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| Titel |
A numerical study of cross-equatorial abyssal ocean currents with a complete representation of the Coriolis force |
| VerfasserIn |
Andrew Stewart, Paul Dellar |
| 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 |
250053613
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| Zusammenfassung |
| Ocean currents that flow close to and across the equator present a challenging dynamical
problem. These currents form an important part of the circulation in the world ocean, yet our
understanding of them remains limited. For example, the Antarctic Bottom Water
(AABW) crosses the equator northward in the western Atlantic off the coast of Brazil.
The AABW thus forms part of the Atlantic Meridional Overturning Circulation
(AMOC), predictions for which vary substantially even in the most recent report of the
Intergovernmental Panel on Climate Change (IPCC). We investigate the behaviour of currents
flowing through cross-equatorial abyssal channels using a set of shallow water
equations that include a complete representation of the Coriolis force. These equations
thus account for the locally horizontal component of the Earth’s rotation vector,
which is commonly neglected in models of the ocean under the so-called “traditional
approximation”. However, weak stratification and almost-horizontal alignment of the
rotation vector combine to maximise non-traditional effects in the abyssal equatorial
ocean.
The movement of fluid across the equator is strongly constrained by the conservation of
potential vorticity (PV) following fluid parcels. A parcel that crosses between hemispheres
experiences a change in sign of the Coriolis parameter f. To conserve its PV, the parcel must
therefore acquire a large relative vorticity to compensate for the change in f. Cross-equatorial
currents therefore tend to generate eddies, and to retroflect back across the equator. In reality,
the PV may be modified by dissipative processes, thereby permitting some form of
cross-equatorial flow.
The conserved potential vorticity acquires a further contribution from the non-traditional
component of the Coriolis force, proportional to the meridional gradient of the bottom
topography. This partially balances the change in f for a current, such as the AABW, that
crosses the equator through an almost zonal channel. The fracture zone off the coast of Brazil
provides a suitable channel for the AABW. Non-traditional effects should thus
enhance cross-equatorial flow, since the fluid need not acquire as much relative
vorticity as it otherwise would need. More intuitively, fluid crossing a zonal equatorial
channel undergoes a smaller change in its distance from the axis of rotation, and is
therefore subject to a smaller change in planetary angular momentum as it crosses the
equator.
We analyse the cross-equatorial flow problem using numerical integration of our shallow
water equations with complete Coriolis force in an idealised steep-sided channel,
and in realistic equatorial bathymetry based on ETOPO1 data. Our scheme is a
generalisation of the Arakawa–Lamb (1981) scheme that exactly conserves total mass,
energy, and potential enstrophy in the absence of explicit dissipation. However, we
include an explicit second-order dissipation to represent the effect of sub-gridscale
eddies.
A substantial portion of a northward-flowing current retroflects as it crosses the equator,
and exits back into the southern hemisphere. Including the complete Coriolis force increases
the cross-equatorial transport, particularly in the case of weak dissipation, when the portion
of the current that enters the northern hemisphere does so as a series of eddies.
Fourier analysis of the time series for the channel exit flux shows that qualitative
features of the behaviour, such as the eddy timescale and the peak outflow, are also
substantially modified. Numerical integrations in the realistic equatorial bathymetry yield
qualitatively similar results to the idealised channels, but the paths of the currents entering
and exiting the actual channel are strongly steered by the bathymetric contours. |
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