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| Titel |
Flow past topography in relation to the Antarctic Circumpolar Current |
| VerfasserIn |
Alistair McVicar, Peter Allison, Matthew Piggott, Arnaud Czaja |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250046109
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| Zusammenfassung |
| The wind-stress impinging upon the Antarctic Circumpolar Current (“ACC”) in the
Southern Ocean is substantial at around 0.2 Nm-2. The associated northward-flowing
Ekman drift must be opposed by a southward geostrophic mass flux below the mixed
layer. However, the nature of the balancing geostrophic flow is hotly debated with
the principal contenders being either transient jets and eddies or the time mean
flow.
This problem is rendered more tractable by using scaled idealised domains in which
multiple sensitivity tests can be evaluated in a computationally efficient manner.
Fluidity–ICOM (http://amcg.ese.ic.ac.uk/) is being used, which utilises unstructured meshes
and a new stable mixed discontinuous/continuous finite element pair (P1DG–P2). This
combination allows small–scale processes in high Reynolds number flows to be modelled and
analysed.
The classical problem of flow past a cylinder is broadly analogous to flow past
topographic highs in the ACC. The model set–up incorporates a barotropic flow in a
non–dimensional, periodic, beta–plane channel. The flow structure is dependent on Reynolds
numbers, the non–dimensional beta parameter and the shape of the obstacles included in
the channel. Both prograde and retrograde flows have been simulated in Reynolds
number and beta-parameter regimes that are equivalent to the Southern Ocean. Low
beta–parameter regimes have been verified against theoretical, laboratory and other modelling
studies.
The high Reynolds number flows lead to the formation of eddies and Rossby waves
down-stream of the cylinder and the creation of jets through the bunching of pressure
contours upstream. Analysis of the transient small–scale eddy structures associated with the
wake provides additional insight into the role of the various forces acting upon the ACC. The
variation of the form drag, frictional stresses and the boundary layer associated with an
obstacle illustrates the change in the dominance of the different terms in the momentum
equation as the flow evolves from laminar to vortex shedding and then to turbulence. New
techniques to decompose the momentum balance into its rotational and divergent parts have
been used to further elucidate the balance of terms and the eddy forces associated with the
Reynolds stresses.
These simulations highlight the importance of jets and eddies in closing the momentum
balance in the Southern Ocean circulation. |
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