The SLP patterns shown in Figure 7.3.1 are deviations from the 1961–90 mean field obtained from
the atmospheric re-analysis of the National Centers
for Environmental Prediction (NCEP; Kalnay et al.,
1996). An advantage of the NCEP products is their
global coverage; however, considerable uncertainties exist as well, as the re-analysed SLP field is only
as good as the input database, and observations are
scarce over vast portions of the tropics and the SH
in particular. Moreover, problems encoding the
observations and other documented technical problems (see the NCEP re-analysis problem list at
http://www.cdc.noaa.gov/cdc/reanalysis/problems.
shtml) cast doubt on the reliability of the results.
A . B. Mullan (1999, personal communication), for
instance, has shown that the NCEP SLP fields do
not agree well with station records throughout
parts of New Zealand and Australia, including
Darwin. If such problems exist in relatively datarich regions, one must suspect the fields in datasparse regions, such as over the tropical and
southern oceans. For this reason, we have chosen
not to present the anomalous SLP field over the
tropics in Figure 7.3.1. Over the NH, the pattern
and magnitude of anomalies in Figure 7.3.1a are
very close to those obtained from other historical
SLP data sets (e.g. the updated data of Trenberth
and Paolino (1980) used by Hurrell and van Loon
(1997) and Thompson et al. (2000), among others), so we have confidence in the NCEP analysis.
Over the SH, less confidence can be placed in the
magnitude of the NCEP anomalies, but the pattern
of change is consistent with the results of studies
using station and gridded SLP records from other
sources (e.g. Hurrell and van Loon, 1994) as well
as with changes and trends in other measures of the
atmospheric circulation (Thompson et al., 2000).
Values of SLP over the elevated terrain of Antarctica are certainly not reliable, however, because of
problems in reducing data to sea level.
The anomalies in the atmospheric circulation
during the WOCE period are strongly related to
variations in modes of natural variability. Over the
NH, these variations have been most pronounced
during the cold season, while those over the SH
have shown less seasonality. Therefore, the spatial
patterns of the most pronounced anomalies in SLP
during the WOCE period are summarized by the
cold season (November–April) map over the NH
(Fig. 7.3.1a) and the annual mean map over the
SH (Fig. 7.3.1b). In addition, to gain insight into
the temporal behaviour of the leading modes of
atmospheric variability and to place the WOCE
period into a longer-term perspective, time series
of several leading atmospheric circulation indices
are shown in the following sections.
7.3.2.2 The North Atlantic
The dominant pattern of atmospheric circulation
variability over the North Atlantic is known as the
North Atlantic Oscillation (NAO), and its wintertime index is shown in Figure 7.3.2a (updated
from Hurrell, 1995a). The NAO, which is linked
to changes in the surface westerlies across the
Atlantic onto Europe, refers to a meridional oscillation in atmospheric mass with centres of action
near Iceland and over the subtropical Atlantic
from the Azores across the Iberian Peninsula. Positive values of the NAO index indicate stronger
than average westerlies over the middle latitudes
with low-pressure anomalies in the Icelandic
region and high-pressure anomalies across the subtropical Atlantic. This phase of the oscillation is
also associated with cold winters over the northwest Atlantic and warm winters over Europe,
as well as wet conditions from Iceland through
Scandinavia and winter dry conditions over southern Europe (Hurrell, 1995a).
A sharp reversal is evident in the index over the
past 30 years and, since 1980, the NAO has
tended to remain in a highly positive phase with
associated negative SLP anomalies over the subpolar North Atlantic and most of the Arctic and
positive SLP anomalies over the subtropical
Atlantic (Fig. 7.3.1a). The most pronounced
anomalies have occurred since the winter of
1988–89 (Hurrell, 1995a; Walsh et al., 1996;
Thompson and Wallace, 1998; Watanabe and
Nitta, 1999) when the index registered its most
positive values in the instrumental record (9170
years). While the signature of the NAO is
strongest during winter, it is the dominant mode
of atmospheric behaviour throughout the year
(Barnston and Livezey, 1987; Rogers, 1990).
