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Influence of the Ocean Currents on Climate
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The fundamental aspects of the influence of Ocean currents on Climate are addressed. Collaboration with Professeur Henk Dijkstra, Director of IMAU laboratory (Institute for Marine and Atmospheric Research Utrecht), Netherlands.
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Aim, Methodology, Application:
We are interessed in the fundamental aspects of wind-driven global oceanic circulation. The methodology of modern theory of Dynamic systems is used. The steady states of the currents and their variability are carried out and studied in the space of the parameters : the Ekman number Ek, the Rossby number e, the Froude number F and the wind stress strength a.
In order to describe the dynamics of the set of non-linear
equations, two methodologies can be used.
The first, and most
often used in fluid mechanics, is to build a time-stepping code in
order to follow the time evolution of the system from various initial
conditions, simulating the physical phenomena to be explained.
But,
only the stable steady states are thus carried out as the numerical
residu due to the approximation of the time integration used plays
the role of a perturbation for the system.
This methodology leads
to the impossibility of calculating the unstable steady states and
having the complete picture of all the steady states necessary to
understand the physical mechanisms involved in the dynamics of the
system under focus.
From the practical point of vue, the waist of
time integration due to the transient before the time-stepping code
converges to the asymptotic stable steady states can be very
important.
Furthermore, the framing of the critical value of a
control parameter at a bifurcation requires an important Cpu budget
as the fluctuations in the neighborhood of the threshold is
proportional to the square root of the value and the critical value
of the control parameter.
We have choosen the second methodology
using the framework of the theory of the Theory of the Dynamical
Systems. It consists of the calculation of the all steady states (as
solutions of the steady equations ) carried out by the Newton method
coupled with a continuation method allowing to follow the branches of
the steady states calculated as a function of the control parameter
even when the system undergoes a bifurcation where the jacobian is
singular.
The global wind-driven circulation with constant density are modelised and simulated. We mainly use a shallow-water 1.5 layer barotropic model constructed with H.A. Dijkstra. The geography of the continents, the bathymetry and the wind-stress forcing used are realistic datas from experiments (for present circulation) or reconstruction (for paleooecanography).
The steady shallow-water equations are discretized spatially
on the domain using a finite difference discretization on a staggered
grid NxM. The steady states are carried out by the Newton method
coupled with a continuation method.
In addition, the
eigenspectrum of the jacobian is calculated along the branch, in
order to carry out the Leading eigen values (i.e. those of the
greatest real part) and vectors associated with transitions and loss
of linear stability.
We apply this methodoly to :
- paleo-oceanography
-
variability and the global influence of the antarctic circumpolar
currenton the dynamics of the global circulation.
Computing Facilities:
No simulations and results without computing facilities!
We thank the two Computing Academic Centers :
SARA, Amsterdam, Netherlands : computing facilities used untill now for paleo-oceanography and the Antarctic Circumpolar Current. (ACC).
IDRIS, Orsay, France : computing facilities which will be used in the near future for the research on the ACC.
Figures:
The following Figures have been done with IDL
software.
Influence of Ocean currents on
Climate:
paleo-oceanography
We are interessed in modelization of paleo
circulation, mainly the cretaceous (See Fig. 1).
This
kind of research is likely to be interested for
paleo-oceanographers, biologists, sedimentologists. As a matter of
fact, the results from modelization appear provide support on the
existence of currents, thus on their influence on Climate system,
propagation of spieces, sediments, etc.
A paper is published [ I ] in Earth & Planetary Sciences Letters journal. The problem of the existence of a Tethys Circumglobal Current (TCC) in the Late Cretaceous continental geometry (Campanian) is addressed. Within an ocean model which is expected to strongly overestimate the wind-driven TCC volume transport, a relatively weak TCC is found for the reconstructed Campanian paleogeography used. As a measure of the strength of the TCC, a Circumglobal Flow Index is introduced. This index is based on volume fluxes through meridional sections within the equatorial part of the domain. The impact of changes in the paleogeography on the TCC is considered by computing steady flows for different shapes of Eurasia. The results on the extent of Eurasia such that a strong TCC appears provide support that no strong wind-driven TCC has existed during the Campanian.
Further details in : [pdf version of I]
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This work was supported by the Netherlands Organization for
Scientific Research (NWO) under a PIONIER grant to HD.
We thank
R.M. DeConto (National Center for Atmospheric Research, Boulder,
Colorado) for providing the Campanian continental geometry and
the wind stress fields through D. Pollard (Earth System Science
Center, Pennsylvania State University).
We thank E.C. Brady
(National Center for Atmospheric Research, Boulder, Colorado) for
providing the bathymetry.
All computations were performed on the
Origin 3800 at the Academic Computing Centre (SARA), Amsterdam, the
Netherlands within the project SC029.
We thank use of these
computing facilities was sponsored by the National Computing
Facilities Foundation with financial support from NWO.
Influence of Ocean currents on Climate:
the
Antarctic Circumpolar Current (A.C.C.)
The Circumpolar region around the
Antarctic between 56 and 62 S is the only area on Earth without any
longitudinal continental boundaries. Thus, the ACC is the only
current like an annular jet constantly powered by the predominant
westerlies. The ACC dynamics is an essential component of the climate
system as the ACC is one of major cold-water reservoir and is
implied as an annular jet in redistribution of physical properties
between the three oceans: Atlantic, Pacific and Indian.
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Fig 2. The annual mean value of the wind stress field interpolated from Trenberth & al (1989) for the present geography of the continents and on a 2°x2° grid. Only one point in two is plotted for clarity and the wind stress values are scaled by the maximum which is about 0.3 Pa. |
Bibliographie
[Brady1998]
E.C. Brady, R.M.
DeConto, S.L. Thompson,
Deep water formation and poleward heat
transport in the warm climate extreme of the Cretaceous (80 Ma),
Geophys. Res. Letters, 25, (1998) 4205-4208.
[Hay1999]
W.W. Hay, R.M.
DeConto, C.N. Wold, K.M. Wilson, S. Voigt, M. Schultz,
A.
Wold-Rossby, W.C. Dullo, A.B. Ronov, A.N. Balukhovsky, E. Söding
Alternative global Cretaceous paleogeography,
in : E. Barrera
and C.C. Johnson, eds.,
Evolution of the Cretaceous
Ocean-Climate system,
(The Geological Society of America,
special paper 332, 1999), 1 - 48.
[Trenberth1989]
K.E. Trenberth,
J.G. Olson, W.G. Large,
A global ocean wind stress
climatology based on {ECMWF} analyses.
Technical report, National
Center for Atmospheric Research, Boulder, CO, U.S.A, 1989.
