I used to be an oceanographer but now I work with computers. Here is a list of my publications from this previous life. Click on the title to view the abstract.
Refereed
2000
- On the Insensitivity of the Wind-Driven Circulation to Bottom Topography
- D. P. Marshall and J. C. Stephens, J. Mar. Res., 59,
1-27.
- Instabilities of Gravity Currents Along a Slope
- S. P. Meacham and J. C. Stephens, J. Phys. Oceanogr., 31,
30-53.
- Dynamical Pathways of Antarctic Bottom Water in the Atlantic
- J. C. Stephens and D. P. Marshall, J. Phys. Oceanogr., 30,
622-640.
1999
- Dynamics of the Mediterranean Salinity Tongue
- J. C. Stephens and D. P. Marshall, J. Phys. Oceanogr., 29,
1425-1441.
Unrefereed
1999
- A Numerical Simulation of the Mediterranean Outflow in the Gulf of Cadiz
- J. C. Stephens and R.W. Hallberg, Uncompleted Manuscript.
1998
- Aspects of the Oceanic Thermohaline Circulation
- J. C. Stephens, PhD thesis, University of Reading, UK, 165pp.
1997
- Linear Stability Analysis of a Gravity Current Over Topography
- J. C. Stephens, Technical Report WHOI-97-10, Summer Program in Geophysical Fluid Dynamics, Woods Hole, MA 02543, USA.
J. C. Stephens, 1998, PhD thesis, University of Reading, UK, 165pp.
Abstract
Two complementary aspects of the thermohaline circulation are considered. In Part I, we examine the linear stability of rotationally-influenced gravity currents with zero potential vorticity, flowing over a topographic slope. Several classes of unstable normal mode are found, and the theory of resonant instabilities provides a simple explanation of the origin of these instabilities and allows one to identify the physical mechanisms primarily responsible for them. Two new types of instability that rely on the presence of topography are found. The first, which is relatively weak, arises as a result of a resonance between gravity waves on the jet and topographic Rossby waves. The second is more powerful, occurs at relatively long wavelengths, and is brought about by a resonance between a vortical wave on the jet and topographic Rossby waves.
In Part II, we develop a 2.5 layer planetary-geostrophic model of the North Atlantic, driven by climatological winds and eastern-boundary ventilation, to investigate the effect of the wind-field in shaping the Mediterranean salinity tongue. The upper-layer depth from our model shows a clear similarity to observations, both in terms of the location and intensity of the subtropical gyre and also the position of the outcropping line in the northern basin. Potential vorticity in layer two reproduces the sweep of potential-vorticity contours south-westwards from the eastern boundary, and provides the pathways along which Mediterranean Water spreads into the model interior. We solve for the steady salinity field in the second layer, including sources of Upper Labrador Sea Water and Antarctic Intermediate Water on the isopycnal surface. The shape and spreading latitude of the model salinity tongues bear a striking resemblance to observations. We also include a simple parameterisation of meddies, which we show act to push the salinity tongue southward.
This manuscript is available as a gzipped postscript file of uncompressed size 33.9 Mb in either a4 or letter format.
D. P. Marshall and J. C. Stephens, 2000, J. Mar. Res., 59,
1-27.
Abstract
An analytical model is developed for the wind-driven circulation in a continuously stratified ocean overlying variable bottom topography. Following Welander (1971), a linear relation between potential vorticity, density, and Bernoulli potential is assumed in the ocean interior, resulting in an adiabatic internal thermocline.
The horizontal structure of the circulation is described by a characteristic equation, obtained by imposing a boundary condition of no-normal flow at the sea floor and a prescribed vertical velocity through the base of the surface Ekman layer. The characteristics, which determine the extent to which bottom topography “steers” the circulation within the upper ocean, are dominated by latitude circles at low latitudes, but are increasingly influenced by the bottom topography at higher latitudes as the thermocline widens and intersects the sea floor. A solution is evaluated for the full three-dimensional circulation in the North Pacific. We find classical Sverdrup gyres within the thermocline, increasingly zonal flows at mid-depths, and weak topographically-bounded gyres within the abyssal ocean.
J. C. Stephens and R. W. Hallberg, 1999, Uncompleted Manuscript.
Abstract
We have conducted numerical simulations of the Mediterranean overflow in the vicinity of the Gulf of Cadiz, using a primitive equation isopycnal model in which the exchange between the Atlantic and the Mediterranean via the Strait of Gibraltar is explicitly resolved. Diapycnal mixing is represented using the recent Richardson number dependent mixing parameterization of Hallberg (99).
At 1/6 degree horizontal resolution and with 16 vertical layers, the model is able to represent the entrainment, descent in the water column, and water mass properties of the overflow current substantially better than in an identical case without the parameterization. Moreover, upon the inclusion of a quadratic drag law and a non-linear equation of state, the similarity of the solutions to observations is striking. At 1/3 degree horizontal resolution, many aspects of the Mediterranean water mass properties are also reproduced satisfactorily.
We observe plausible transports and properties of Mediterranean Water through the model Strait of Gibraltar, with little dependence on horizontal resolution. This suggests the possibility that future climate simulations using layered models attempt to explicitly resolve this exchange. In combination with a similar mixing scheme we expect to see fundamental improvements in the fidelity of deep and intermediate water masses.
