440
very sparse, in particular outside· the tropical regions where longer time
scales are appropriate. In fact, it is fair to say that noteworthy theoretical studies of aperiodic variations of large-scale current systems have
only been performed for the tropical processes associated with ENSO (see
e. g. the review by Neelin et al. 1994). Here modelling efforts were strongly
guided and verified by observations. On the contrary, other areas of recent research on temporal variability, such as the thermohaline circulation
(e. g. Rahmstorf 1995) or coupled modes of the ocean-atmosphere system
(see e. g. Latif et al. 1996, this volume), still lack observational support.
Our investigation on the existence of multiple equilibria and the temporal variability of zonally periodic ocean currents - with the Antarctic
Circumpolar Current (ACC) as the only member of this species - has
a similarly staggering foundation. There are virtually no observations to
support our results, even numerical models which comprise the appropriate
dynamical and thermohaline physics (i. e. eddy resolving primitive equation physics) have not been employed to study our topic. Therefore, apart
from the pure investigation of a dynamical system, the results of this paper may serve to analyse and interpret future numerical experiments and
observational data. In addition, the paper points out some benefits and
limitations of low-order modelling.
To summarize the main results: we have developed a low-order image of
wind forced flow in periodic zonal channel, governed by quasigeostrophic
,a-plane dynamics. The investigation supplements and extends previous
studies in this area of Charney-DeVore type models in so far as it applies oceanic parameters and includes stratification and non-linear wave
interactions. We have derived and discussed the dynamical balance and
stability of the existing steady states and unraveled the complex aperiodic and chaotic temporal behavior. This analysis could be performed for
the entire range of model parameters (topographic height, friction coefficients, channel dimensions and reduced gravity) since the model equations
are completely solved by analytical techniques. The model described here
has nine degrees of freedom, it is embedded in a more elaborate low-order
model with 21 degrees of freedom (Volker 1995) which resolves a larger
number of wave modes.
Finally, we have tested the balance and temporal behavior of this loworder model against 'observed data' obtained from a numerical simulation
with an eddy resolving quasigeostrophic model in the identical configuration. We accept the hypothesis that the slow temporal variability of
very sparse, in particular outside· the tropical regions where longer time
scales are appropriate. In fact, it is fair to say that noteworthy theoretical studies of aperiodic variations of large-scale current systems have
only been performed for the tropical processes associated with ENSO (see
e. g. the review by Neelin et al. 1994). Here modelling efforts were strongly
guided and verified by observations. On the contrary, other areas of recent research on temporal variability, such as the thermohaline circulation
(e. g. Rahmstorf 1995) or coupled modes of the ocean-atmosphere system
(see e. g. Latif et al. 1996, this volume), still lack observational support.
Our investigation on the existence of multiple equilibria and the temporal variability of zonally periodic ocean currents - with the Antarctic
Circumpolar Current (ACC) as the only member of this species - has
a similarly staggering foundation. There are virtually no observations to
support our results, even numerical models which comprise the appropriate
dynamical and thermohaline physics (i. e. eddy resolving primitive equation physics) have not been employed to study our topic. Therefore, apart
from the pure investigation of a dynamical system, the results of this paper may serve to analyse and interpret future numerical experiments and
observational data. In addition, the paper points out some benefits and
limitations of low-order modelling.
To summarize the main results: we have developed a low-order image of
wind forced flow in periodic zonal channel, governed by quasigeostrophic
,a-plane dynamics. The investigation supplements and extends previous
studies in this area of Charney-DeVore type models in so far as it applies oceanic parameters and includes stratification and non-linear wave
interactions. We have derived and discussed the dynamical balance and
stability of the existing steady states and unraveled the complex aperiodic and chaotic temporal behavior. This analysis could be performed for
the entire range of model parameters (topographic height, friction coefficients, channel dimensions and reduced gravity) since the model equations
are completely solved by analytical techniques. The model described here
has nine degrees of freedom, it is embedded in a more elaborate low-order
model with 21 degrees of freedom (Volker 1995) which resolves a larger
number of wave modes.
Finally, we have tested the balance and temporal behavior of this loworder model against 'observed data' obtained from a numerical simulation
with an eddy resolving quasigeostrophic model in the identical configuration. We accept the hypothesis that the slow temporal variability of