while quasibiennial variability dominated in the
early record. Cook et al. (1998) used tree ring data
from Europe and the eastern USA to reconstruct
the NAO index back to 1701 and found similar
concentrations of spectral power, but they also
identified a multidecadal oscillation with a period
near 70 years. Osborn et al. (1999), however,
question the reliability of the Cook et al. (1998)
reconstruction based on the small percentage of
variance it captures in the 1824–73 portion of the
Jones et al. (1997) index, although problems with
the early instrumental data cannot be ruled out.
Most recently, Stockton and Glueck (1999) used
tree ring data from Morocco and Finland, in addition to Greenland ice core data, to reconstruct a
NAO index 555 years in length. They find that
while there could have been several periods of consistently high or low index values such as those
evident in Figure 7.3.2a, the reconstructed values
during such periods are not as great in magnitude
as the most recent values, such as during the 1960s
and since the 1980s, in the instrumental records.
The fact that the highest observed values of the
index (post 1989) occur after the Stockton–Glueck
chronology ends suggests that these recent high
values may indeed be unprecedented.
7.3.2.3 The North Pacific
Over the North Pacific, a simple and robust index
to measure climate variability is the area-weighted
mean SLP averaged over most of the extratropical
North Pacific ocean (Fig. 7.3.2b, updated from
Trenberth and Hurrell, 1994). This North Pacific
(NP) index reflects changes in the intensity of the
Aleutian low pressure centre, and it is closely related
to a well-known pattern of natural atmospheric
circulation variability termed the Pacific–North
American (PNA) pattern (Wallace and Gutzler,
1981). A feature of the time series is a striking
regime shift after about 1976, which has been particularly evident during the winter half of the year
(November to March). The regime shift is characterized by a deeper than normal Aleutian low-pressure system, accompanied by stronger than normal
westerly winds across the central North Pacific
and enhanced southerly flow along the west coast of
North America (e.g. Trenberth and Hurrell, 1994).
Consequently, over much of the past two decades,
there have been increases in surface air temperatures and Sea Surface Temperatures (SSTs) over
much of western North America and the eastern
North Pacific but decreases in SSTs over the central North Pacific. While positive values of the NP
index were recorded for three consecutive winters
beginning in 1988–89, as well as during the winter
season of 1993–94, the index has remained negative in recent years, including the WOCE period,
continuing the tendency apparent since the mid1970s. Moreover, while the largest pressure falls
over the North Pacific were evident during winter
from the mid-1970s through the 1980s, the largest
SLP anomalies during the 1990s have occurred during spring. From March through May, SLP anomalies in excess of 93 hPa reflect an eastward shifted
and intensified Aleutian low (not shown), and it is
this springtime anomaly that contributes the most
to the cold season differences in Figure 7.3.1a.
Numerous recent studies have suggested that
the mid-1970s change in the atmospheric circulation is but a single realization of lower-frequency
interdecadal variations over the North Pacific.
Most of these studies have been based on analyses
of historical SST records, which show subtly different patterns of variability for interannual versus
decadal and longer time scales (Kawamura, 1994;
Deser and Blackmon, 1995; Zhang et al., 1997;
Enfield and Mestas-Nuñez, 1999). The interannual
variability over the Pacific is dominated by the El
Niño-Southern Oscillation (ENSO) phenomenon,
with the strongest SST signals of one sign along
the equator over the central and eastern Pacific
and a boomerang-shaped pattern of weaker SST
signals of opposite sign over the middle latitudes of
both hemispheres. The higher-latitude changes in
the Pacific arise from teleconnections that form an
atmospheric bridge to force the anomalous surface
fluxes that drive the SST anomalies (e.g. Lau and
Nath, 1996; Trenberth et al., 1998). The lowerfrequency SST variability, on the other hand, is
less equatorially confined in the central and eastern Pacific, and it is relatively more prominent
over the extratropical North Pacific. The corresponding SLP signature is also stronger over the
North Pacific, and its wintertime counterpart in
the middle troposphere more closely resembles the
PNA pattern (see also Zhang et al., 1996).
