global warming caused by human activities. Or they can have the opposite effect of accentuating it.” This underscores the importance of collecting consistent and long-term weather records, as only data records that span at least one full cycle of the PDO will reveal the coherent “signal” of climate change that is contained within all natural climate variability.

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