Reports

Report - Participation in the Trimester "Mathematical Developments in Geophysical Fluid Dynamics" in Paris by Philomène Dufour (July 26)

I am a PhD student at the University of Hamburg and a member of the Collaborative Research Centre TRR 181 Energy Transfers in Atmosphere and Ocean (sub-project T2: Ocean Surface Layer Energetics). From the 29.06.2026 to the 03.07.2026, I participated in the third workshop of the trimester programme Mathematical Developments in Geophysical Fluid Dynamics, entitled Idealised Mathematical Models for Geophysical Flows, held at the Institut Henri Poincaré in Paris.

After attending the introductory school at the CIRM in Marseille in April, near the beautiful site of the Calanques, I went to Paris to participate in the third workshop of this programme.

I was particularly pleased that this workshop was held in Paris, as it was an opportunity to see some of my old friends from the Master's programme at Sorbonne University and to come back to this city to which I am particularly attached.

The third workshop lasted one week and took place at the Institut Henri Poincaré, in the Latin Quarter of Paris, which is well known for its intellectual and student atmosphere. We were sitting in the Charles Hermite amphitheatre, one of the oldest lecture halls of the institute, which has hosted many renowned mathematicians and physicists.

During this workshop, I attended many talks and presented my poster on an attempt at deriving the $\alpha$-modified Craik–Leibovich equations using geometric generalised Lagrangian mean theory. This work aims to develop a mathematical model describing wave–mean current interactions while accounting for the effects of turbulence through an $\alpha$-regularisation. Because the poster combines ideas from Craik–Leibovich theory, $\alpha$-models of turbulence and geometric GLM, it attracted researchers with rather different backgrounds, including oceanographers, applied mathematicians and specialists in mechanics. These discussions helped me to better identify which aspects of the work are the most accessible to different audiences and which questions naturally arise from each community.

I was very happy to see that this subject raised the interest of many people; it encouraged me a lot to continue. As the poster was displayed throughout the whole week, it also enabled me to continue discussions with people afterwards. This experience also highlighted the importance of poster sessions for presenting ongoing work, receiving feedback and establishing new scientific contacts.

Although the event in Paris was completely free of charge, lunches were organised directly next to the amphitheatre, which greatly facilitated discussions and informal interactions. One of the highlights of the week was undoubtedly the cocktail on the rooftop of the Maison Poincaré, where discussions continued in a relaxed atmosphere. Participants were also offered free admission to the Maison Poincaré's mathematics museum. I visited it with a few other participants and enjoyed discovering its interactive exhibitions.

The scientific programme covered a broad range of themes. The talks ranged from reduced models for strongly stratified turbulence, derived using asymptotic analysis, to the treatment of boundary conditions in wave–structure interactions. They showcased a variety of mathematical tools, including turbulence modelling, asymptotic methods, geometric mechanics, and numerical simulations. Although the topics were diverse, they all shared the common objective of improving our understanding of the fundamental mechanisms governing geophysical flows. At this stage of my PhD, this workshop helped me better understand how my research fits within the broader field of geophysical fluid dynamics.

In the following week, there will be a satellite conference, *Perspectives and Challenges in Geophysical Fluid Dynamics*, at the École normale supérieure. I am looking forward to meeting other members of the TRR and many other researchers there.

The workshop also gave me the opportunity to learn about future events of the community. Several participants recommended the Advanced Summer School on Mathematical Fluid Dynamics in Cargèse next year, which I hope to attend.

Report - Liège Colloquium on Submesoscale Processes in the Ocean by Jabeen Safeer (May 26)

Jabeen Safeer, a PhD student at the University of Hamburg, attended the 57th International Liège Colloquium on Ocean Dynamics at the University of Liège in Belgium at the end of May. Here she shares her experiences.

The Liège Collocuium on Ocean Dynamics is one of the longest-running series in physical oceanography, bringing together leading researchers from across the globe to advance the understanding of ocean dynamics. This year’s colloquium revisited submesoscale dynamics of the ocean - a decade on from the 48th Colloquium in 2016 - taking stock of new developments across a range of oceanographic disciplines, including observational, modelling, and theoretical approaches. I was accompanied by Evridiki Chrysagi - my supervisor and also a TRR 181 member - and Arooba Nawaz, a master’s student at the Universität Hamburg.

