The Atmospheric Circulation - The Arctic Climate System - page 132

Figure 4.23. Monthly values of the Pacific Decadal Oscillation (PDO) index for the
1900-2011 period, the bold line shows the index values smoothed with a 121-month
running average (by permission of Creative Commons Attribution-Share Alike 3.0
Unreported license (CC BY-SA 3.0), data from http://jisao.washington.edu/pdo/PDO.
latest . (See plate section for color version.)
The most important of these ENSO-like patterns is the Pacific Decadal Oscillation
(PDO). The term was coined by Steven Hare in 1996 while researching connections
between Alaska salmon production cycles and Pacific climate (Mantua et al., 1997 ).
The index of the PDO is defined as the leading principal component of North Pacific
monthly SST variability poleward of 20 o N. When SSTs are anomalously high along
the north Pacific Coast but anomalously low in the interior North Pacific, the PDO
is said to be in its warm (positive) phase. In the warm PDO phase, SLPs are below
average over the northern North Pacific - that is, the Aleutian Low is unusually
deep. In the cold (negative) PDO phase, the anomaly patterns are broadly reversed,
with positive SST anomalies in the interior North Pacific, negative SST anomalies
along the North American coast, and a weaker Aleutian Low.
During the twentieth century, each PDO phase has typically lasted for twenty
to thirty years. Cold PDO regimes prevailed from 1890-1924 and again from
1947-1976, while warm PDO regimes dominated from 1925-1946 and from 1977
through the mid-1990s ( Figure 4.23 ). The PDO may now be entering a new neg-
ative phase. Much attention has been paid to the Pacific climate regime shift that
transpired during the winter of 1976-1977, linked to the shift in the PDO from it
cold to its warm phase, which had substantial impacts on marine ecosystems of
the North Pacific. This appears to have been but one in a series of climate shifts
linked to the PDO (Hare and Mantua, 2000 ). Air temperature anomalies over Alaska
are known to be strongly tied to the phase of the PDO (Hartmann and Wendler,
2003 ). Compared to the negative PDO phase (1951-1975), mean annual and sea-
sonal air temperatures during the positive phase period (1977-2001) examined in