Mantua et al. (1997). The relationship is such that
cooler than average SSTs occur during periods of
lower than average SLP over the central North
Pacific and vice versa. The PDO index (not shown)
hints even more strongly that the mid-1970s
regime has continued through the 1990s. In addition, there is evidence of shifts in the polarity of
the PDO in the mid-1920s and the mid-1940s.
Minobe (1997) examined a number of different
climate indices around the Pacific, including
more than 300 years of tree ring data from North
America, and concluded that the interdecadal variability has a dominant time scale of 50–70 years,
roughly consistent with the implied periods in the
short record of Figure 7.3.2b. Recent evidence suggests that the phase of the PDO may also play a
key role in modulating ENSO teleconnections
across North America on interdecadal time scales
(Gershunov and Barnett, 1998), though direct
effects on the Atlantic Ocean have proved elusive.
7.3.2.4 The tropical Pacific
The decadal changes over the North Pacific have
been linked to variations in the tropics (Nitta and
Yamada, 1989; Trenberth, 1990; Trenberth and
Hurrell, 1994; Zhang et al., 1997), and several
modelling studies have confirmed that North
Pacific atmospheric variability is controlled in part
by anomalous tropical Pacific SST forcing (e.g.
Kitoh, 1991; Chen et al., 1992; Graham et al.,
1994; Kumar et al., 1994; Lau and Nath, 1994;
Miller et al., 1994). Fluctuations in tropical Pacific
SSTs are related to the occurrence of El Niño. The
atmospheric phenomenon tied to El Niño is
termed the Southern Oscillation (SO), which is a
global-scale ‘see-saw’ or standing wave in atmospheric mass (thus evident in SLP), involving
exchanges of air between the eastern and western
hemispheres centred in tropical and subtropical
latitudes. The oscillation is characterized by the
inverse variations in SLP at Darwin (12.4°S,
130.9°E) in northern Australia and Tahiti (17.5°S,
149.6°W) in the south Pacific: the annual mean
pressures at these two stations are anticorrelated
at ϳ90.8. A simple index of the SO is, therefore,
often defined by the normalized Tahiti minus
Darwin SLP anomalies, or simply by the normalized
negative Darwin record (Fig. 7.3.2c; updated from
Trenberth and Hoar, 1996). During an El Niño
event, the SLP tends to be higher than usual at
Darwin and lower than usual at Tahiti. Negative
values of the SO index, therefore, are typically
associated with warmer than average SSTs in the
central and eastern tropical Pacific and weaker
than average easterly trade winds.
Although the SO has a period of 2–7 years, the
strength of the oscillation has varied over the past
100 years: strong variations from 1880 to the
1920s and after about 1950 and, except for the
strong event during 1939–42, weaker variations
from the mid-1920s to 1950 (Fig. 7.3.2c). Like the
recent behaviour of the NAO, a remarkable feature of the SO index, however, is the amplitude of
decadal and longer-term variability in recent years,
which is lacking from earlier periods. In particular,
a pronounced change toward more negative values
is evident since the mid-1970s, including prolonged warm conditions from 1990–95, the beginning of the WOCE period. One interpretation of
this change is given by Trenberth and Hoar (1996,
1997). They have statistically analysed the SO
index shown in Figure 7.3.2c to assess the likelihood that its behaviour in the past 20 years or so
is part of a natural decadal time scale variation.
Their conclusion was that both the recent trend
for more warm ENSO events since 1976 and the
prolonged warm conditions since 1990 are unexpected given the previous record, with a probability of occurrence about once in 2000 years.
7.3.2.5 The southern hemisphere
The leading pattern of variability over the SH
exists year round and is characterized by equivalent barotropic fluctuations in the zonal mean circulation between middle and high latitudes with a
node near 45°S (e.g. Rogers and van Loon, 1982;
Szeredi and Karoly, 1987; Yoden et al., 1987;
Kidson, 1988; Karoly, 1990; Hartmann and Lo,
1998; Thompson and Wallace, 2000). An annual
index of this mode, based on 850 hPa geopotential
height data from the NCEP re-analyses, is shown
in Figure 7.3.2d (D.W.J. Thompson, 1999, personal communication). The mode explains 27% of
month-to-month variations in lower tropospheric
heights poleward of 20°S over the SH (Thompson
et al., 2000), and its time series shows a secular
trend over the past few decades toward the highindex polarity, consistent with the lower than average SLPs during the 1990s at high latitudes (Fig.
7.3.1b) and stronger westerlies in middle latitudes.
