Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 30

Figure 2.1 Values of the
NAO from the late 1800s to
the late 1900s. The coldest
European winters on record
(in the 1940s and the 1960s)
coincide with negative
phases of the NAO.

1860 1880 1900 1920 1940 1960 1980 2000

of particularly mild winters across Europe. As noted in Section 2.7 , the NAO and
the Arctic Oscillation (AO) are intimately linked.
An in-depth analysis of the NAO is presented in a special publication of the
American Geophysical Union (Hurrell et al. 2003 ). This work deals not only
with the dynamic climatology of the NAO but also provides an account of its
ecological and economic consequences.

2.3 The North Pacific Oscillation (NPO)/Pacific
Decadal Oscillation (PDO)
Zhang et al.( 1997 ) describe the PDO as a long-lived El Ni˜ o-like pattern
of Pacific climate variability. The term was coined by Mantua et al.( 1997 )
in a study investigating the role of sea surface temperature on salmon
behavior in the North Pacific Ocean. The index used to assess the PDO is derived
from monthly sea surface temperature (SST) anomalies north of 208 N in the
Pacific Ocean. Other studies have provided other names including the Pacific
Decadal Variation (PDV) and the Interdecadal Pacific Oscillation (IPO).
The PDO is actually represented by a shift of SST that occurs on a 20 to 30
year cycle. It is in its warm or positive phase when the northwest SST anomalies
are negative, while the SST anomalies in the eastern tropical Pacific Ocean are
positive. The cool or negative phase is the reverse of this, with positive anoma-
lies in the NW and negative in the tropical zone. In effect, it represents a change
in the location of cool and warm water masses that impart their influence upon
the atmosphere in a variety of ways. This, in turn has impacts upon the western
parts of the United States.
Figure 2.2 shows the occurrence of positive and negative phases of the PDO.
A number of studies (Mantua et al. 1997 ; Minobe 1997 ) suggest that there have
been just two full PDO cycles in the last 100 years. A cool PDO cycle occurred
from 1890 to 1924 and from 1947 to 1976, while the warm cycle was in effect
from 1925 to 1946 and from 1977 to the late 1990s. It is thought that a possible
change to the cool PDO phase began at that time.
The impact upon climate at the time of the various phases of PDO has been the
topic of a number of studies. Of particular interest are those studies relating the
PDO to the climate of the American Southwest. In a series of locally distributed
articles (also available at www.srh.weather.gov/abq/feature/PDO_NM.htm),