Climatic and Environmental Processes Decadal
Forcing Factors NAO
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
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.
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.
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.
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.
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,
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.
Images from NOAA, Lamont-Doherty