These findings support earlier studies of recent
decadal changes in the atmospheric circulation
SECTION 7 INSIGHTS FOR THE FUTURE
562
cooler than average SSTs occur during periods of
lower than average SLP over the central North
Pacific and vice versa. The PDO index (not shown)
hints even more strongly that the mid-1970s
regime has continued through the 1990s. In addition, there is evidence of shifts in the polarity of
the PDO in the mid-1920s and the mid-1940s.
Minobe (1997) examined a number of different
climate indices around the Pacific, including
more than 300 years of tree ring data from North
America, and concluded that the interdecadal variability has a dominant time scale of 50–70 years,
roughly consistent with the implied periods in the
short record of Figure 7.3.2b. Recent evidence suggests that the phase of the PDO may also play a
key role in modulating ENSO teleconnections
across North America on interdecadal time scales
(Gershunov and Barnett, 1998), though direct
effects on the Atlantic Ocean have proved elusive.
7.3.2.4 The tropical Pacific
The decadal changes over the North Pacific have
been linked to variations in the tropics (Nitta and
Yamada, 1989; Trenberth, 1990; Trenberth and
Hurrell, 1994; Zhang et al., 1997), and several
modelling studies have confirmed that North
Pacific atmospheric variability is controlled in part
by anomalous tropical Pacific SST forcing (e.g.
Kitoh, 1991; Chen et al., 1992; Graham et al.,
1994; Kumar et al., 1994; Lau and Nath, 1994;
Miller et al., 1994). Fluctuations in tropical Pacific
SSTs are related to the occurrence of El Niño. The
atmospheric phenomenon tied to El Niño is
termed the Southern Oscillation (SO), which is a
global-scale ‘see-saw’ or standing wave in atmospheric mass (thus evident in SLP), involving
exchanges of air between the eastern and western
hemispheres centred in tropical and subtropical
latitudes. The oscillation is characterized by the
inverse variations in SLP at Darwin (12.4°S,
130.9°E) in northern Australia and Tahiti (17.5°S,
149.6°W) in the south Pacific: the annual mean
pressures at these two stations are anticorrelated
at ϳ90.8. A simple index of the SO is, therefore,
often defined by the normalized Tahiti minus
Darwin SLP anomalies, or simply by the normalized
negative Darwin record (Fig. 7.3.2c; updated from
Trenberth and Hoar, 1996). During an El Niño
event, the SLP tends to be higher than usual at
Darwin and lower than usual at Tahiti. Negative
values of the SO index, therefore, are typically
associated with warmer than average SSTs in the
central and eastern tropical Pacific and weaker
than average easterly trade winds.
Although the SO has a period of 2–7 years, the
strength of the oscillation has varied over the past
100 years: strong variations from 1880 to the
1920s and after about 1950 and, except for the
strong event during 1939–42, weaker variations
from the mid-1920s to 1950 (Fig. 7.3.2c). Like the
recent behaviour of the NAO, a remarkable feature of the SO index, however, is the amplitude of
decadal and longer-term variability in recent years,
which is lacking from earlier periods. In particular,
a pronounced change toward more negative values
is evident since the mid-1970s, including prolonged warm conditions from 1990–95, the beginning of the WOCE period. One interpretation of
this change is given by Trenberth and Hoar (1996,
1997). They have statistically analysed the SO
index shown in Figure 7.3.2c to assess the likelihood that its behaviour in the past 20 years or so
is part of a natural decadal time scale variation.
Their conclusion was that both the recent trend
for more warm ENSO events since 1976 and the
prolonged warm conditions since 1990 are unexpected given the previous record, with a probability of occurrence about once in 2000 years.
7.3.2.5 The southern hemisphere
The leading pattern of variability over the SH
exists year round and is characterized by equivalent barotropic fluctuations in the zonal mean circulation between middle and high latitudes with a
node near 45°S (e.g. Rogers and van Loon, 1982;
Szeredi and Karoly, 1987; Yoden et al., 1987;
Kidson, 1988; Karoly, 1990; Hartmann and Lo,
1998; Thompson and Wallace, 2000). An annual
index of this mode, based on 850 hPa geopotential
height data from the NCEP re-analyses, is shown
in Figure 7.3.2d (D.W.J. Thompson, 1999, personal communication). The mode explains 27% of
month-to-month variations in lower tropospheric
heights poleward of 20°S over the SH (Thompson
et al., 2000), and its time series shows a secular
trend over the past few decades toward the highindex polarity, consistent with the lower than average SLPs during the 1990s at high latitudes (Fig.
7.3.1b) and stronger westerlies in middle latitudes.
These findings support earlier studies of recent
decadal changes in the atmospheric circulation
SECTION 7 INSIGHTS FOR THE FUTURE
562