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Jonas Nycander
Department of Meteorology
Stockholm University
S-106 91 Stockholm, Sweden
phone: +46-(0)8-164336
email: jonas@misu.su.se
Fundamentals of Ocean Circulation
As most people know, the atmospheric circulation is thermally driven by
the differential heating from the sun. Thermodynamically speaking, it is
a heat engine. However, contrary to what many people think, this is not
so for the ocean circulation, which is mechanically driven.
This was demonstrated about a century ago by the Swedish oceanographer
Sandström, who performed a simple laboratory experiment. In a vessel
filled with water he introduced a heating source and a cooling source, in
the form of metal tubes with runnig hot or cold water inside. He found that
when the heating was at a lower level than the cooling, a vigorous circulation
was excited between the two levels. If, on the other hand, they were at the
same level, the circulation was very weak, and confined to a thin layer.
Thus, if there were only the heating and cooling of the ocean surface
by the sun and the atmosphere, but no mechanical forcing, there would exist
a significant flow only in a thin layer just below the surface, driven by
molecular heat conduction. The rest of the ocean would be filled with stagnant
cold water. But in reality there are strong ocean currents at great depths.
According to the conventinal picture of the overturning circulation of
the ocean, dense and cold water sinks to the bottom at narrow convection
sites in the North Atlantic and near Antarctica, and rises again in a broad
upwelling in the tropics and midlatitudes.
As the water rises it must also be heated, in order to attain the higher
temperature prevailing at smaller depths. This can only be accomplished
by turbulent mixing, since the molecular heat conductivity is much too low.
The turbulence and the mixing are caused by breaking internal waves. These
waves are excited by winds and by tidal flows that encounter rough topography
on the bottom of the ocean. In this view, therefore,
the deep circulation of the ocean is driven by the small-scale mixing, and
ultimately by winds and tides.
My research
- Internal tides
By using linear wave theory, it is possible to compute the radiation of
internal waves that are generated by tides over bottom topography. The figure
shows the result of such a computation for the global ocean, and is taken
from a recent paper ( pdf file
).
The color represents radiated power, in units of Watts per square meter,
and the scale is logarithmic; for example, -3 represents 1 mW/m2.
The input data used in the computation were the bottom topography with a
resolution of 2 nautical miles, the tidal velocity field, and the
stratification of the ocean.
About half of the internal wave energy was in this computation generated
in points where the bottom slope is supercritical (i.e. the slope is greater
than the slope of the internal wave rays). At these points the linear theory
on which the computation was based is not valid. Will the nonlinear effects
caused by supercriticality increase or decrease the radiated power compared
to the prediction of linear theory? This is currently an area of very active
research, but the answer in still not known. Personally, I strongly believe
that the nonlinear effects will decrease the radiation. The arguments for
this are presented in a recent paper
( pdf file ), which also contains
an analytic solution to a simple test problem that gives some (but not very
strong) support for this hypothesis. Much more remains to do in this field.
- Analysis of ocean circulation
Oceanic currents mainly flow along isopycnal surfaces (i.e. surfaces
of constant density). However, from many points of view, the small
component of the flow which is perpendicular to these surfaces is the most
interesting one, since it necessarily involves irreversible energy
transformations, connected with mixing. This is, for example, the most
important aspect of the conveyor belt circulation.
In a recent paper
( pdf file ),
we propose calculating a stream function as function of depth and density
as a new way of analysing the thermodynamic character of the
overturning circulation. Since depth is linearly
related to pressure, and density is the inverse of specific volume,
the circulation is in effect displayed in a classical thermodynamic
pV-diagramme. The sign of an overturning cell in this diagramme
directly shows whether it is driven mechanically by large-scale wind
stress, or 'thermally' by heat conduction and small scale mixing.
The figure below shows this stream function for the high-resolution global
ocean model OCCAM, with potential density and depth on the axes.
The color represents the stream function in Sverdrups
(106 m3/s), with clockwise circulation in negative
overturning cells. Three main cells are seen in the figure: 1) a tropical
thermal cell in the warmest and shallowest waters, 2) a mechanically driven
cell (i.e. the sinking water is lighter than the rising water) dominating
depths between 200 m and 2000 m, 3) a thin thermal cell in the very
densest waters. Only the third cell is compatible with the conventional
ideas about the conveyor belt circulation.
- Vortex Dynamics
Vortices are common in many parts of the ocean. They can be generated by
instabilities in frontal currents (for example the Gulf Stream rings), by
intrusion of water into a basin where the temperature and salinity is different
(for example the outflow of Meditteranean water into the Atlantic, which creates
Meddies), by flow over topography, or by other mechanisms. I have tried
to understand various properties (such as stability or drift speed) of such
vortices.
In this paper, a very general
variational principle was used to understand the properties of vortices
attached to seamounts. It was found that there exists a large class of
stationary and stable anticyclonic vortices that are attached to a given
localized seamount of arbitrary shape. If the
seamount is circular there are also stable cyclones, but these are destabilized
by noncircularities in the topographic shape, unlike the anticyclones.
In this paper, the same variational
principle is applied to flow in closed basins with sloping boundaries. It is
shown that weak (Ro << 1) cyclonic basin flow with monotonic radial profile of
potential vorticity (PV) is nonlinearly stable, even if the basin has an
irregular shape. By contrast, anticyclonic flow can only be shown to be stable
if the basin is circular. If the basin is non-circular, anticyclonic flows can
therefore be expected to be unstable, even if the PV-profile is monotonic.