We reached the city on the afternoon of May 24th and were greeted by the sight of the transparent, monumental vault of the Gare de Liege-Guillemins, built entirely out of glass and steel. The conference venue was the University of Liège's lecture hall, with neoclassical walls inscribed with the exceptional heritage of Wallonia, the French-speaking region of Belgium. The conference started with a session on remote sensing of submesoscale dynamics, which shed light on ongoing efforts to utilise high-resolution SWOT satellite data to reconstruct various ocean variables. Following the scientific sessions, we participated in a hands-on training session hosted by EUMETSAT (European Organisation for the Exploitation of Meteorological Satellites), focused on monitoring submesoscale ocean processes using Earth observation data. It provided us with new information and tools to work directly with freely available satellite-derived ocean data from the Copernicus Marine Service. The poster presentations were scheduled for the second day, along with an icebreaker event. Besides, the posters were on display all week, which provided me with a very relaxed environment to talk about my poster and discuss my work with several people. The remaining days featured keynote talks on frontal instabilities, wave-front/eddy interactions, multiscale processes, submesoscale dynamics at the boundaries and physical-biological interactions. I found it fascinating to learn that polar oceanographers use seal-borne CTD sensors to study Southern Ocean dynamics. This was just one example that highlighted for me how essential creativity is in conducting meaningful scientific research, as I discovered many throughout the colloquium. Although the five days were packed with talks and activities, I found each talk to be very informative and gained several insights relevant to my own research. Outside of the scientific sessions, we spent our evenings walking through the beautiful streets of Liège and sampling a variety of local restaurants. The most delicious culinary experience we had was in an Afghan restaurant. The renowned Liège waffles also warrant a special mention. I am grateful to TRR 181 for supporting this trip, and to Evridiki and Arooba for the company, both in the lecture hall and our evening explorations of Liege.

LES Simulations of Energy Fluxes in the Surface

“However at Submesoscales, a lack of observations means that it is not yet clear which process dominate in energy dissipation.
Josh Pein, PhD T2

I am a physical oceanographer working as a PhD at the University of Hamburg under the supervision of Dr. Nils Brüggemann (Universität Hamburg), Dr. Jeff Carpenter (Helmholtz Zentrum Geesthacht), Dr. Lars Czeschel (Universität Hamburg).

I am investigating the energetics in the oceanic surface mixed layer.

I studied a dual major in `Environmental Sciences` as well as `Atmospheric and Ocean Sciences a the University of Cape Town, a true amalgamation of the earth sciences. Following a successful research cruise in the Southern Ocean in 2015, I moved my studies to the IfM in Hamburg.

I am a member of the TRR subproject T2 “Ocean Surface Layer Energetics”. The importance of the upper-ocean Surface Mixed Layer (SML), an interface between the ocean and Atmosphere goes without saying. It is responsible for communicating atmospheric fluxes into the ocean interior, and is the most energetic part of the ocean! Processes in the SML interact to produce a variety of energy transfers. However at Submesoscales, a lack of observations means that it is not yet clear which process dominate in energy dissipation. Consequently, climate models often artificially create or dissipate energy. T2 seeks to rectify this! Using a combination of observations and large eddy simulations, the main aim of our subproject is to identify, quantify and parameterize these dominant processes. Ultimately, this will expand our understanding of the conceptual energy cycle of the ocean, providing more energetically consistent surface mixed layer parameterisations for climate models.

I am responsible for running, and the analysis of the LES. One set up of interest, and common place in the upper ocean, are oceanic fronts. Often close to thermal wind balance, not quite in equilibrium, they are unstable to a “family” of possible submesoscale instabilities.

The figure below, produced from one of our runs, is an example of such a set up. It shows the evolution of a baroclinic front in the mixed layer. The colour scale gives the buoyancy and the white contours indicate the associated eastward jet.

After 6 hours symmetric instability develops at the southern flank of the jet, as the relative vorticity of the background flow reduces the potential vorticity below zero (a necessary condition for symmetric instability). After 24h we can see the development of baroclinic instability on a much larger scale. The development is not symmetric around the background jet as the symmetric instability has already re-stratified large parts of the southern flank. We are especially interested in the impact of the so called ‘secondary instabilities’, such as Kelvin-Helmholtz instability, which accompany symmetric and baroclinic instabilities. In order to explore the role of the ‘secondary instabilities’ for the mixing and energy dissipation in the mixed layer, our LES simulations demand grid resolutions of (O)1m.

High-resolution data for a better understanding of energy budgets

I am driven by the translation of large amounts of data into palpable results that improve the understanding of a system while also allowing the identification of further knowledge gaps.
Larissa Schultze, Postdoc T2

My name is Larissa Schultze and I am a Postdoc at the Helmholtz-Zentrum Geesthacht. I am passionate about data and I am eager learn about and implement methods that support the analysis of collected measurements and of simulation results. I am driven by the translation of large amounts of data into palpable results that improve the understanding of a system while also allowing the identification of further knowledge gaps.

Within the TRR 181, I work with principal investigator Jeff Carpenter in the subproject T2, in which we tackle the energy transfers of the surface mixed layer. I make use of observational methods and numerical modelling to study stratification, turbulence and mixing in shallow seas. The observational approach focuses on the processing and analysis of high-resolution data collected by autonomous underwater gliders equipped with an instrument package for small-scale turbulence measurements. Generally, the gliders are controlled via satellite and are able to uninterruptedly collect data for several weeks even under adverse weather conditions. The gliders are able to measure physical properties ranging from the surface of the water column until approximately a thousand meters depth. This, for example, advances knowledge of turbulence levels, mixing rates and heat transfers across the water column during storms. As for the numerical modelling, I conduct Large Eddy Simulations using PALM (Parallelized Large Eddy Simulation Model for atmospheric and oceanic flows) to improve the understanding of wind-wave dynamics.

Hunting fronts

SML fronts also host various frontal instabilities which are considered as routes to mixing and energy dissipation in the energy cascade.
Jen-Ping Peng, PhD T2

Hi, my name is Jen-Ping Peng. I am a PhD student of the subproject T2: “Energy budget of the ocean surface mixed layer” under supervision of Dr. Lars Umlauf at the Leibniz Institute for Baltic Sea Research (IOW) in Warnemünde. I investigate the oceanic surface mixed layer (SML), typically known to have substantial turbulent mixing driven by vertical surface forcing such as wind stress and surface buoyancy fluxes. However, the processes inside the SML are considerably complicated by strong horizontal density gradients (e.g., fronts, filaments), which may induce restratification that competes with mixing. SML fronts also host various frontal instabilities which are considered as routes to mixing and energy dissipation in the energy cascade. We address surface-layers fronts and their associated restratification and mixing processes based on the data collected from several cruises in different areas of the ocean.

The analysis of data from research cruises is one of the main tasks of my PhD. The TRR181 cruises took place in the Benguela upwelling system (South-East Atlantic Ocean) in 2016, closely coordinated with subproject L3 using drifters, and in the Central Baltic Sea in 2017 and 2018. These two study areas are characterized by the rich presence of fronts and filaments, ideally suited for the investigation of the processes studied in this subproject. I participated in two research campaigns in the central Baltic Sea. Together with our T2 colleagues from HZG, we were hunting fronts with specialized instrumentations, including turbulence microstructure profilers, a Scanfish, a research catamaran, and ocean gliders.

I am currently analyzing data obtained from the Benguela upwelling system toward a better understanding of the formation and decay of an upwelling filament, and related instabilities and mixing. I am also involved in the analysis of a related data set that we collected in a frontal region in the central Baltic Sea.

Focussing on the ocean surface mixed layer

Our scope is to investigate the sub-mesoscale structures and the surface mixed layer instabilities in order to develop new parameterisations of energy consistent pathways.
Evridiki Chrysagi, PhD student in T2

My name is Evridiki and I’m PhD candidate working with Prof. Dr. Hans Burchard, in subproject T2. Our research will be focused mainly on the ocean surface mixed layer which is a highly complex and energetic region. The upper ocean is characterized by a relative shallow mixing layer with weak stratification due to turbulent mixing. Our scope is to investigate the sub-mesoscale structures and the surface mixed layer instabilities in order to develop new parameterisations of energy consistent pathways, associated with these motions. For that we will use the General Estuarine Transport Model (GETM) which includes turbulence closure models provided by GOTM, diagnostic tools for the numerical mixing and dissipation but also adaptive vertical coordinates that can resolve the sub-mesoscale features. The configurations will include idealized high resolution simulations as well as hindcast simulations of the Central Baltic Sea. In order to validate the model, the results will be combined with field observations.