below), passed on by the dense northern overflows
through Denmark Strait and the Faroe Bank
Channel (see Saunders, Chapter 5.6). As the net
result of these changes, the steric height in the central Labrador Sea in the mid-1990s was typically
6–9 cm lower than in the late 1960s.
As already described, these changes have a
value in tracing out the rates and pathways by
which LSW-spreads across the basin. However,
the major importance of these changes is likely to
lie in their influence on the Atlantic gyre circulation itself. As Curry and McCartney (2001) point
out, the main North Atlantic Current is driven by
the gradient of potential energy anomaly (PEЈ)
across the mutual boundary between the subtropical and subpolar gyres. Since PEЈ reflects the vertical density structure and heat content of the upper
ocean to well below the wind-driven layer, it follows that coordinated changes of opposite sign in
the production and characteristics of the mode
waters in each gyre will have the potential to drive
deep-seated changes in the PEЈ gradient, and hence
in the strength of the Atlantic gyre circulation. If
these changes in the density and heat content of
mode waters are attributable to the NAO, then the
amplification of the NAO to extreme values over
the past three or four decades is likely to have
been followed by a corresponding multidecadal
spin-up of the Atlantic gyre circulation.
From the observed PEЈ differences between the
centres of the two gyres, Curry and McCartney
calculate that the long-term increase in the NAO
index to the mid-1990s was accompanied by a
30% increase in the 0–2000 dbar eastgoing baroclinic mass transport along the gyre–gyre boundary, from 50 Sv (1 Sv:10
6 m
3 s
91 ) in 1970 to
65 Sv in the mid-1990s (Fig. 7.3.7, see Plate 7.3.7,
p. 588). Both subpolar and subtropical gyres contributed equally to the changes in their transport
index. Thus in response to the NAO, the North
Atlantic gyre circulation during WOCE is likely to
have been at its strongest for more than a century.
Needless to say, as Curry and McCartney point
out, reality is more complex than this simple partnership between the circulation indices of ocean
and atmosphere.
1 Though local wind and buoyancy forcing was
the principal influence on potential energy
anomaly in the subpolar gyre, the passage of
Great Salinity Anomalies (Dickson et al., 1988;
Belkin et al., 1998) had a temporary damping
effect on transport as they passed through the
Labrador Basin in 1968–72 and 1980–84 (see
also Curry et al., 1998).
2 Though wind and buoyancy forcing did contribute to PEЈ changes in the subtropical gyre,
the PEЈ history of that basin seems mainly to
reflect thermocline ‘heave’ as a lagged lowfrequency baroclinic adjustment to changing
wind stresses further east.
3 Since advection moves the heat content anomalies around after their formation, the PEЈ pattern becomes a subtle quadripole rather than a
dipole, with opposite dynamic height departures
in the western and eastern parts of either gyre.
Nonetheless, this neat and convincing synthesis of
the observational record by Curry and McCartney
does appear to justify the view that decadal change
in the ocean may be the result of a coupled system,
not merely the response of a passive ocean to a
decadally evolving atmosphere; and they certainly
emphasize the unusual nature of the Atlantic circulation and its mode waters during WOCE.
7.3.6 Altered patterns of exchange in
Nordic Seas
The changes observed in the hydrographic character of the Greenland, Norwegian and Icelandic
Seas at all levels during WOCE were among the
starkest observed anywhere. In part these variations reflect decadal changes in the intensity and
depth of vertical exchange in the convection centre
of the Greenland Sea – part of the coordinated panAtlantic pattern of convective change described
earlier (Fig. 7.3.5). But the altered properties of
the deep and bottom waters in the Greenland Sea
suggest a more complex origin.
As Swift et al. (1983) point out, the deep water
of the Greenland Sea (GSDW) is the coldest and
freshest deep-water component in the Arctic
Ocean/Nordic Seas system, while that of the Arctic
Ocean (AODW) is the warmest and most saline.
Other local deep waters are a mixture of the two.
Since the deep waters filling the Greenland Sea
Basin are very much more homogeneous in space
than their observed variation in time, the implication seems clear – that cooling of waters in the
deep Greenland Sea can only be carried out from
above, by convection, while warming can only be
effected from outside the basin, by lateral exchange
7.3 The World during WOCE
569
Dickson et al.
through Denmark Strait and the Faroe Bank
Channel (see Saunders, Chapter 5.6). As the net
result of these changes, the steric height in the central Labrador Sea in the mid-1990s was typically
6–9 cm lower than in the late 1960s.
