not be specified (so long as it is not zero and the
flux is down gradient).
At given latitude, Sverdrup transport occurs
within both the central ventilated region and the
western, unventilated pool region. The width of
the western pool region and the ratio of the meridional transports in the two regions depends upon a
number of parameters, including the stratification
and the ratio of the meridional scale of the wind
stress (the half width of the subtropical wind cell,
roughly) and the scale of the variation of the Coriolis parameter, proportional to the earth’s radius
but dependent upon latitude. This ratio can be
rather small, and thus on a given isopycnal layer
only a small fraction of the Sverdrup transport
occurs within the directly ventilated region of the
thermocline, the rest being within the western pool
region. Thus only a fraction of the sea surface is
within the ‘mouth’ of a Montgomery streamtube,
typically the central and eastern half of a subtropical gyre. This may be evident in Iselin’s subduction
diagram (Fig. 5.3.1a). Notice that the central profiles (in depth and along the sea surface) are
closely similar over a substantial range of temperatures, roughly 8°C to 16°C. By comparison, the
western profiles are not closely similar, and indeed
the best match is over the range 15°C to 18°C
where the western profile overlaps the central profile. That is, the western profile in depth is more
like the central (surface or depth) profiles than the
western surface profile. This suggests that to the
extent that western thermocline water has an origin at the sea surface in the North Atlantic, then
that origin is in the central or eastern subtropical
surface layer. This eastward bias of subduction has
observable consequences in the evolution of interannual Sea Surface Temperature (SST) anomalies
(discussed in Section 5.3.4).
Subduction theory has been extended and
applied in many directions since the seminal LPS
contribution. This chapter attempts a summary of
the progress toward three main questions:
1 What are the surface layer dynamics of the subduction process? (Section 5.3.2)
2 What are the predictions of a continuous (as
opposed to a layered) model? And how applicable is the LPS adiabatic dynamics to the ocean
and to numerical model results? (Section 5.3.3)
3 What is the thermocline response to interannual
variability of surface forcing? (Section 5.3.4).
An excellent review of subduction theory up until
about 1990 is by Huang (1991). The monograph
by Pedlosky (1996) is a masterful treatment of
many aspects of large-scale ocean circulation theory, including especially tropical–subtropical connections associated with subduction that are
not much discussed here (and see also Liu and
Philander, Chapter 4.4).
5.3.2 Surface-layer dynamics and
thermodynamics of the subduction
process
The first steady models of the ventilated thermocline took subduction as an observed fact and then
proceeded to find the consequences for the Sverdrup flow of the thermocline. It was clear, though,
that the subduction process (the passage of water
from the surface layer to the main thermocline)
must involve the combined effects of upper ocean
dynamics, which are highly diabatic, and the gyre
circulation. It was expected, too, that the details of
where and how much water is subducted must be
of consequence to the resulting circulation, and
would certainly affect tracer distributions. Indeed,
some of the first comparisons of observed, largescale tracer inventories and tracer ages with the
Ekman pumping rate (Sarmiento, 1983; Jenkins,
1987) disclosed that the subduction rate on some
isopycnal layers was well in excess of the Ekman
pumping rate onto those surfaces, implying a
subduction process beyond that envisioned by
Montgomery (1938).
The subduction rate may be defined as the rate
at which water flows downward across a surface
defined by the depth of the late winter mixed layer
(Cushman-Roisin, 1987; Williams, 1991; Marshall
and Nurser, 1992; Marshall et al., 1993a). If that
surface were level, then only a vertical velocity
would support a flux, and at depths close to the
sea surface, the vertical velocity would be due
mainly to Ekman pumping. However, in some
regions, especially near western boundary currents, the winter mixed-layer depth has a significant horizontal gradient, and in that case a purely
horizontal velocity will also carry water from the
surface layer into the main thermocline, i.e. cause
subduction. This process has been termed ‘lateral
induction’ (Huang, 1991). Over the region just
south and east of the Gulf Stream, lateral induction has been estimated to produce about as much
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
360
flux is down gradient).
