Orateurs invités, résumés des exposés et slides

Vertical mixing is often regarded as the Achilles' heel of ocean models. Here we review recent progress in
parameterizing vertical ocean mixing beneath the surface boundary layer. We also highlight persistent gaps in
connecting the resolved large-scale flow with parameterized mixing in global ocean models.

Impact of finescale currents on biogeochemical cycles in a changing ocean

Marina Levy

Finescale currents, O(1-100 km, days-months), are actively involved in the transport and transformation of 
biogeochemical tracers in the ocean. In this presentation, I will discuss current knowledge of their overall impact 
on large-scale biogeochemical cycling on the time scale of years, which remain poorly understood due to the 
multi-scale nature of the problem. I will present the history of the study of these impacts, which have started 
roughly 30 years ago. I will examine how eddy-fluxes and upscale connections enter the large-scale balance of 
biogeochemical tracers. I will show that the overall contribution of eddy-fluxes to primary production and carbon
export may not be as large as for oxygen ventilation. I will highlight the importance of finescales to low-frequency 
natural variability through upscale connections. I will show that finescales may buffer the negative effects of climate 
change on the functioning of biogeochemical cycles. These different points will be illustrated using recent (mostly >
2018) work conducted at LOCEAN-IPSL. I will call for significant interdisciplinary efforts to properly account for the 
cross-scale effects of finescales on biogeochemical cycles in climate projections.

Bifurcations et singularités

Bérengère Dubrulle

slides 

En turbulence, on observe de nombreuses bifurcations globales dans des milieux
très turbulents, que ce soit dans des expériences de laboratoires ou dans des
simulations numériques, en 2D comme en 3D, avec ou sans scalaire ou vecteurs
actifs.  Ces bifurcations donnent lieu à des changements très brutaux des
circulations.  Des telles bifurcations (appelées aussi points de bascule par les
climatologues) sont aussi suspectées (comme dans la circulation Nord Atlantique)
ou craintes (comme dans le système climatique) en geo-sciences.  Après avoir
rappelé brièvement quelques modèles et propriétés intéressantes de ces
bifurcations, notamment en ce qui concerne leur persistance, je discuterai d’un
nouveau type de bifurcation globale, dans lequel la bifurcation est provoquée
par une (quasi-) singularité.  Je résumerai les propriétés de ces bifurcations
et discuterai de leur intérêt ou applicabilité en turbulence ou en climat.

Clustering transition in natural convection induced by radiatively heated particles

Remi Zamansky

We consider the heating of particles in a dilute suspension, for instance by
radiation, and the convective flow caused by the buoyancy fluctuations. For very
small particle inertia, the macroscopic behavior of the system is the result of
the many thermal plumes that are generated independently of each other. On the
other hand, when the inertia of the particles is sufficient, we observe that the
thermal plumes tend to segregate the particles and that the locally higher
particle concentrations induce more intense thermal plumes which in turn lead to
more segregated particles and so on. This feedback loop is manifested by a sharp
modification of the behavior of the system. In particular, we observe that the
system becomes independent of the average particle concentration. This
observation enables estimating the critical exponent characterizing the system
close to this clustering transition. This behavior is associated with a
continuous change in the slope of the power law of the energy spectra from -1 to
-5/3 as the particle inertia increases. Interestingly, we note that the
clustering transition can append for realistic conditions, typically for a
concentration of 200 drops per cm^3, with 50 micrometers in diameter and
subjected to a radiative flux of 1000w/m^2.

Jet stream variability and predictability

Gwendal Rivière

 slides

Jet stream fluctuations and meanders play a key role in the formation of
mid-latitude surface weather extremes. The mid-latitude jet stream is mainly
eddy-driven and the underlying processes will be first recalled in the
talk. Then an overview of the main processes and forcings involved in the
variability of eddy-driven jet streams in the present climate will be presented,
together with their tendencies in future climate projections. Finally the talk
will focus on the impact of diabatic processes on the jet streams and how they
may be at the origins of uncertainties in short-term and medium-range numerical
weather predictions.

Attribution des événements extrêmes : pourquoi est-ce difficile ? Quelles pistes pour progresser ?

Laurent Terray

 slides

"Attributing and projecting heatwaves is hard: we can do better". Tel est le
titre de la dernière publication de notre collègue Geert Jan van Oldenborgh, un
des pionniers sur l’attribution des extrêmes, qui nous a quitté en Octobre
2021. Dans cette contribution, je souhaite m’inscrire dans la suite de son
injonction faite à notre communauté et continuer à identifier et discuter les
verrous scientifiques propres à l’attribution des extrêmes et les
stratégies/méthodes scientifiques mises en œuvre pour les surmonter. Pour ce
faire, j’utiliserai deux objets scientifiques très étudiés en ce moment : 1. la
canicule extrême de juin 2021 dans l’ouest du Canada et au nord-ouest des
Etats-Unis 2. L’augmentation des épisodes de canicule estivale sur l’Europe de
l’ouest au cours des dernières décennies. Pour conclure, j’évoquerai la question
"how can we do better", en essayant de proposer que nous devrions réfléchir à
une approche plus holistique et intégrée que celles déployées actuellement.