This draft manuscript is available as a gzipped postscript file of uncompressed size 18 Mb here.
J. C. Stephens and D. P. Marshall, 1999, J. Phys. Oceanogr., 29,
1425-1441.
Abstract
A reduced-gravity planetary-geostrophic model of the North Atlantic, consisting of two active layers overlying a motionless abyss is developed to investigate the effect of the windfield in shaping the dynamics of the Mediterranean salinity tongue. The model is driven by climatological winds and eastern-boundary ventilation in a basin of realistic geometry and includes a parameterisation of meddies.
The upper-layer depth from our model shows a clear similarity to observations, both in terms of the location and intensity of the subtropical gyre and also the position of the outcropping line in the northern basin. Potential vorticity in layer two reproduces the sweep of potential-vorticity contours southwestwards from the eastern boundary and extending westwards into the interior, and provide the pathways along which Mediterranean Water spreads into the model interior.
We solve for the steady salinity field in the second layer, including sources of Upper Labrador Sea Water and Antarctic Intermediate Water on the isopycnal surface. The shape and spreading latitude of the model salinity tongues bear a striking resemblance to observations. Both the windforcing, and the occurrence of a mean transport of Mediterranean Water away from the eastern-boundary, are crucial in obtaining a realistic salinity tongue. The salinity tongues are remarkably stable to variations in the Peclet number.
We also include a simple parameterisation of meddies in the model. Where meddies are dissipated locally by collisions with topographic seamounts, for example, they may generate large recirculations extending across to the western boundary. The net effect of these recirculations is to shift the salinity tongue equatorward.
J. C. Stephens, 1997, Technical Report WHOI-97-10, Summer Program in Geophysical Fluid Dynamics, Woods Hole, MA 02543, USA.
Abstract
Gravity currents are important for transporting heat, salt and momentum in oceanic, atmospheric and other environments. The Denmark Straits overflow, for example, in the North Atlantic transports significant quantities of cold water equatorward, whilst the Mediterranean Undercurrent is an important source of heat and salt in the eastern North Atlantic. On a smaller scale, gravity currents may be observed in lakes if the deformation radius is small enough that they feel the Earth’s rotation.
A study of a two-fronted current on a slope in a one layer reduced gravity system was considered by Griffiths et al. (1982). They discovered that a zero potential vorticity current was unstable to linearised perturbations over a finite range of wavenumbers. The instabilities take the form of normal modes that couple the free streamlines of the current and extract both kinetic and potential energy from the flow. In the long wavelength limit, these become sinuous modes.
Here we extend the study conducted by Griffiths et al. to a two layer system. It is found that the presence of the upper layer in combination with the slope serves to modify the shear instability and stabilises the flow to long wavelength perturbations.
The contents of this manuscript have been incorporated into the above paper “Instabilities Of Gravity Currents Along a Slope”, which contains a more complete discussion of such instabilities.
S. P. Meacham and J. C. Stephens, 2000, J. Phys. Oceanogr., 31, 30-53.
Abstract
This work examines the linear stability of rotationally influenced density currents with zero potential vorticity flowing over a sloping sea-floor at the base of an ocean of finite depth. This configuration serves as a crude model of a type of current that is common in the ocean. The normal modes of the jet are classified according to the physical process primarily responsible for the mode in the limit of long along-jet wavelengths. Several classes of unstable normal mode are found. The theory of resonant instabilities provides a simple explanation of the origin of these instabilities and allows one to identify the physical mechanisms primariliy responsible for the instabilities. In addition to previously recognised instabilities such as that noted by Griffiths, Killworth and Stern (1982), two new types of instability that rely on the presence of topography are found. The first, which is relatively weak, arises as a result of a resonance between gravity waves on the jet and topographic Rossby waves. The second is more powerful, occurs at relatively long wavelengths, and is brought about by resonance between a vortical wave on the jet and topographic Rossby waves.
J. C. Stephens and D. P. Marshall, 2000, J. Phys. Oceanogr., 30,622-640.
Abstract
A reduced-gravity model is developed to represent the flow of Antarctic Bottom Water (AABW) over realistic bathymetry in the Atlantic domain. The model dynamics are based on the steady, planetary-geostrophic, shallow-water equations, including a linear bottom friction and a prescribed uniform upwelling through the top of the model layer.
The model solutions are broadly consistent with observations of the distribution and transport of AABW. The flows occur predominantly along potential vorticity contours, which are in turn broadly oriented along bathymetric contours. The characteristic weak flow across potential vorticity contours of the Stommel-Arons model is present as a small addition to the stronger forced mode along potential vorticity contours. As a consequence, mass balance is maintained not by hypothesised western boundary currents as in the Stommel-Arons model, but by the interplay between topographic slope currents and interior recirculations. In particular, we find a transition in the flow of AABW from the western side of the Brazil Basin south of the equator, to the western flank of the Mid-Atlantic Ridge north of the equator. This is also consistent with an analytical result derived by extending Parson’s mechanism to an abyssal layer overlying arbitrary bathymetry. We suggest that our results provide a more convincing zero-order picture than the Stommel-Arons model for the circulation of AABW, and perhaps for abyssal water masses in general.
