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Abyssal Circulation
7.7 Vertical Structure of the Abyssal Circulation
Formulation
The Stommel-Arons theory represents the abyssal region as a single,
homogeneous layer in which the horizontal velocity is independent of depth.
It follows that at each geographical location the flow must have only a single
direction. Yet, as we noted in Section 7.1 (see Fig. 7.1.1), the motion in the
abyss is layered with interleaving tongues of water from different polar sources
apparently flowing in opposite directions. In order to explain this behavior a
baroclinic model of the abyss is needed in which the vertical structure of the
abyssal flow is determined by the dynamics of the abyss rather than being
imposed by the specification of small number of layers to represent the flow.
The results of the previous section in which the model of Kawase is discussed
suggest that the baroclinic dynamics of the large-scale Rossby waves is essential
in understanding the behavior of the interior abyssal flow outside the western
boundary current. It was observed there that the propagation of signals from
the source region follows an intricate, indirect path so that the thermal
anomalies actually enter the interior from the eastern boundary and only then
move westward to put the interior into motion. In fact, the results of the
classical Stommel-Arons theory prepare us for the indirect relationship
between the direction of the motion in the interior and the position of the
sources. This is even more evident when the flow is baroclinic and thus has
more degrees of freedom in the vertical.
At the same time, instead of restricting the driving of the abyssal region to
areas in which the abyss develops its own thermal anomalies, in this section we
return to the starting point of the Stommel-Arons theory in which the
thermocline dynamics specifies a vertical velocity at its base which drives the
abyss. It is likely that some combination of the two models of the driving
would be most realistic, but for simplicity we choose the Stommel-Arons
approach in the expectation that limiting the domain of motion to those
regions which are self-driven by the abyssal anomalies is overly restrictive.
In the Stommel-Arons theory the upwelling velocity into the thermocline,
w0 , is often taken for illustrative purposes to be a constant. For the barotropic
model this presents little loss of generality. As noted in our earlier discussion,
Stommel and Arons were keenly aware that w0 is probably not spatially
constant and suggested in fact that since theoretical considerations suggest it to
be inversely proportional to the depth scale of the thermocline, it should be
largest in eastern and equatorial regions and certainly not be spatially uniform.
The combination of non-uniform upwelling and baroclinic wave dynamics is
seen to yield an intriguing suggestion about the relative importance of the
baroclinic to barotropic response of the abyss to the upwelling into the
thermocline.
Abyssal Circulation
7.7 Vertical Structure of the Abyssal Circulation
Formulation
The Stommel-Arons theory represents the abyssal region as a single,
homogeneous layer in which the horizontal velocity is independent of depth.
It follows that at each geographical location the flow must have only a single
direction. Yet, as we noted in Section 7.1 (see Fig. 7.1.1), the motion in the
abyss is layered with interleaving tongues of water from different polar sources
apparently flowing in opposite directions. In order to explain this behavior a
baroclinic model of the abyss is needed in which the vertical structure of the
abyssal flow is determined by the dynamics of the abyss rather than being
imposed by the specification of small number of layers to represent the flow.
The results of the previous section in which the model of Kawase is discussed
suggest that the baroclinic dynamics of the large-scale Rossby waves is essential
in understanding the behavior of the interior abyssal flow outside the western
boundary current. It was observed there that the propagation of signals from
the source region follows an intricate, indirect path so that the thermal
anomalies actually enter the interior from the eastern boundary and only then
move westward to put the interior into motion. In fact, the results of the
classical Stommel-Arons theory prepare us for the indirect relationship
between the direction of the motion in the interior and the position of the
sources. This is even more evident when the flow is baroclinic and thus has
more degrees of freedom in the vertical.
At the same time, instead of restricting the driving of the abyssal region to
areas in which the abyss develops its own thermal anomalies, in this section we
return to the starting point of the Stommel-Arons theory in which the
thermocline dynamics specifies a vertical velocity at its base which drives the
abyss. It is likely that some combination of the two models of the driving
would be most realistic, but for simplicity we choose the Stommel-Arons
approach in the expectation that limiting the domain of motion to those
regions which are self-driven by the abyssal anomalies is overly restrictive.
In the Stommel-Arons theory the upwelling velocity into the thermocline,
w0 , is often taken for illustrative purposes to be a constant. For the barotropic
model this presents little loss of generality. As noted in our earlier discussion,
Stommel and Arons were keenly aware that w0 is probably not spatially
constant and suggested in fact that since theoretical considerations suggest it to
be inversely proportional to the depth scale of the thermocline, it should be
largest in eastern and equatorial regions and certainly not be spatially uniform.
The combination of non-uniform upwelling and baroclinic wave dynamics is
seen to yield an intriguing suggestion about the relative importance of the
baroclinic to barotropic response of the abyss to the upwelling into the
thermocline.