Interdecadal fluctuations in the dominant pattern of North Pacific SLP variability, which is well
represented by the NP index in Figure 7.3.2b, have
closely paralleled those in the leading North
Pacific SST pattern, as represented by the index of
the so-called Pacific Decadal Oscillation (PDO) of
7.3 The World during WOCE
561
Dickson et al.
early record. Cook et al. (1998) used tree ring data
from Europe and the eastern USA to reconstruct
the NAO index back to 1701 and found similar
concentrations of spectral power, but they also
identified a multidecadal oscillation with a period
near 70 years. Osborn et al. (1999), however,
question the reliability of the Cook et al. (1998)
reconstruction based on the small percentage of
variance it captures in the 1824–73 portion of the
Jones et al. (1997) index, although problems with
the early instrumental data cannot be ruled out.
Most recently, Stockton and Glueck (1999) used
tree ring data from Morocco and Finland, in addition to Greenland ice core data, to reconstruct a
NAO index 555 years in length. They find that
while there could have been several periods of consistently high or low index values such as those
evident in Figure 7.3.2a, the reconstructed values
during such periods are not as great in magnitude
as the most recent values, such as during the 1960s
and since the 1980s, in the instrumental records.
The fact that the highest observed values of the
index (post 1989) occur after the Stockton–Glueck
chronology ends suggests that these recent high
values may indeed be unprecedented.
7.3.2.3 The North Pacific
Over the North Pacific, a simple and robust index
to measure climate variability is the area-weighted
mean SLP averaged over most of the extratropical
North Pacific ocean (Fig. 7.3.2b, updated from
Trenberth and Hurrell, 1994). This North Pacific
(NP) index reflects changes in the intensity of the
Aleutian low pressure centre, and it is closely related
to a well-known pattern of natural atmospheric
circulation variability termed the Pacific–North
American (PNA) pattern (Wallace and Gutzler,
1981). A feature of the time series is a striking
regime shift after about 1976, which has been particularly evident during the winter half of the year
(November to March). The regime shift is characterized by a deeper than normal Aleutian low-pressure system, accompanied by stronger than normal
westerly winds across the central North Pacific
and enhanced southerly flow along the west coast of
North America (e.g. Trenberth and Hurrell, 1994).
Consequently, over much of the past two decades,
there have been increases in surface air temperatures and Sea Surface Temperatures (SSTs) over
much of western North America and the eastern
North Pacific but decreases in SSTs over the central North Pacific. While positive values of the NP
index were recorded for three consecutive winters
beginning in 1988–89, as well as during the winter
season of 1993–94, the index has remained negative in recent years, including the WOCE period,
continuing the tendency apparent since the mid1970s. Moreover, while the largest pressure falls
over the North Pacific were evident during winter
from the mid-1970s through the 1980s, the largest
SLP anomalies during the 1990s have occurred during spring. From March through May, SLP anomalies in excess of 93 hPa reflect an eastward shifted
and intensified Aleutian low (not shown), and it is
this springtime anomaly that contributes the most
to the cold season differences in Figure 7.3.1a.
Numerous recent studies have suggested that
the mid-1970s change in the atmospheric circulation is but a single realization of lower-frequency
interdecadal variations over the North Pacific.
Most of these studies have been based on analyses
of historical SST records, which show subtly different patterns of variability for interannual versus
decadal and longer time scales (Kawamura, 1994;
Deser and Blackmon, 1995; Zhang et al., 1997;
Enfield and Mestas-Nuñez, 1999). The interannual
variability over the Pacific is dominated by the El
Niño-Southern Oscillation (ENSO) phenomenon,
with the strongest SST signals of one sign along
the equator over the central and eastern Pacific
and a boomerang-shaped pattern of weaker SST
signals of opposite sign over the middle latitudes of
both hemispheres. The higher-latitude changes in
the Pacific arise from teleconnections that form an
atmospheric bridge to force the anomalous surface
fluxes that drive the SST anomalies (e.g. Lau and
Nath, 1996; Trenberth et al., 1998). The lowerfrequency SST variability, on the other hand, is
less equatorially confined in the central and eastern Pacific, and it is relatively more prominent
over the extratropical North Pacific. The corresponding SLP signature is also stronger over the
North Pacific, and its wintertime counterpart in
the middle troposphere more closely resembles the
PNA pattern (see also Zhang et al., 1996).
Interdecadal fluctuations in the dominant pattern of North Pacific SLP variability, which is well
represented by the NP index in Figure 7.3.2b, have
closely paralleled those in the leading North
Pacific SST pattern, as represented by the index of
the so-called Pacific Decadal Oscillation (PDO) of
7.3 The World during WOCE
561
Dickson et al.