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Pacific Decadal Oscillation (PDO)

The Pacific Decadal Oscillation is a long-term variability in temperature over the Pacific Ocean that is located above 20 degrees north latitude. The warm/positive phase fluctuates with the cool/negative phase over a period of 20-30 years. Like the Interdecadal Pacific Oscillation (IPO), the PDO is not particularly well understood due to the long timeframes of variability involved, though reconstructions as far back as the mid-1600s have been completed using tree ring data. Currently, the PDO is thought to be related to El Nino-Southern Oscillation cycling as well as to something called stochastic atmospheric forcing.

Stochastic atmospheric forcing (SF for short) is a term that needs to be broken down to be understood. Atmospheric forcing refers to the role of gases in the atmosphere in regulating the energy received from the sun. So, positive forcing refers to the trapping of energy by the atmosphere while negative forcing means the energy is being released into space. This is important because changes in greenhouse gas concentrations could affect the atmospheric forcing of the PDO and thus change weather patterns in the North Pacific Ocean.

In the natural sciences, a stochastic process is one that is not deterministic, which simply means the current state of the system cannot tell you anything about future states. The only way to approximate what a future state will be is through statistical modeling. The stochastic nature of the atmosphere results from the passage of storms, which alter ocean temperatures and atmospheric conditions that control forcing.

Thus, stochastic atmospheric forcing refers to energy regulation by the atmosphere that is unpredictable. From that definition, it should be clear why the PDO is not well understood. The process, as we currently understand it, appears to be random, with 20-30 year fluctuations between positive and negative phases. Current understanding of the PDO allows for an approximate prediction that varies by about 4 seasons.

Impact of the Pacific Decadal Oscillation

Precipitation along the west coast of North America is the main impact of the Pacific Decadal Oscillation. During the positive/warm phase, precipitation in the southwestern United States will be above normal with cooler than average temperatures. In the northern part of the continental U.S. and into Canada and Alaska, temperatures are above normal with dry conditions extending across all of North America to the east coast. During the cool/negative phase, precipitation will be below normal in the southeastern United States and above normal north of that. Temperature variations are opposite to those outlined above.

Patterns of temperature and precipitation associated with PDO events are actually similar to El Nino-Southern Oscillation events. When both are in a warm phase, extreme precipitation occurs in the southeastern United States and when both are in a cool phase extreme drought occurs. When the two are out of phase, they tend to negate one another and lead to milder conditions.

Summary of PDO Phase Conditions

Location

Warm/Positive

Cool/Negative

Tropical Pacific

·         Above avg. temperatures

·         Below avg. temperatures

Northwestern North America

·         Above avg. temperatures

·         Below avg. precipitation

·         Below avg. snow pack

·         Below avg. temperatures

·         Above avg. precipitation

·         Above avg. snow pack

Southwestern North America

·         Below avg. temperatures

·         Above avg. precipitation

·         Above avg. temperatures

·         Below avg. precipitation

Northeastern North America

·         Above avg. temperatures

·         Below avg. precipitation

·         Below avg. temperatures

·         Above avg. precipitation

Southeastern North America

·         Below avg. temperatures

·         Above avg. precipitation

·         Above avg. temperatures

·         Below avg. precipitation

Modified from: Mantua N. The Pacific Decadal Oscillation and Climate Forecasting for North America. http://www.atmos.washington.edu/~mantua/REPORTS/PDO/PDO_cs.htm

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