4.2.1 Processes in the ocean interior
In this chapter we concentrate on the interior circulation of the ocean away from the equator and,
for the most part, away from all boundaries. We
will also generally concentrate on flows at scales
larger than the Rossby radius.
Our present knowledge about such flows
comes from a mixture of hard in-situ observations
and shrewd theoretical analysis. This combination works best in the top kilometre of the ocean
where current velocities are relatively large. At
greater depths the mean velocity (often less than
0.5 cm s
91
) is usually masked by the mesoscale
eddy field (ϳ5 cm s
91 ) and by tides (ϳ5 cm s
91 ).
As a result our knowledge of the deep ocean is still
far from complete.
Observations of tracer distributions and mixing
processes in the ocean show that the transport
is primarily along density surfaces. Observations
and theoretical studies indicate that, away from the
equator and boundaries, the currents are basically
in geostrophic balance. This means that in the
momentum equation, which describes the dynamical behaviour of the ocean, the main balance is
between the Coriolis force and the horizontal pressure field.
Another key result from both observations and
theory is that at oceanic scales the potential energy
of the ocean is much larger than its kinetic energy.
As a result, the processes that ultimately determine
the circulation are the ones that are most effective
at changing the potential energy of the ocean.
Such processes usually occur at the boundaries.
Examples are the changes in sea level produced by
convergences and divergences of the wind-driven
Ekman transport and by heating, cooling, precipitation and evaporation at the ocean surface. The
circulation can also be driven by inflows or outflows at the boundary of a basin.
Within the ocean, the most important process is
probably the deep convection normally occurring
at high latitudes (Morawitz et al., 1996; Schott
et al., 1996; see also Lazier et al., Chapter 5.5).
Interior vertical mixing is generally weak, but it
may become important in deep ocean basins, where
other processes are negligible, or in regions where
there are large breaking internal tides (Egbert,
1997). The inertial terms in the momentum equation
are also generally considered to be small. However,
in regions of strong currents, such as the Gulf
Stream and Kuroshio, there are often large recirculation regions that are almost certainly due to the
effect of these terms.
4.2.1.1 The response to changes in forcing
Our understanding of how the ocean responds to a
change in the forcing depends primarily on theory
(Anderson and Gill, 1975; Gill, 1982) but is supported by many observations. Initially there is an
imbalance between the new pressure field and the
Coriolis term. This imbalance produces inertial
oscillations that may last for a few days but eventually die or propagate away. Geostrophic balance
then takes over, and Kelvin and Rossby waves
dominate the response at large scales for periods
of many months. Eventually these die away, leaving a residual steady-state circulation.
The conservation laws associated with mass,
energy and angular momentum severely limit the
possible steady states of the ocean circulation.
4.2
The Interior Circulation of the Ocean
D. J.Webb and N. Suginohara
205
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER
In this chapter we concentrate on the interior circulation of the ocean away from the equator and,
for the most part, away from all boundaries. We
will also generally concentrate on flows at scales
larger than the Rossby radius.
Our present knowledge about such flows
comes from a mixture of hard in-situ observations
and shrewd theoretical analysis. This combination works best in the top kilometre of the ocean
where current velocities are relatively large. At
greater depths the mean velocity (often less than
0.5 cm s
91
) is usually masked by the mesoscale
eddy field (ϳ5 cm s
91 ) and by tides (ϳ5 cm s
91 ).
As a result our knowledge of the deep ocean is still
far from complete.
Observations of tracer distributions and mixing
processes in the ocean show that the transport
is primarily along density surfaces. Observations
and theoretical studies indicate that, away from the
equator and boundaries, the currents are basically
in geostrophic balance. This means that in the
momentum equation, which describes the dynamical behaviour of the ocean, the main balance is
between the Coriolis force and the horizontal pressure field.
Another key result from both observations and
theory is that at oceanic scales the potential energy
of the ocean is much larger than its kinetic energy.
As a result, the processes that ultimately determine
the circulation are the ones that are most effective
at changing the potential energy of the ocean.
Such processes usually occur at the boundaries.
Examples are the changes in sea level produced by
convergences and divergences of the wind-driven
Ekman transport and by heating, cooling, precipitation and evaporation at the ocean surface. The
circulation can also be driven by inflows or outflows at the boundary of a basin.
Within the ocean, the most important process is
probably the deep convection normally occurring
at high latitudes (Morawitz et al., 1996; Schott
et al., 1996; see also Lazier et al., Chapter 5.5).
Interior vertical mixing is generally weak, but it
may become important in deep ocean basins, where
other processes are negligible, or in regions where
there are large breaking internal tides (Egbert,
1997). The inertial terms in the momentum equation
are also generally considered to be small. However,
in regions of strong currents, such as the Gulf
Stream and Kuroshio, there are often large recirculation regions that are almost certainly due to the
effect of these terms.
4.2.1.1 The response to changes in forcing
Our understanding of how the ocean responds to a
change in the forcing depends primarily on theory
(Anderson and Gill, 1975; Gill, 1982) but is supported by many observations. Initially there is an
imbalance between the new pressure field and the
Coriolis term. This imbalance produces inertial
oscillations that may last for a few days but eventually die or propagate away. Geostrophic balance
then takes over, and Kelvin and Rossby waves
dominate the response at large scales for periods
of many months. Eventually these die away, leaving a residual steady-state circulation.
The conservation laws associated with mass,
energy and angular momentum severely limit the
possible steady states of the ocean circulation.
4.2
The Interior Circulation of the Ocean
D. J.Webb and N. Suginohara
205
OCEAN CIRCULATION AND CLIMATE
Copyright © 2001 Academic Press
ISBN 0-12-641351-7
All rights of reproduction in any form reserved
CHAPTER