2.6.5 The Northern Hemisphere excluding the North Pacific
Ocean
The atmospheric circulation over the Northern Hemisphere has exhibited anomalous
behaviour over the past several decades. In particular, the dominant patterns
of atmospheric variability in the winter half-year have tended to be strongly
biased to one phase. Thus SLP has been lower than average over the mid- and
high latitudes of the North Atlantic Ocean, as well as over much of the Arctic,
while it has been higher than average over the sub-tropical oceans, especially
the Atlantic. Moreover, in the past thirty years, changes in these leading patterns
of natural atmospheric variability appear to be unusual in the context of the
observational record.

Figure 2.30: December to March North Atlantic Oscillation (NAO) indices,
1864 to 2000, and Arctic Oscillation (AO) indices, 1900 to 2000, updated
from Hurrell (1995) and updated from Thompson and Wallace (2000) and Thompson
et al. (2000b), respectively. The indices were normalised using the means
and standard deviations from their common period, 1900 to 2000, smoothed
twice using a 21-point binomial filter where indicated and then plotted
according to the years of their Januarys. |
The dominant pattern of atmospheric circulation variability over the North
Atlantic is known as the NAO, and its wintertime index is shown in Figure
2.30 (updated from Hurrell, 1995). As discussed in the SAR, positive values
of the NAO give stronger than average westerlies over the mid-latitudes of the
Atlantic with low SLP anomalies in the Icelandic region and over much of the
Arctic and high SLP anomalies across the sub-tropical Atlantic and into southern
Europe. The positive, enhanced westerly, phase of the NAO is associated with
cold winters over the north-west Atlantic and warm winters over Europe, Siberia
and eastern Asia (Thompson and Wallace, 2001) as well as wet conditions from
Iceland to Scandinavia and dry winters over southern Europe. A sharp reversal
is evident in the NAO index starting around 1970 from a negative towards a positive
phase. Since about 1985, the NAO has tended to remain in a strong positive phase,
though with substantial interannual variability. Hurrell (1996) and Thompson
et al. (2000a) showed that the recent upward trend in the NAO accounts for much
of the regional surface winter half-year warming over northern Europe and Asia
north of about 40°N over the past thirty years, as well as the cooling over
the north-west Atlantic (see Section 2.2.2.3). Moreover,
when circulation changes over the North Pacific are also considered, much of
the pattern of the Northern Hemisphere winter half-year surface temperature
changes since the mid-1970s can be explained. This can be associated with changes
in the NAO, and in the PNA atmospheric pattern related to ENSO or the PDO (Graf
et al., 1995; Wallace et al., 1995; Shabbar et al., 1997; Thompson and Wallace,
1998, 2000).
The changes in atmospheric circulation over the Atlantic are also connected
with much of the observed pressure fall over the Arctic in recent years (Walsh
et al., 1996). Other features related to the circulation changes include the
strengthening of sub-polar westerlies from the surface of the North Atlantic
up, in winter as high as the lower stratosphere (Thompson et al., 2000a) and
pronounced regional changes in precipitation patterns (Hurrell, 1995; Dai et
al., 1997b; Hurrell and van Loon 1997; Section 2.5.2.1).
Associated precipitation increases have resulted in the notable advance of some
Scandinavian glaciers (Hagen et al., 1995), while decreases to the south of
about 50oN have contributed to the further retreat of Alpine glaciers (Frank,
1997; see also Section 2.2.5.3)
The NAO is regarded (largely) by some as the regional expression of a zonally
symmetrical hemispheric mode of variability characterised by a seesaw of atmospheric
mass between the polar cap and the mid-latitudes in both the Atlantic and Pacific
Ocean basins (Thompson and Wallace, 1998, 2001). This mode has been named the
AO (Figure 2.30). The time-series of the NAO and
AO are quite similar: the correlation of monthly anomalies of station data SLP
series of NAO and AO is about 0.7 (depending on their exact definitions and
epochs) while seasonal variations shown in Figure 2.30
have even higher correlations. The NAO and AO can be viewed as manifestations
of the same basic phenomenon (Wallace, 2000).
Changes and decadal fluctuations in sea-ice cover in the Labrador and Greenland
Seas, as well as over the Arctic, appear well correlated with the NAO (Chapman
and Walsh, 1993; Maslanik et al., 1996; McPhee et al., 1998; Mysak and Venegas,
1998; Parkinson et al., 1999; Deser et al., 2000). The relationship between
the SLP and ice anomaly fields is consistent with the idea that atmospheric
circulation anomalies force the sea-ice variations (Prisenberg et al., 1997).
Feedbacks or other influences of winter ice anomalies on the atmosphere have
been more difficult to detect, although Deser et al. (2000) suggest that a local
response of the atmospheric circulation to the reduced sea-ice cover east of
Greenland in recent years is also apparent (see also Section
2.2.5.2).
A number of studies have placed the recent positive values of the NAO into
a longer-term perspective (Jones et al., 1997a; Appenzeller et al., 1998; Cook
et al., 1998; Luterbacher et al., 1999; Osborn et al., 1999) back to the 1700s.
The recent strength of the positive phase of the NAO seems unusual from these
reconstructions but, as in Figure 2.28, these proxy
data reconstructions may underestimate variability. An extended positive phase
occurred in the early 20th century (Figure 2.30),
particularly pronounced in January (Parker and Folland, 1988), comparable in
length to the recent positive phase. Higher-frequency variability of the NAO
also appears to have varied. Hurrell and van Loon (1997) showed that quasi-decadal
(6 to 10 year) variability has become more pronounced over the latter half of
the 20th century, while quasi-biennial variability dominated in the early instrumental
record.
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