Ocean – sea ice interactions at increasing resolution

Camille Lique

 slides

The decrease of the summer sea ice extent in the Arctic is one of the sentinel
index for climate change and climate models predict that the decrease will
further intensify in the future.  The mechanisms responsible for this decrease
are not yet fully elucidated: while in IPCC-type models, the trend in sea ice
extent is almost solely driven by increased air temperature, observations,
idealized process models and realistic high resolution models suggest that the
ocean may drive part of the sea ice melting, through both thermodynamical and
dynamical processes. This discrepancy highlights the need to scrutinize the
ocean-sea ice interactions.
In this talk, we will discuss the time and space scales of the different
processes involved in the interplay between ocean and sea ice. In particular, we
will highlight how the presence of of (sub-)mesoscale eddies can impact the
variations of the ocean surface heat flux, as well as the sea ice vorticity. We
will then discuss future avenue for model development in order to reduce the
uncertainty associated with the projected changes in the Arctic Basin.

Challenges and prospects in ocean modelling for climate applications

Julie Deshayes

Building up from Fox-Kemper et al. 2019 review on ocean general circulation
models (doi: 10.3389/fmars.2019.00065), I propose to review current challenges
and prospects in ocean modelling for climate applications. Indeed, the latter
specifically require long integrations for calibration, equilibration, and to
ensure a statistically robust analysis of climate modes of variability (up to
centennial timescale in this context). I would also outline, in parallel,
ongoing efforts in improving the ocean component of the two French climate
models.

Wind-wave interactions and air-sea fluxes using theoretical models

Alex Ayet

When wind blows over the ocean, short wind-waves (of wavelength smaller than 10m)
are generated, rapidly reaching an equilibrium with the overlying turbulence
(at heights lower than 10 m). Understanding this equilibrium is key to many
applications since it determines (i) air–sea fluxes of heat, momentum and gas,
essential for numerical models; (ii) energy loss from wind to waves, which
regulates how swell is generated and how energy is transferred to the ocean
mixed layer and; (iii) the ocean surface roughness, visible from remote sensing
measurements. Describing this equilibrium is still a controversial topic due,
among others, to the multi-scale nature of a realistic ocean surface and to wave
breaking, which changes its topology. Here we will review theoretical models
describing the interaction between wind and waves and present a new framework
allowing to better describe the multi-scale coupling between the turbulent and
wave spectra. This would allow accounting for the influence of environmental
parameters on momentum and heat fluxes, and opens new paths both from a
theoretical perspective and for the analysis of experimental data.

Parameterization of non-orographic gravity waves in large-scale models: Formalism, impact and test against direct observations.

Francois Lott

 slides

The parameterization of non-orographic gravity waves breaking in the atmospheric
component of the large scale Coupled Model developed at IPSL for the 6th IPCC
assessment report (IPSLCM6 ) will be reviewed. For the gravity waves emitted by
front it will be shown that the GW sources can be derived from spontaneous
adjustment theory, providing that the sources are placed at multiple altitudes
in the troposphere and lower stratosphere. The representation of gravity waves
issued from convection is derived from a much simpler theory, where the
variability of the latent heating due to convection is distributed over a large
ensemble of spatio-temporal harmonics each harmonic forcing the corresponding
waves. In these two different schemes, the large ensemble of waves needed is
realised by stochastic methods.

The parameterizations are shown to reduce systematic errors, for instance on the
quasi-biennal oscillation in the tropical stratosphere. In this respect careful
diagnostics are also made of the large scale equatorial waves, and to verify
that the parameterizations are not spuriously balancing errors on element of
dynamics that should be explicitly solved by the model.

Finally, direct comparison with constant level balloon flights done in November
2019-February 2020 and in the lower tropical stratosphere during the strateole-2
campaign are made. The atmospheric conditions under the balloons location at
each time are derived from the ERA5 reanalysis and used to activate the
non-orographic gravity waves schemes. We show that the momentum fluxes predicted
by the schemes compare well with the observed values, and with highly
significant correlations between the day-to-day variabilities. The correlations
obtained are for the operational values of the non-dimensional “tuning”
parameters of the schemes, and have values that could be well improved,
regarding the number of degrees of freedom present in the dataset.

Slides des orateurs non-invités:
Jérémie Bec, Spontaneous stochasticity or a tale on the “true” butterfly effect, slides
Achim Wirth, Fluctuating Air-Sea Interaction, slides
Laurent Li,  Machine learning to optimize climate projection over China with multi-model ensemble simulations, slides
Nicolas Lanchon, Turbulence d’ondes dans un fluide stratifié avec et sans mode de cavité, slides
Luminata Danaila, Interaction between large scales and temperaturefluctuations during the 2003 heat wave, slides