As already described, these changes have a
value in tracing out the rates and pathways by
which LSW-spreads across the basin. However,
the major importance of these changes is likely to
lie in their influence on the Atlantic gyre circulation itself. As Curry and McCartney (2001) point
out, the main North Atlantic Current is driven by
the gradient of potential energy anomaly (PEЈ)
across the mutual boundary between the subtropical and subpolar gyres. Since PEЈ reflects the vertical density structure and heat content of the upper
ocean to well below the wind-driven layer, it follows that coordinated changes of opposite sign in
the production and characteristics of the mode
waters in each gyre will have the potential to drive
deep-seated changes in the PEЈ gradient, and hence
in the strength of the Atlantic gyre circulation. If
these changes in the density and heat content of
mode waters are attributable to the NAO, then the
amplification of the NAO to extreme values over
the past three or four decades is likely to have
been followed by a corresponding multidecadal
spin-up of the Atlantic gyre circulation.
From the observed PEЈ differences between the
centres of the two gyres, Curry and McCartney
calculate that the long-term increase in the NAO
index to the mid-1990s was accompanied by a
30% increase in the 0–2000 dbar eastgoing baroclinic mass transport along the gyre–gyre boundary, from 50 Sv (1 Sv:10
6 m
3 s
91 ) in 1970 to
65 Sv in the mid-1990s (Fig. 7.3.7, see Plate 7.3.7,
p. 588). Both subpolar and subtropical gyres contributed equally to the changes in their transport
index. Thus in response to the NAO, the North
Atlantic gyre circulation during WOCE is likely to
have been at its strongest for more than a century.
Needless to say, as Curry and McCartney point
out, reality is more complex than this simple partnership between the circulation indices of ocean
and atmosphere.
1 Though local wind and buoyancy forcing was
the principal influence on potential energy
anomaly in the subpolar gyre, the passage of
Great Salinity Anomalies (Dickson et al., 1988;
Belkin et al., 1998) had a temporary damping
effect on transport as they passed through the
Labrador Basin in 1968–72 and 1980–84 (see
also Curry et al., 1998).
2 Though wind and buoyancy forcing did contribute to PEЈ changes in the subtropical gyre,
the PEЈ history of that basin seems mainly to
reflect thermocline ‘heave’ as a lagged lowfrequency baroclinic adjustment to changing
wind stresses further east.
3 Since advection moves the heat content anomalies around after their formation, the PEЈ pattern becomes a subtle quadripole rather than a
dipole, with opposite dynamic height departures
in the western and eastern parts of either gyre.
Nonetheless, this neat and convincing synthesis of
the observational record by Curry and McCartney
does appear to justify the view that decadal change
in the ocean may be the result of a coupled system,
not merely the response of a passive ocean to a
decadally evolving atmosphere; and they certainly
emphasize the unusual nature of the Atlantic circulation and its mode waters during WOCE.
7.3.6 Altered patterns of exchange in
Nordic Seas
The changes observed in the hydrographic character of the Greenland, Norwegian and Icelandic
Seas at all levels during WOCE were among the
starkest observed anywhere. In part these variations reflect decadal changes in the intensity and
depth of vertical exchange in the convection centre
of the Greenland Sea – part of the coordinated panAtlantic pattern of convective change described
earlier (Fig. 7.3.5). But the altered properties of
the deep and bottom waters in the Greenland Sea
suggest a more complex origin.
As Swift et al. (1983) point out, the deep water
of the Greenland Sea (GSDW) is the coldest and
freshest deep-water component in the Arctic
Ocean/Nordic Seas system, while that of the Arctic
Ocean (AODW) is the warmest and most saline.
Other local deep waters are a mixture of the two.
Since the deep waters filling the Greenland Sea
Basin are very much more homogeneous in space
than their observed variation in time, the implication seems clear – that cooling of waters in the
deep Greenland Sea can only be carried out from
above, by convection, while warming can only be
effected from outside the basin, by lateral exchange
7.3 The World during WOCE
569
Dickson et al.