408
put forward almost half a century ago as a narrow parallel to the balance of zonal atmospheric flow by an oceanographer and a meteorologist
(Munk and Palmen 1951). In view of the apparent inability of friction at
the bottom or lateral coasts to extract the eastward momentum which is
generously imparted to the ocean in the ACC belt by the strong surface
wind, they suggested that it could flux out of the fluid system into the
solid earth by the bottom form stress mechanism. This stress arises from
a systematically higher pressure amplitude on the luv compared to the lee
side of the submarine ridges blocking the flow at great depths in the Drake
Passage and the other oceans which the ACC has to cross. The mechanism can hardly be measured: the pressure difference across a ridge would
correspond to only a few centimeters of surface displacement (the role of
the barotropic and baroclinic pressure fields in balancing the ACC is however much more complicated, see e. g. Olbers et al. 1992). Experiments by
numerical models of the ACC (McWilliams et al. 1978, Wolff and Olbers
1989, Wolff et al. 1991, Marshall et al. 1993, The FRAM Group 1991 and
others) have clearly shown that eddies do not transport eastward momentum out of the current system to enable lateral frictional loss. They rather
concentrate the jets and - by interfacial form stress - they also help to
transport momentum downward to the bottom. Furthermore, the entire
flow arranges a pressure field such that the bottom form stress is by far
more effective than friction to extract momentum.
This strong resemblance in the balance of forces is encouragement to
look for more correspondences of the ACC physics with the physical features discovered for zonal atmospheric flows. Outstanding theoretical issues in this field of atmospheric science are the occurrence of multiple
steady states, their stability and their conjectured role in temporal variability. This problem is addressed in the present paper for a simple system
of fluid flow which in most of its physical aspects resembles the ACC but
is still understandable with analytical treatment.
The concept of multiple equilibria in a severely truncated 'low-order'
image (the CdV model) of the atmospheric circulation was put forward
by Egger, Charney, Wijn-Nielsen and others in a series of papers (Egger
1978, Charney and DeVore 1979, Wijn-Nielsen 1979, Charney et al. 1981)
to explain the variability of atmospheric large-scale flow, in particular the
occurrence and transition between blocked and unblocked situations in the
midlatitudes of the northern hemisphere. It is appealing to connect these
'GroBwetterlagen' with the steady regimes of a low-order subsystem of the
put forward almost half a century ago as a narrow parallel to the balance of zonal atmospheric flow by an oceanographer and a meteorologist
(Munk and Palmen 1951). In view of the apparent inability of friction at
the bottom or lateral coasts to extract the eastward momentum which is
generously imparted to the ocean in the ACC belt by the strong surface
wind, they suggested that it could flux out of the fluid system into the
solid earth by the bottom form stress mechanism. This stress arises from
a systematically higher pressure amplitude on the luv compared to the lee
side of the submarine ridges blocking the flow at great depths in the Drake
Passage and the other oceans which the ACC has to cross. The mechanism can hardly be measured: the pressure difference across a ridge would
correspond to only a few centimeters of surface displacement (the role of
the barotropic and baroclinic pressure fields in balancing the ACC is however much more complicated, see e. g. Olbers et al. 1992). Experiments by
numerical models of the ACC (McWilliams et al. 1978, Wolff and Olbers
1989, Wolff et al. 1991, Marshall et al. 1993, The FRAM Group 1991 and
others) have clearly shown that eddies do not transport eastward momentum out of the current system to enable lateral frictional loss. They rather
concentrate the jets and - by interfacial form stress - they also help to
transport momentum downward to the bottom. Furthermore, the entire
flow arranges a pressure field such that the bottom form stress is by far
more effective than friction to extract momentum.
This strong resemblance in the balance of forces is encouragement to
look for more correspondences of the ACC physics with the physical features discovered for zonal atmospheric flows. Outstanding theoretical issues in this field of atmospheric science are the occurrence of multiple
steady states, their stability and their conjectured role in temporal variability. This problem is addressed in the present paper for a simple system
of fluid flow which in most of its physical aspects resembles the ACC but
is still understandable with analytical treatment.
The concept of multiple equilibria in a severely truncated 'low-order'
image (the CdV model) of the atmospheric circulation was put forward
by Egger, Charney, Wijn-Nielsen and others in a series of papers (Egger
1978, Charney and DeVore 1979, Wijn-Nielsen 1979, Charney et al. 1981)
to explain the variability of atmospheric large-scale flow, in particular the
occurrence and transition between blocked and unblocked situations in the
midlatitudes of the northern hemisphere. It is appealing to connect these
'GroBwetterlagen' with the steady regimes of a low-order subsystem of the