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Climate Science: Investigating Climatic and Environmental Processes
Decadal Processes (<102 years)
Forcing Factors

Image of PDO phases Scientists examining sea surface temperatures (SST) for patterns in the heating and cooling of the ocean systems have identified variability that influence climate and fisheries. One, centered in the north Pacific and most commonly called the Pacific Decadal Oscillation or PDO, seems to have return periods of 15 to 25 years, and of 50 to 70 years. The other, the North Atlantic Oscillation (NAO), has a dominant period of 12 years (Deser, 1993 ), and as its name implies, it is centered in the North Atlantic.

Similar to and interacting with interannual phenomenas such as ENSO, these decadal-scale processes appear to play a major role in regional and global climate dynamics.

Pacific Decadal Oscillation- PDO
The term PDO was coined by fisheries scientist Steven Hare in 1996 at the University of Washington where he was studying sea surface temperature (SST) data of the northern Pacific. He observed that during certain periods (now called positive phase), a large pool of colder than average sea surface water in the central north Pacific appeared along a narrow band of warmer sea surface temperatures along the west coast of North America. During the negative phase of PDO, the opposite was observed: a warm pool of sea surface waters in the central north Pacific and cold SSTs along the west coast. The most recent warm phase began in 1977 and may have finished in 1999. In theory, ENSO overlays the PDO's longer pulse.

Not all scientists are convinced PDO is actually an oscillating feature since research on the phenomena is less than a decade old. It has also been suggested by some researchers that PDO may be part of other features such as ENSO, or define the structure of the dynamic differently. (Gershunov, 1999) Meanwhile research continues to track such climate changes in the Pacific. See More on the PDO.

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Internal ocean variability can be tracked through paleoclimatic data which provide evidence for dramatic climatic events not seen in the limited instrumental records. Surface conditions like soil moisture, vegetion and albedo, can also play a role at this time scale.

How Measured
Instrumental records are limited in their ability to examine decadal to centennial scale climate variability. Paleo proxies, including tree rings, ice cores and corals, are now being used in an effort to determine the climate dynamics and forces involved.

North Atlantic Oscillation- NAO
The North Atlantic Oscillation or NAO has been studied by the Lamont-Doherty Earth Observatory and other researchers for a number of years. Researchers have identified a dynamic process that has particularly important impact on European climate.

NAO varies from year to year (interannual variability) but has a roughly decadal pattern with a dominant period of 12 years (Deser, 1993 ).

During its positive phase, NAO shows a stronger than usual subtropical high pressure center around the Azores and a deeper than normal Icelandic low, with increased pressure generating more and stronger winter storms crossing the Atlantic Ocean on a more northerly track. Europe tends towards warm and wet winters while northern Canada and Greenland will usually have winters are cold and dry, with the eastern United States generally experiencing mild, wet winter conditions.

During its negative index phase, NAO is characterized by weak subtropical highs and weak Icelandic lows, with fewer and weaker winter storms that cross on a more west-east pathway, bringing cold air and snowy weather conditions to the U.S. east coast during the winter months, cold air in northern Europe, and moist air to the Mediterranean.

Image of NAO Index

Image of phases of NAO
Images from NOAA, Lamont-Doherty Earth Observatory

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