comprises a large fraction of the 1000–2000-dbar
layer in the South Atlantic and a lesser fraction of
this layer in the southern North Atlantic. For this
reason, the large warmings (and compensating
increases in salinity) along isopycnals in the South
Atlantic, and the lesser warmings along isopycnals
in the southern North Atlantic, are suggestive of
changes in AAIW, especially since the warmings in
the southern North Atlantic are more pronounced
in the western part (Fig. 7.3.4b, see Plate 7.3.4b,
p. 588), where the presence of AAIW is strongest.
Likewise, the coolings along isopycnals in the
northern North Atlantic are suggestive of changes
in LSW (Fig. 7.3.4b, see Plate 7.3.4b, p. 588).
Our study has broadened the geographical extent
of the warmings at intermediate depths reported by
previous authors (Roemmich and Wunsch, 1984;
Parrilla et al., 1994; Bryden et al., 1996; Joyce and
Robbins, 1996; Arhan et al., 1998; Levitus, 1989;
Joyce et al., 1999) by showing that these warmings
extend into the South Atlantic. We find that the
isopycnal-displacement mechanism, which is of
widespread significance in the North Atlantic
(Levitus, 1989), is not the only one of importance.
Changes on isopycnals also form a large and widespread component of the changes observed on isobars. However, unlike the displacement mechanism,
changes along isopycnals respond very slowly to
changes in forcing, so that by serving as integrators over long time scales, they might be the more
sensitive indicator of long-term climate change
(suggestion of anonymous reviewer). In this light,
the isopycnal warmings we observe, extending
back into the 1920s, may well indicate that recent
changes in the climate of the ocean are a response
to climatic forcing over longer periods than we
have previously realized.
7.3.5 Spin-up of the North Atlantic gyre
circulation
From the subtropics northwards over the northern
North Atlantic and Nordic Seas, the status of the
marine environment during WOCE is overwhelmingly determined by the extreme long-term behaviour of the NAO.
As described earlier (Section 7.3.2.2), the WOCE
field phase occurred at a most unusual interval in
the climatic history of the North Atlantic, at the
end of a period in which the principal atmospheric
forcing mode in that sector had amplified over
three or four decades to its most extreme positive
state (generally) in a 176-year instrumental record
and possibly much longer. In the present context,
the importance of this extreme climatic shift lies in
the wide range of variables attributed to the
changing NAO, which have the potential to cause
change in the marine environment. These include
variations in wind speed, latent and sensible heat
flux (Cayan, 1992a,b,c), evaporation/precipitation
(Cayan and Reverdin, 1994; Hurrell, 1995a), the
distribution, prevalence and intensity of Atlantic
storms (Rogers, 1990, 1994, 1997; Hurrell, 1995b;
WASA Group, 1997; Alexandersson et al., 1998),
hence effects on the wave climate (Bacon and
Carter, 1993; Kushnir et al., 1997; WASA Group,
1997; Carter, 1999; Cotton and Challenor, 1999),
sea surface temperature (Cayan, 1992c; Hansen
and Bezdek, 1996), the strength of the Labrador
Current (Myers et al., 1989), the characteristics
and distribution of water masses (Lazier, 1995;
McCartney et al., 1996, 1997; Houghton, 1996;
Joyce and Robbins, 1996; Molinari et al., 1997,
Sy et al., 1997; Joyce et al., 1999; Curry et al.,
1998; Curry and McCartney, 2001), the extent of
the marginal ice zone (Fang and Wallace, 1994;
Mysak et al., 1996; Deser et al., 2000), Davis Strait
ice volume (Deser and Blackmon, 1993), the iceberg flux past Newfoundland (Drinkwater, in
Rhines, 1994), and the intensity of deep convection
at the main Atlantic sites (Greenland Sea, Labrador
Sea and Sargasso; Dickson et al., 1996; Talley,
1996b; Dickson, 1997; Joyce et al., 2000).
Certain of these responses seem more critical
than others. In particular we highlight here the
radical interannual and interdecadal changes in the
production of the convectively formed mode
waters of the west Atlantic (Labrador Sea Water
and the Eighteen-Degree Water), which Dickson
et al. (1996) suggest to be part of a coordinated panAtlantic pattern of convective activity, driven by
the changing NAO. Specifically they suggest that
convective activity at all three main sites evolved
over decades to a long-term extreme state – in
phase but of differing sign – during the NAO minimum of the 1960s, at which time the ventilation of
the Greenland Sea and Sargasso was at a maximum and that of the Labrador Sea was tightly
capped. Since then until the mid-1990s, all three
centres have evolved rather more rapidly towards
their opposite extreme states, in which convection
in the Greenland Sea and Sargasso is suppressed
7.3 The World during WOCE
567
Dickson et al.
layer in the South Atlantic and a lesser fraction of
this layer in the southern North Atlantic. For this
reason, the large warmings (and compensating
increases in salinity) along isopycnals in the South
Atlantic, and the lesser warmings along isopycnals
in the southern North Atlantic, are suggestive of
changes in AAIW, especially since the warmings in
the southern North Atlantic are more pronounced
in the western part (Fig. 7.3.4b, see Plate 7.3.4b,
p. 588), where the presence of AAIW is strongest.