Thompson and Wallace (1998, 2000) and
Thompson et al. (2000) have recently drawn attention to the fact that the NAO may be the regional
manifestation of a hemispheric mode of variability
characterized by exchanges of atmospheric mass
between the polar cap and the middle latitudes, as
shown by the leading pattern of variability in the
hemispheric pressure field. They name this mode
7.3 The World during WOCE
559
Dickson et al.
the atmospheric re-analysis of the National Centers
for Environmental Prediction (NCEP; Kalnay et al.,
1996). An advantage of the NCEP products is their
global coverage; however, considerable uncertainties exist as well, as the re-analysed SLP field is only
as good as the input database, and observations are
scarce over vast portions of the tropics and the SH
in particular. Moreover, problems encoding the
observations and other documented technical problems (see the NCEP re-analysis problem list at
http://www.cdc.noaa.gov/cdc/reanalysis/problems.
shtml) cast doubt on the reliability of the results.
A . B. Mullan (1999, personal communication), for
instance, has shown that the NCEP SLP fields do
not agree well with station records throughout
parts of New Zealand and Australia, including
Darwin. If such problems exist in relatively datarich regions, one must suspect the fields in datasparse regions, such as over the tropical and
southern oceans. For this reason, we have chosen
not to present the anomalous SLP field over the
tropics in Figure 7.3.1. Over the NH, the pattern
and magnitude of anomalies in Figure 7.3.1a are
very close to those obtained from other historical
SLP data sets (e.g. the updated data of Trenberth
and Paolino (1980) used by Hurrell and van Loon
(1997) and Thompson et al. (2000), among others), so we have confidence in the NCEP analysis.
Over the SH, less confidence can be placed in the
magnitude of the NCEP anomalies, but the pattern
of change is consistent with the results of studies
using station and gridded SLP records from other
sources (e.g. Hurrell and van Loon, 1994) as well
as with changes and trends in other measures of the
atmospheric circulation (Thompson et al., 2000).
Values of SLP over the elevated terrain of Antarctica are certainly not reliable, however, because of
problems in reducing data to sea level.
The anomalies in the atmospheric circulation
during the WOCE period are strongly related to
variations in modes of natural variability. Over the
NH, these variations have been most pronounced
during the cold season, while those over the SH
have shown less seasonality. Therefore, the spatial
patterns of the most pronounced anomalies in SLP
during the WOCE period are summarized by the
cold season (November–April) map over the NH
(Fig. 7.3.1a) and the annual mean map over the
SH (Fig. 7.3.1b). In addition, to gain insight into
the temporal behaviour of the leading modes of
atmospheric variability and to place the WOCE
period into a longer-term perspective, time series
of several leading atmospheric circulation indices
are shown in the following sections.
7.3.2.2 The North Atlantic
The dominant pattern of atmospheric circulation
variability over the North Atlantic is known as the
North Atlantic Oscillation (NAO), and its wintertime index is shown in Figure 7.3.2a (updated
from Hurrell, 1995a). The NAO, which is linked
to changes in the surface westerlies across the
Atlantic onto Europe, refers to a meridional oscillation in atmospheric mass with centres of action
near Iceland and over the subtropical Atlantic
from the Azores across the Iberian Peninsula. Positive values of the NAO index indicate stronger
than average westerlies over the middle latitudes
with low-pressure anomalies in the Icelandic
region and high-pressure anomalies across the subtropical Atlantic. This phase of the oscillation is
also associated with cold winters over the northwest Atlantic and warm winters over Europe,
as well as wet conditions from Iceland through
Scandinavia and winter dry conditions over southern Europe (Hurrell, 1995a).
A sharp reversal is evident in the index over the
past 30 years and, since 1980, the NAO has
tended to remain in a highly positive phase with
associated negative SLP anomalies over the subpolar North Atlantic and most of the Arctic and
positive SLP anomalies over the subtropical
Atlantic (Fig. 7.3.1a). The most pronounced
anomalies have occurred since the winter of
1988–89 (Hurrell, 1995a; Walsh et al., 1996;
Thompson and Wallace, 1998; Watanabe and
Nitta, 1999) when the index registered its most
positive values in the instrumental record (9170
years). While the signature of the NAO is
strongest during winter, it is the dominant mode
of atmospheric behaviour throughout the year
(Barnston and Livezey, 1987; Rogers, 1990).
Thompson and Wallace (1998, 2000) and
Thompson et al. (2000) have recently drawn attention to the fact that the NAO may be the regional
manifestation of a hemispheric mode of variability
characterized by exchanges of atmospheric mass
between the polar cap and the middle latitudes, as
shown by the leading pattern of variability in the
hemispheric pressure field. They name this mode
7.3 The World during WOCE
559
Dickson et al.