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Dynamics of the Leeuwin Current: Part 2. Impacts of mixing, friction, and advection on a buoyancy-driven eastern boundary current over a shelf
Citation
Benthuysen, J and Furue, R and McCreary, JP and Bindoff, NL and Phillips, HE, Dynamics of the Leeuwin Current: Part 2. Impacts of mixing, friction, and advection on a buoyancy-driven eastern boundary current over a shelf, Dynamics of Atmospheres and Oceans, 65 pp. 39-63. ISSN 0377-0265 (2014) [Refereed Article]
Copyright Statement
Copyright 2013 Elsevier
DOI: doi:10.1016/j.dynatmoce.2013.10.004
Abstract
The boundary currents over the Western Australian continental
shelf and slope consist of the poleward flowing Leeuwin Current
(LC) and the equatorward flowing Leeuwin Undercurrent (LUC).
Key properties of the LC are its poleward strengthening, deepening
to the south, and shelfbreak intensification. The alongshore
flow reverses direction below about 300 m, forming the LUC at
greater depths. To investigate the processes that cause these features,
we obtain solutions to an idealized, regional ocean model
of the South Indian Ocean. Solutions are forced by relaxing surface
density to a prescribed, meridionally varying density profile
∗(y) with a timescale of ıt. In addition, vertical diffusion is intensified
near the ocean surface. This diffusion establishes the minimum
thickness over which density is well-mixed. We define this thickness
as the "upper layer". Solutions are obtained with and without a
continental shelf and slope off Western Australia and for a range of
values of ıt and mixing parameters. Within this upper layer, there
is a meridional density gradient that balances a near-surface, eastward
geostrophic flow. The eastward current downwells near the
eastern boundary, leading to westward flow at depth. The upper
layer’s meridional structure and zonal currents crucially depend
on coastal processes, including the presence of topography near
the eastern boundary. Kelvin waves inhibit the upper layer from
deepening at the coast. Rossby waves propagate the coastal density
structure offshore, hence modifying the interior currents. A
comparison of the solutions with or without a continental shelf
and slope demonstrate that topographic trapping of Rossby waves
is a necessary process for maintaining realistic eastern boundary
current speeds. Significant poleward speeds occur only onshore of
where the upper layer intersects the slope, that is, at a grounding
line. Its poleward transport increases when surface-enhanced vertical
mixing is applied over a greater depth. When the timescale ıt
is sufficiently short, the poleward current is nearly barotropic. The
current’s spatial structure over the shelf is controlled by horizontal
mixing, having the structure of a Munk layer. Increasing vertical
diffusion deepens the upper layer thickness and strengthens the
alongshore current speed. Bottom drag leads to an offshore flow
along the bottom, reducing the net onshore transport and weakening
the current’s poleward acceleration. When ıt is long, poleward
advection of buoyancy forms a density front near the shelf break,
intensifying poleward speeds near the surface. With bottom drag,
a bottom Ekman flow advects density offshore, shifting the jet core
offshore of the shelf break. The resulting cross-shelf density gradient
reverses the meridional current’s direction at depth, leading to
an equatorward undercurrent.
Item Details
Item Type: | Refereed Article |
---|---|
Keywords: | Leeuwin Current, oceanography |
Research Division: | Earth Sciences |
Research Group: | Oceanography |
Research Field: | Physical oceanography |
Objective Division: | Environmental Management |
Objective Group: | Marine systems and management |
Objective Field: | Oceanic processes (excl. in the Antarctic and Southern Ocean) |
UTAS Author: | Benthuysen, J (Dr Jessica Benthuysen) |
UTAS Author: | Bindoff, NL (Professor Nathan Bindoff) |
UTAS Author: | Phillips, HE (Associate Professor Helen Phillips) |
ID Code: | 89039 |
Year Published: | 2014 (online first 2013) |
Web of Science® Times Cited: | 14 |
Deposited By: | IMAS Research and Education Centre |
Deposited On: | 2014-02-24 |
Last Modified: | 2017-11-01 |
Downloads: | 0 |
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