At given latitude, Sverdrup transport occurs
within both the central ventilated region and the
western, unventilated pool region. The width of
the western pool region and the ratio of the meridional transports in the two regions depends upon a
number of parameters, including the stratification
and the ratio of the meridional scale of the wind
stress (the half width of the subtropical wind cell,
roughly) and the scale of the variation of the Coriolis parameter, proportional to the earth’s radius
but dependent upon latitude. This ratio can be
rather small, and thus on a given isopycnal layer
only a small fraction of the Sverdrup transport
occurs within the directly ventilated region of the
thermocline, the rest being within the western pool
region. Thus only a fraction of the sea surface is
within the ‘mouth’ of a Montgomery streamtube,
typically the central and eastern half of a subtropical gyre. This may be evident in Iselin’s subduction
diagram (Fig. 5.3.1a). Notice that the central profiles (in depth and along the sea surface) are
closely similar over a substantial range of temperatures, roughly 8°C to 16°C. By comparison, the
western profiles are not closely similar, and indeed
the best match is over the range 15°C to 18°C
where the western profile overlaps the central profile. That is, the western profile in depth is more
like the central (surface or depth) profiles than the
western surface profile. This suggests that to the
extent that western thermocline water has an origin at the sea surface in the North Atlantic, then
that origin is in the central or eastern subtropical
surface layer. This eastward bias of subduction has
observable consequences in the evolution of interannual Sea Surface Temperature (SST) anomalies
(discussed in Section 5.3.4).
Subduction theory has been extended and
applied in many directions since the seminal LPS
contribution. This chapter attempts a summary of
the progress toward three main questions:
1 What are the surface layer dynamics of the subduction process? (Section 5.3.2)
2 What are the predictions of a continuous (as
opposed to a layered) model? And how applicable is the LPS adiabatic dynamics to the ocean
and to numerical model results? (Section 5.3.3)
3 What is the thermocline response to interannual
variability of surface forcing? (Section 5.3.4).
An excellent review of subduction theory up until
about 1990 is by Huang (1991). The monograph
by Pedlosky (1996) is a masterful treatment of
many aspects of large-scale ocean circulation theory, including especially tropical–subtropical connections associated with subduction that are
not much discussed here (and see also Liu and
Philander, Chapter 4.4).
5.3.2 Surface-layer dynamics and
thermodynamics of the subduction
process
The first steady models of the ventilated thermocline took subduction as an observed fact and then
proceeded to find the consequences for the Sverdrup flow of the thermocline. It was clear, though,
that the subduction process (the passage of water
from the surface layer to the main thermocline)
must involve the combined effects of upper ocean
dynamics, which are highly diabatic, and the gyre
circulation. It was expected, too, that the details of
where and how much water is subducted must be
of consequence to the resulting circulation, and
would certainly affect tracer distributions. Indeed,
some of the first comparisons of observed, largescale tracer inventories and tracer ages with the
Ekman pumping rate (Sarmiento, 1983; Jenkins,
1987) disclosed that the subduction rate on some
isopycnal layers was well in excess of the Ekman
pumping rate onto those surfaces, implying a
subduction process beyond that envisioned by
Montgomery (1938).
The subduction rate may be defined as the rate
at which water flows downward across a surface
defined by the depth of the late winter mixed layer
(Cushman-Roisin, 1987; Williams, 1991; Marshall
and Nurser, 1992; Marshall et al., 1993a). If that
surface were level, then only a vertical velocity
would support a flux, and at depths close to the
sea surface, the vertical velocity would be due
mainly to Ekman pumping. However, in some
regions, especially near western boundary currents, the winter mixed-layer depth has a significant horizontal gradient, and in that case a purely
horizontal velocity will also carry water from the
surface layer into the main thermocline, i.e. cause
subduction. This process has been termed ‘lateral
induction’ (Huang, 1991). Over the region just
south and east of the Gulf Stream, lateral induction has been estimated to produce about as much
SECTION 5 FORMATION AND TRANSPORT OF WATER MASSES
360