5.3.1 A little of the background on
oceanic subduction
The formation and circulation of thermocline
water masses is a central theme of large-scale,
physical oceanography. The formation half of the
phenomenon usually involves a combination of
horizontal and downward flow that has a resemblance to the relative movement of tectonic plates.
The formation of thermocline water masses by
combined vertical and horizontal flow has come to
be termed ‘subduction’ (Luyten et al., 1983). This
chapter reviews the progress made toward understanding the oceanic subduction process and its
consequences, with some emphasis upon contributions made during the 1990s.
Though the intent here is to highlight recent
progress, it is fitting to point out that the roots of
our modern ideas are clearly found in the first
basin-scale surveys of the Atlantic Ocean conducted during the 1920s and 1930s. The Meteor
expedition, described in the monograph by Wüst
(1935), revealed the large-scale, three-dimensional
patterns of temperature, salinity and oxygen concentration over the Atlantic Ocean. These patterns
were highly suggestive of water mass formation at
the sea surface in specific and rather confined
regions, followed by downward and horizontal
flow into the thermocline or abyssal ocean.
Regarding the Subantarctic Intermediate Water,
Wüst (1935, p. 3) noted that ‘The vertical structure of the Subantarctic Intermediate Water, with
its horizontal spreading at depth, is analogous to
a vertical figure of the horizontal arrangement
of temperature and salinity at the surface of the
formation region. The upper portions of the Intermediate Water conform to the northern while the
lower ones conform to the southern parts of
the formation area.’ Wüst (1935, p. 60) noted an
effort by Wattenberg during the mid-1920s to
infer the current speed along the spreading path of
intermediate water by observing the distance
between salinity and oxygen anomalies that were
presumed to be annual pulses imposed by seasonality at the sea surface. The data available at that
time were probably not adequate to this demanding task, in part because the subduction process
works as a ‘demon’ that admits mainly winter
waters to the main thermocline (the demon is
described further below). Iselin (1939) was performing a similar analysis of North Atlantic properties at about the same time, and his famous
Temperature/Salinity (T/S) diagram (Fig. 5.3.1), in
which vertical and horizontal profiles were superposed, reveals what seems to be a distributed
source of thermocline water at the sea surface, and
a downward and southward flow of water over at
least a portion of the North Atlantic subtropical
gyre. Iselin noted that the best correlation between
surface and thermocline T/S properties was with
winter surface conditions (as if water at the surface in summer did not contribute significantly to
thermocline properties). A second and perhaps
more fundamental point is that the T/S properties
of subducted waters are evidently not grossly
altered by vertical or horizontal mixing after thermocline waters leave the surface layer, i.e. after they
are subducted into the thermocline. This implies a
crucial idealization for thermocline models, namely,
5.3
Subduction
James F. Price
357
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
oceanic subduction
The formation and circulation of thermocline
water masses is a central theme of large-scale,
physical oceanography. The formation half of the
phenomenon usually involves a combination of
horizontal and downward flow that has a resemblance to the relative movement of tectonic plates.
The formation of thermocline water masses by
combined vertical and horizontal flow has come to
be termed ‘subduction’ (Luyten et al., 1983). This
chapter reviews the progress made toward understanding the oceanic subduction process and its
consequences, with some emphasis upon contributions made during the 1990s.
Though the intent here is to highlight recent
progress, it is fitting to point out that the roots of
our modern ideas are clearly found in the first
basin-scale surveys of the Atlantic Ocean conducted during the 1920s and 1930s. The Meteor
expedition, described in the monograph by Wüst
(1935), revealed the large-scale, three-dimensional
patterns of temperature, salinity and oxygen concentration over the Atlantic Ocean. These patterns
were highly suggestive of water mass formation at
the sea surface in specific and rather confined
regions, followed by downward and horizontal
flow into the thermocline or abyssal ocean.
Regarding the Subantarctic Intermediate Water,
Wüst (1935, p. 3) noted that ‘The vertical structure of the Subantarctic Intermediate Water, with
its horizontal spreading at depth, is analogous to
a vertical figure of the horizontal arrangement
of temperature and salinity at the surface of the
formation region. The upper portions of the Intermediate Water conform to the northern while the
lower ones conform to the southern parts of
the formation area.’ Wüst (1935, p. 60) noted an
effort by Wattenberg during the mid-1920s to
infer the current speed along the spreading path of
intermediate water by observing the distance
between salinity and oxygen anomalies that were
presumed to be annual pulses imposed by seasonality at the sea surface. The data available at that
time were probably not adequate to this demanding task, in part because the subduction process
works as a ‘demon’ that admits mainly winter
waters to the main thermocline (the demon is
described further below). Iselin (1939) was performing a similar analysis of North Atlantic properties at about the same time, and his famous
Temperature/Salinity (T/S) diagram (Fig. 5.3.1), in
which vertical and horizontal profiles were superposed, reveals what seems to be a distributed
source of thermocline water at the sea surface, and
a downward and southward flow of water over at
least a portion of the North Atlantic subtropical
gyre. Iselin noted that the best correlation between
surface and thermocline T/S properties was with
winter surface conditions (as if water at the surface in summer did not contribute significantly to
thermocline properties). A second and perhaps
more fundamental point is that the T/S properties
of subducted waters are evidently not grossly
altered by vertical or horizontal mixing after thermocline waters leave the surface layer, i.e. after they
are subducted into the thermocline. This implies a
crucial idealization for thermocline models, namely,
5.3
Subduction
James F. Price
357
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER