Energetics of ocean turbulence from realistic modelling and observations

Abstract:

Ocean turbulence and associated energetic pathways remain partly understood in the real ocean, mostly because of scale overlaps between several dynamical regimes, including mesoscale eddies, submesoscale vortices, and inertia gravity waves. Since ocean models resolve now part of the turbulent kinetic energy spectrum and high-resolution observations are becoming available in large numbers, opportunities are becoming available to compute of accurate turbulence statistics and characterise energy pathways.

The first part of this seminar introduces a spectral framework in the frequency-wavenumber domain to
characterise what spatio-temporal scales gain or loose kinetic energy through the inverse cascade process
occurring in geostrophic turbulence. A focus is made on nonlinear scale interactions associated with relative
vorticity advection, evaluated from sea level anomaly (SLA) time series, that yield inverse cascades of
energy towards larger spatial scales and longer temporal scales. Those inverse cascades may partly
explain the self-generation of long-term variability induced by oceanic mesoscale eddies, and noticed at
oceanic midlatitudes in recent eddy-resolving simulations. In the midlatitude North Pacific, two dynamical
regimes are distinguished: high-frequency frontal Rossby waves (FRWs), probably generated by baroclinic
instability, that transfers energy toward a lower-frequency regime, involving westward-propagating
mesoscale eddies (WMEs). Examples of using the same spectral framework to understand energetic
pathways in equatorial ocean turbulence and submesoscale turbulence are also given.
The second part of this seminar focuses on turbulent processes occurring in the range of scales 1-100 km.
At those scales balanced dynamics, composed of mesoscale and submesoscale vortices, overlap with the
dynamics of inertia-gravity waves and stratified turbulence, with substantial contribution from divergent
motions. In order to understand, in situ observations are analysed around New Caledonia using structure
functions on upper ocean velocities, on which a Helmholtz decomposition is also performed, and on surface
tracers. Those results are used to discuss the turbulence regimes likely to be at work in this region. The
reservoir of Available Potential Energy (APE) for Mixed-Layer Instabilities (MLI) over the world's oceans is
also investigated using high-resolution shipboard measurements of temperature and salinity.

Guillaume recently started position as a Research Associate at the Climate Change Research Centre,
UNSW, Australia. He received a Master of Science from Claude Bernard University, Lyon (France) and a
Master of Engineering from École Centrale Lyon, Écully (France) in 2011. In 2012, he had a short work
experience in the aeronautic industry before switching to research in geophysics. In early 2016, he
completed a PhD in Physical Oceanography from Paul Sabatier University, Toulouse, while being based
most of the time at the Laboratory of Glaciology and Geophysics of the Environment, Grenoble. From 2016
to 2018, he worked as a Post-doctoral Research Associate at the Laboratory of Studies in Geophysics and
Spatial Oceanography.
Guillaume studies oceanic turbulent flows with the support of high-resolution ocean models and
observations. He mostly focuses on the impact of mesoscale oceanic eddies (O 100 km) on the climate
system through their feedback with the atmosphere and their capacity to generate long-term fluctuation,
sometimes called eddy-driven variability or intrinsic variability. Oceanic eddies may hamper the detection
and the attribution of long-term changes in some regions of the ocean, while also affecting the atmospheric
dynamics and weather patterns at midlatitudes. At even smaller scales (O 1-10 km), he also studies
submesoscale motions, characterised by fronts and filaments, and propagating internal waves, generated
by the interaction of the barotropic tides with steep topographic features. To do so, he uses existing
observations from in situ shipboard measurements of velocities, temperature and salinity, as well as from
autonomous gliders and satellite altimeters. He eventually contributes to develop several numerical tools in
Python for signal processing and analysing large geophysical datasets, within the community effort Pangeo.