This striking difference between cyclonic and anticyclonic flows is confirmed
by both numerical simulations and laboratory experiments
with non-circular basins. They demonstrate that cyclonically forced flows
nicely follow the isobaths, while corresponding anticyclonically forced flows
develop strong cross-slope flows. This agrees with the real ocean, where
cyclonic flows tend to follow isobaths closely, while anticyclonic flows
(such as the Gulf Stream) separate more easily from the topography.
Recent publications and manuscripts under review
- Nycander, J. 2009
Energy conversion, mixing energy and neutral surfaces
with a nonlinear equation of state
Journal of Physical Oceanography (submitted)
pdf file
- Zarroug, M., Nycander, J. and Döös, K. 2009
Energetics of tidally-generated internal waves for nonuniform stratification
Tellus A (in press)
pdf file
- Bahrami, F., Nycander, J. and Alikhani, R. 2009
Existence of energy maximizing vortices in a three-dimensional
quasigeostrophic shear flow with bounded height.
Nonlinear Analysis: Real World Applications (in press)
pdf file
- Magnusson, L., Nycander, J. and Källen, E. 2009
Flow-dependent versus flow-independent initial perturbations for ensemble
prediction.
Tellus 61A, 194-209.
pdf file
- Nost, O.A., Nilsson, J. and Nycander, J. 2008
On the asymmetry between cyclonic and anticyclonic flow in basins with sloping boundaries.
Journal of Physical Oceanography 38, 771-787.
pdf file
- Jönsson, B., Döös, K., Nycander, J. and Lundberg, P. 2008
Standing waves in the Gulf of Finland and their relationship to the
basin-wide Baltic seiches.
Journal of Geophysical Research 113, C03004, doi:10.1029/2006JC003862.
pdf file
- Magnusson, L., Källen, E. and Nycander, J. 2008
Initial state perturbations in ensemble forecasting.
Nonlinear Processes in Geophysics 15, 751-759.
pdf file
- Nycander, J., Hogg, A.M. and Frankcombe, L.M. 2008
Open boundary conditions for nonlinear channel flow.
Ocean Modelling 24, 108-121.
pdf file
- Turnewitsch, R., Reyss, J.-L., Nycander, J., Waniek, J.J., and
Lampitt, R.S. 2008
Internal tides and sediment dynamics in the deep sea - evidence from
radioactive 234Th/238U disequilibria.
Deep-Sea Research I 55, 1727-1747.
pdf file
- Döös, K., Nycander, J. and Coward, A. C. 2008
Lagrangian decomposition of the Deacon Cell.
Journal of Geophysical Research 113, C07028, doi:10.1029/2007JC004351.
pdf file
- Nycander, J., Nilsson, J., Döös, K. and Broström, G. 2007
Thermodynamic analysis of ocean circulation.
Journal of Physical Oceanography 37, 2038-2052.
pdf file
- Jakobsson, M. et al. 2007
The early Miocene onset of a ventilated circulation regime in the Arctic
Ocean.
Nature 447, 986-990.
- Bahrami, F. and Nycander, J. 2007
Existence of energy minimizing vortices attached to a flat-top seamount.
Nonlinear Analysis: Real World Applications 8, 288-294.
pdf file available by email
- Nycander, J. 2006
Tidal generation of internal waves from a periodic array of steep ridges.
Journal of Fluid Mechanics 567, 415-432.
pdf file
- Nycander, J. 2005
Generation of internal waves in the deep ocean by tides.
Journal of Geophysical Research 110, C10028, doi:10.1029/2004JC002487.
pdf file
- Benilov, E.S., Nycander, J. and Dritschel, D.G. 2004
Destabilisation of barotropic flows by small-scale topography.
Journal of Fluid Mechanics 517, 359-374.
pdf file
- Marchal, O. and Nycander, J. 2004
Nonuniform upwelling in a shallow-water model of the Antarctic Bottom Water
in the Brazil Basin.
Journal of Physical Oceanography 34, 2492-2513.
pdf file
- Nycander, J. and LaCasce, J.H. 2004
Stable and unstable vortices attached to seamounts.
Journal of Fluid Mechanics 507, 71-94.
pdf file
- Döös, K., Nycander, J. and Sigray, P. 2004
Slope dependent friction in a barotropic model.
Journal of Geophysical Research 109, C01008, doi:10.1029/2002JC001517.
pdf file
- Nycander, J. and Döös, K. 2003
Open boundary conditions for barotropic waves.
Journal of Geophysical Research 108 (C5), 37.
pdf file
- Nycander, J. 2003
Stable vortices as maximum or minimum energy flows.
In: O.U. Velasco Fuentes, Julio Sheinbaum, Jose Luis Ochoa Torre (Eds.),
Nonlinear Processes in Geophysical Fluid Dynamics, Kluwer, Dordrecht.
pdf file
- Nycander, J. and Emamizadeh, B. 2003
Variational problem for vortices attached to seamounts.
Nonlinear Analysis 55, 15-24.
pdf file
- Naulin, V, Rasmussen, J.J. and Nycander, J. 2003
Transport barriers and edge localized mode-like bursts in a plasma model with turbulent equipartition profiles.
Physics of Plasmas 10, 1075-1082.
pdf file
Erratum.
Physics of Plasmas 10, 3804.
pdf file
- Nycander, J., Döös, K. and Coward, A.C. 2002
Chaotic and regular trajectories in the Antarctic Circumpolar Current.
Tellus 54A, 99-106.
pdf file
Here is a link to the home
page of my wife Maria Nycander. There you can find an interesting report
(in Swedish) about the grades of boys and girls in Swedish schools.