382
Abyssal Circulation
hemisphere. The tongues of the oxygen maxima are particularly important in
identifying the poles as the sources of the deep water.
It is virtually impossible on the basis of direct observations to sketch the
horizontal structure of this layered circulation with confidence, but the signal
of the circulation is easily seen in a complex series of western boundary
currents responsible both for carrying part of the flow from the polar sources
and for closing the abyssal circulation occurring in the ocean interior.
Figure 7.1.2 from Warren (1981) shows the signal in the hydrographic fields of
potential temperature, salinity, silica, and oxygen of the western boundary
currents associated with the Antarctic bottom water and the southward flowing
North Atlantic deep water above. These cross sections also emphasize the
strong topographic features at the depths of the abyssal circulation where the
Mid-Atlantic Ridge is seen to separate the Atlantic basin into separate
subbasins.
The bottom topography evidently has a strong influence on the flow.
Observations of deep flows (e.g., Warren and Owens 1988; McCartney et al.
1991; Friedrichs and Hall 1993) each demonstrate the important role of
topography in shaping the structure of the deep circulation. In particular the
passages from one subbasin to another through breaks in the midocean ridge
systems strongly condition the shape of the flow. Figure 7.1.3 from McCartney
et al. (1991) shows a bulging, isolated pool of water with potential temperature
below 1.8 °C trapped on the eastern side of the gap in the Mid-Atlantic Ridge
formed by the Vema fracture zone at 11 °N in the Atlantic. Friedrichs and Hall
(1993) associate this region with a recirculating gyre which they infer is fed
through the Vema fracture zone in the deep water, as shown in Fig. 7.1.4.
Thus the abyssal circulation possesses simultaneously global scales
associated with the overall response to the production of cold water in polar
regions and its subsequent spread though the abyss, as well as subplanetary
scale motions as a consequence of the complex bottom topography of the
ocean, of which the Mid-Atlantic Ridge and its system of gaps and faults is a
typical example.
The modern theory of the abyssal circulation truly begins with a series of
remarkable papers by Stommel (1958) and Stommel and Arons (1960a,b). In
fact, the dynamical theory which they developed is actually a straightforward
application of Sverdrup theory as described in Chapter 1, but its contextual
framework is very original. The application and synthesis of the familiar ideas
of Sverdrup theory in the context of the abyssal circulation, driven by
prescribed sources of deep water in polar regions, leads to very counterintuitive
results, and the theory in this form is familiarly called the Stommel-Arons
theory.
A principal component of the Stommel-Arons theory and all subsequent
theories is the notion that the water that rises out of the abyss to replace the
water which has sunk at the poles does so over broad lateral scales. The water
that sinks does so in local, semienclosed regions of the polar oceans, for
example, as in the Weddell Sea, and the evidence of the sinking can be read in
Abyssal Circulation
hemisphere. The tongues of the oxygen maxima are particularly important in
identifying the poles as the sources of the deep water.
It is virtually impossible on the basis of direct observations to sketch the
horizontal structure of this layered circulation with confidence, but the signal
of the circulation is easily seen in a complex series of western boundary
currents responsible both for carrying part of the flow from the polar sources
and for closing the abyssal circulation occurring in the ocean interior.
Figure 7.1.2 from Warren (1981) shows the signal in the hydrographic fields of
potential temperature, salinity, silica, and oxygen of the western boundary
currents associated with the Antarctic bottom water and the southward flowing
North Atlantic deep water above. These cross sections also emphasize the
strong topographic features at the depths of the abyssal circulation where the
Mid-Atlantic Ridge is seen to separate the Atlantic basin into separate
subbasins.
The bottom topography evidently has a strong influence on the flow.
Observations of deep flows (e.g., Warren and Owens 1988; McCartney et al.
1991; Friedrichs and Hall 1993) each demonstrate the important role of
topography in shaping the structure of the deep circulation. In particular the
passages from one subbasin to another through breaks in the midocean ridge
systems strongly condition the shape of the flow. Figure 7.1.3 from McCartney
et al. (1991) shows a bulging, isolated pool of water with potential temperature
below 1.8 °C trapped on the eastern side of the gap in the Mid-Atlantic Ridge
formed by the Vema fracture zone at 11 °N in the Atlantic. Friedrichs and Hall
(1993) associate this region with a recirculating gyre which they infer is fed
through the Vema fracture zone in the deep water, as shown in Fig. 7.1.4.
Thus the abyssal circulation possesses simultaneously global scales
associated with the overall response to the production of cold water in polar
regions and its subsequent spread though the abyss, as well as subplanetary
scale motions as a consequence of the complex bottom topography of the
ocean, of which the Mid-Atlantic Ridge and its system of gaps and faults is a
typical example.
The modern theory of the abyssal circulation truly begins with a series of
remarkable papers by Stommel (1958) and Stommel and Arons (1960a,b). In
fact, the dynamical theory which they developed is actually a straightforward
application of Sverdrup theory as described in Chapter 1, but its contextual
framework is very original. The application and synthesis of the familiar ideas
of Sverdrup theory in the context of the abyssal circulation, driven by
prescribed sources of deep water in polar regions, leads to very counterintuitive
results, and the theory in this form is familiarly called the Stommel-Arons
theory.
A principal component of the Stommel-Arons theory and all subsequent
theories is the notion that the water that rises out of the abyss to replace the
water which has sunk at the poles does so over broad lateral scales. The water
that sinks does so in local, semienclosed regions of the polar oceans, for
example, as in the Weddell Sea, and the evidence of the sinking can be read in