Likewise, the coolings along isopycnals in the
northern North Atlantic are suggestive of changes
in LSW (Fig. 7.3.4b, see Plate 7.3.4b, p. 588).
Our study has broadened the geographical extent
of the warmings at intermediate depths reported by
previous authors (Roemmich and Wunsch, 1984;
Parrilla et al., 1994; Bryden et al., 1996; Joyce and
Robbins, 1996; Arhan et al., 1998; Levitus, 1989;
Joyce et al., 1999) by showing that these warmings
extend into the South Atlantic. We find that the
isopycnal-displacement mechanism, which is of
widespread significance in the North Atlantic
(Levitus, 1989), is not the only one of importance.
Changes on isopycnals also form a large and widespread component of the changes observed on isobars. However, unlike the displacement mechanism,
changes along isopycnals respond very slowly to
changes in forcing, so that by serving as integrators over long time scales, they might be the more
sensitive indicator of long-term climate change
(suggestion of anonymous reviewer). In this light,
the isopycnal warmings we observe, extending
back into the 1920s, may well indicate that recent
changes in the climate of the ocean are a response
to climatic forcing over longer periods than we
have previously realized.
7.3.5 Spin-up of the North Atlantic gyre
circulation
From the subtropics northwards over the northern
North Atlantic and Nordic Seas, the status of the
marine environment during WOCE is overwhelmingly determined by the extreme long-term behaviour of the NAO.
As described earlier (Section 7.3.2.2), the WOCE
field phase occurred at a most unusual interval in
the climatic history of the North Atlantic, at the
end of a period in which the principal atmospheric
forcing mode in that sector had amplified over
three or four decades to its most extreme positive
state (generally) in a 176-year instrumental record
and possibly much longer. In the present context,
the importance of this extreme climatic shift lies in
the wide range of variables attributed to the
changing NAO, which have the potential to cause
change in the marine environment. These include
variations in wind speed, latent and sensible heat
flux (Cayan, 1992a,b,c), evaporation/precipitation
(Cayan and Reverdin, 1994; Hurrell, 1995a), the
distribution, prevalence and intensity of Atlantic
storms (Rogers, 1990, 1994, 1997; Hurrell, 1995b;
WASA Group, 1997; Alexandersson et al., 1998),
hence effects on the wave climate (Bacon and
Carter, 1993; Kushnir et al., 1997; WASA Group,
1997; Carter, 1999; Cotton and Challenor, 1999),
sea surface temperature (Cayan, 1992c; Hansen
and Bezdek, 1996), the strength of the Labrador
Current (Myers et al., 1989), the characteristics
and distribution of water masses (Lazier, 1995;
McCartney et al., 1996, 1997; Houghton, 1996;
Joyce and Robbins, 1996; Molinari et al., 1997,
Sy et al., 1997; Joyce et al., 1999; Curry et al.,
1998; Curry and McCartney, 2001), the extent of
the marginal ice zone (Fang and Wallace, 1994;
Mysak et al., 1996; Deser et al., 2000), Davis Strait
ice volume (Deser and Blackmon, 1993), the iceberg flux past Newfoundland (Drinkwater, in
Rhines, 1994), and the intensity of deep convection
at the main Atlantic sites (Greenland Sea, Labrador
Sea and Sargasso; Dickson et al., 1996; Talley,
1996b; Dickson, 1997; Joyce et al., 2000).
Certain of these responses seem more critical
than others. In particular we highlight here the
radical interannual and interdecadal changes in the
production of the convectively formed mode
waters of the west Atlantic (Labrador Sea Water
and the Eighteen-Degree Water), which Dickson
et al. (1996) suggest to be part of a coordinated panAtlantic pattern of convective activity, driven by
the changing NAO. Specifically they suggest that
convective activity at all three main sites evolved
over decades to a long-term extreme state – in
phase but of differing sign – during the NAO minimum of the 1960s, at which time the ventilation of
the Greenland Sea and Sargasso was at a maximum and that of the Labrador Sea was tightly
capped. Since then until the mid-1990s, all three
centres have evolved rather more rapidly towards
their opposite extreme states, in which convection
in the Greenland Sea and Sargasso is suppressed
7.3 The World during WOCE
567
Dickson et al.