4.5
Quantification of the Deep Circulation
Nelson G. Hogg
259
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
4.5.1 Deep circulation in the framework
of WOCE
During the design phase of the World Ocean Circulation Experiment (WOCE) it was recognized
that various strategies were needed to ‘divide and
conquer’ the massive problem of making a significant step forward in our understanding of the
global circulation. One of these was a series of
‘subject’ meetings to contrast with the earlier ‘sector’ meetings at which different ocean basins had
been the topic. One such meeting was held at
Woods Hole in 1986 and concerned the subject of
the deep circulation and its interaction with topography. Advice was sought on a variety of issues
from the use of tracers, to the roles of diapycnal
mixing, interaction with a rough bottom, the relative importance of advection and diffusion, etc.
A set of objectives was formulated and further
refined at the Core Project 3 meeting at the Royal
Society later in the year (WCRP, 1987). Ultimately, the list of objectives for improving our
knowledge of the deep circulation became:
¥ to explore and fill out the system of Deep Western Boundary Currents (DWBCs) and quantify
their transports, on a global basis;
¥ to quantify the interior circulation, away from
the DWBCs, including the role of mixing, deep
passages, etc.;
¥ to quantify the time-dependent production of
deep and bottom waters;
¥ to determine the importance of topographic
form stress; and
¥ to study the dissipation of kinetic energy through
frictional processes at the bottom.
At the writing of this chapter it is fair to say that
the above list remains pretty much intact as important research areas: WOCE has made significant
contributions to the first two topics, which will be
outlined below; little advance has been made on
the last three.
A guiding framework for the design of an experimental approach to understanding the deep circulation is provided by the circulation scheme devised
by Henry Stommel (1958) and further embellished
in the series of papers with Arnold Arons and
others. This posits that there are a small number
(two) of deep water formation areas in the polar
regions of the ocean and that the water made dense
there must spread equatorward in DWBCs on a
rotating spherical planet (Fig. 4.5.1). Recognizing
that there is a thermocline separating warm surface
water from deep cold water over much of the
globe, Stommel further hypothesized that this
is maintained, against the downward diffusion
of heat, by a uniform upwelling that supplies the
upper ocean with the water needed to replace that
which sinks at high latitudes. Such an upward vertical velocity at the top of the abyssal layer implies
vortex stretching. Conservation of potential vorticity then demands poleward flow in the interior.
With this scheme in mind researchers during
WOCE sought to fulfil the first two objectives listed
above. Progress toward quantifying and fleshing
out the system of DWBCs on a global basis (Section
4.5.2) was achieved through strategic placement of
moored arrays along the western boundaries of the
world’s oceans. Recognizing that quantification of
the deep circulation on a global basis was well
beyond our means, research on this objective was
Quantification of the Deep Circulation
Nelson G. Hogg
259
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
4.5.1 Deep circulation in the framework
of WOCE
During the design phase of the World Ocean Circulation Experiment (WOCE) it was recognized
that various strategies were needed to ‘divide and
conquer’ the massive problem of making a significant step forward in our understanding of the
global circulation. One of these was a series of
‘subject’ meetings to contrast with the earlier ‘sector’ meetings at which different ocean basins had
been the topic. One such meeting was held at
Woods Hole in 1986 and concerned the subject of
the deep circulation and its interaction with topography. Advice was sought on a variety of issues
from the use of tracers, to the roles of diapycnal
mixing, interaction with a rough bottom, the relative importance of advection and diffusion, etc.
A set of objectives was formulated and further
refined at the Core Project 3 meeting at the Royal
Society later in the year (WCRP, 1987). Ultimately, the list of objectives for improving our
knowledge of the deep circulation became:
¥ to explore and fill out the system of Deep Western Boundary Currents (DWBCs) and quantify
their transports, on a global basis;
¥ to quantify the interior circulation, away from
the DWBCs, including the role of mixing, deep
passages, etc.;
¥ to quantify the time-dependent production of
deep and bottom waters;
¥ to determine the importance of topographic
form stress; and
¥ to study the dissipation of kinetic energy through
frictional processes at the bottom.
At the writing of this chapter it is fair to say that
the above list remains pretty much intact as important research areas: WOCE has made significant
contributions to the first two topics, which will be
outlined below; little advance has been made on
the last three.
A guiding framework for the design of an experimental approach to understanding the deep circulation is provided by the circulation scheme devised
by Henry Stommel (1958) and further embellished
in the series of papers with Arnold Arons and
others. This posits that there are a small number
(two) of deep water formation areas in the polar
regions of the ocean and that the water made dense
there must spread equatorward in DWBCs on a
rotating spherical planet (Fig. 4.5.1). Recognizing
that there is a thermocline separating warm surface
water from deep cold water over much of the
globe, Stommel further hypothesized that this
is maintained, against the downward diffusion
of heat, by a uniform upwelling that supplies the
upper ocean with the water needed to replace that
which sinks at high latitudes. Such an upward vertical velocity at the top of the abyssal layer implies
vortex stretching. Conservation of potential vorticity then demands poleward flow in the interior.
With this scheme in mind researchers during
WOCE sought to fulfil the first two objectives listed
above. Progress toward quantifying and fleshing
out the system of DWBCs on a global basis (Section
4.5.2) was achieved through strategic placement of
moored arrays along the western boundaries of the
world’s oceans. Recognizing that quantification of
the deep circulation on a global basis was well
beyond our means, research on this objective was