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| Titel |
The role of the complete Coriolis force in cross-equatorial transport of the Antarctic Bottom Water |
| VerfasserIn |
Andrew Stewart, Paul Dellar |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032276
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| Zusammenfassung |
| We investigate the equatorial crossing of the Antarctic Bottom Water using a shallow water
model that includes the complete Coriolis force. Most theoretical models of the
atmosphere and ocean neglect the component of the Coriolis force associated with
the horizontal component of the Earth’s rotation vector, the so-called traditional
approximation.
This approximation is typically justified on the basis that ratio of the ocean depth to the
Rossby radius of deformation is negligibly small, H-Rd -ª 1. However, the steep topography
and weak stratification in the abyssal ocean magnify the role of the non-traditional
component of the Coriolis force. This is most pronounced in equatorial regions, where the
traditional component of the Coriolis force is weakest and the non-traditional component
is strongest. The inclusion of the complete Coriolis force gives rise to a range of
very long sub-inertial waves, whose frequencies lie below the inertial frequency, in
the two-layer shallow water equations. These waves have a dramatically different
structure to their traditional counterparts, particularly when the stratification is
weak.
We focus on the flow of the Antarctic Bottom Water from the Brazil Basin in the western
South Atlantic to the Guiana Basin in the western North Atlantic. In this region, the current
traverses a deep channel directed westwards and very slightly northwards across the
equator. Previous attempts to model this flow have struggled to explain why the
cross-equatorial transport is so high, with around 2.0–2.2 Sv exiting at the northern end of the
channel.
We present analytical and numerical solutions of the non-traditional shallow water
equations for the cross-equatorial flow of the Antarctic Bottom Water. We obtain analytical
solutions by considering the steady-state flow of a single layer of shallow water through a
northwesterly channel with a simple geometry. We assume zero potential vorticity, as it may
be shown that fluid whose potential vorticity q has sign opposite to that of the Coriolis
parameter f is subject to symmetric instability, and measurements confirm that the potential
vorticity is indeed close to zero near the equator. The solution reveals that the transport of
fluid through the channel is strongly affected by the non-traditional component of the Coriolis
force. The geometry of the channel and the position of the current are also found
to have a substantial effect on the solution. The combination of non-traditional
effects with the bathymetry of the channel through which the Antarctic Bottom
Water flows can increase the cross-equatorial transport by 50% or more, for realistic
parameters.
To determine how realistic equatorial topography might influence the transport of the
Antarctic Bottom Water, we conduct large-scale numerical simulations of the two-layer
shallow water equations, including the complete Coriolis force. The flow into the basin is
prescribed at the southern boundary, and the time-dependent shallow water equations are
integrated to a quasi-steady state. We discuss some of the computational complications
associated with the additional Coriolis terms, and we compare the transports predicted by the
traditional and non-traditional equations. |
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