Ariane, a Lagrangian analysis application for ocean circulation models
developed by Bruno Blanke and Nicolas Grima
General introduction
Hello!
My name is Bruno Blanke.
I am a researcher at CNRS (Centre National de la Recherche Scientifique)
More precisely, I am a physical oceanographer
and I conduct my research at the Laboratoire d’Océanographie Physique et Spatiale,
the LOPS,
in Brest (France).
For many years,
I have been interested in the Lagrangian description
of ocean circulation.
For this,
I wrote a program
dedicated to the computation of trajectories in the ocean,
Ariane.
The objective of this online course
is to present, in eight parts,
the rationale
and the main functions of this Lagrangian analysis program.
In this course,
we will first present some contextual generalities about the tool.
Then, we will take a look at the differences
between an Eulerian description
and a Lagrangian description of velocities.
It will then be useful to introduce
the equations used in ocean modelling,
with a quick overview
of the main historical milestones.
We will then present the details of the trajectory calculation,
made by Ariane.
We will then discuss the ideas behind
some diagnostic analyses that can be performed
with these trajectories,
first for a qualitative approach,
then for a quantitative approach.
Finally, we will discuss the notion
of Lagrangian stream function,
specific to Ariane.
Let us start by presenting Ariane in one sentence:
Ariane is a program for computing trajectories, in an oceanic velocity field provided by the user.
An article published in 2018
outlines the scientific background
of numerical computation of trajectories in the ocean
and its applications.
In this article,
the Ariane tool is presented,
along with other programs,
with numerous bibliographic references
that expand the understanding
of Lagrangian methods and issues.
Let’s go back over a little mythology.
'Aria(d)ne' in the program name
is a direct reference to Ariadne’s thread.
In Greek mythology,
Ariadne is the daughter of Minos,
the king of Crete.
She falls in love with Theseus,
a mythical king of Athens.
She helps Theseus to escape from the labyrinth of the Minotaur
a half-man, half-bull monster,
thanks to a ball of thread she gives him.
This unwound thread
can be seen as a trajectory,
more or less complex,
in the environment of a labyrinth
much like a turbulent velocity field.
The origins of the Ariane code
date back to spring 1992.
At the end of my PhD on the modelling of tropical ocean dynamics,
I thought it would be useful
to be able to conveniently calculate
the motion of water masses at the surface and in the interior of the ocean.
In this PhD,
subsurface geostrophic convergence,
equatorial upwelling
and surface Ekman divergence
were indeed interesting dynamic features to illustrate.
In practice,
the first available reference to the calculation of trajectories
with the Ariane formalism
is found in another thesis,
published at the end of 1992,
with the illustration of particle trajectories
in the Alboran Basin,
downstream of the Strait of Gibraltar.
This thesis, by Sabrina Speich, is entitled:
Study of the forcing of ocean circulation by straits: the case of the Alboran Sea.
Ariane is a utility for Lagrangian analysis
of the velocity field computed
and archived by an ocean general circulation model,
a priori three-dimensional.
More generally,
Ariane computes the trajectories of virtual particles
in the velocity field provided by the user.
Since 2007,
thanks to Nicolas Grima,
Ariane has been distributed under a CeCILL license.
Companion online course materials by Nicolas
explain the more technical aspects
of the installation, compilation
and use of the Ariane program.
CeCILL is an acronym for
CEA,
CNRS,
INRIA,
logiciel libre [open software].
This is a free license under French law,
compatible with the GNU-GPL license,
and proposed by three major French research organisations,
including the CNRS,
which employs Nicolas and me.
The Ariane tool was developed mainly to
analyse velocity fields computed by the OPA/NEMO community ocean model.
It was then successfully adapted
to ROMS/AGRIF.
This version was adjusted for the SYMPHONIE model.
There are also adaptations
to the MARS3D model
and to the MOM model,
which have had variable feedback.
The number of requests to access the Ariane source code
now amounts to several hundreds of people.
If we make a count over the last 8 years
of scientific publications referencing Ariane,
because researchers use it,
we see that the majority of these publications
deal with physical oceanography.
A significant part of these publications,
which is increasing with time,
deals with biogeochemistry and ecosystems.
This indicates the tool's versatility,
its flexibility and
its adaptability to
fields of study that are constantly evolving and even expanding.
The Ariane tool integrates two essential functions.
The first is the calculation of trajectories,
three-dimensional by default,
with an original calculation core
based on an analytical approach.
The other function is
the construction of online diagnostics,
with the automatic calculation of initial positions
on control sections
for so-called quantitative applications,
and the calculation of Lagrangian transport components,
which we will discuss in detail in the last part of this course.
Even though the two functions,
trajectory calculations and
first Lagrangian diagnoses
are formally well separated in their principles and roles,
their intertwining in the Ariane code is strong,
mainly because the Lagrangian diagnoses
take full advantage of the way the trajectories are computed.
Ariane works with a velocity field provided by the user.
It is essential to keep in mind that
Ariane does not create any additional information
about ocean dynamics.
All the information is contained in the Eulerian data provided by the user.
On the other hand,
Ariane allows us to illustrate, interpret and analyse
this velocity field in a complementary way,
thanks to a Lagrangian vision.