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T2: Energy budget of the ocean surface mixed layer

Principal investigators: Dr. Jeff Carpenter (Helmholtz Center Geesthacht), Dr. Lars Umlauf (Leibniz Center for Baltic Research), Prof. Hans Burchard (Leibniz Center for Baltic Research)

Sea surface temperature on 25 September 2007 from an eddy-resolving GETM simulation of the Baltic Sea (left), and AVHRR remote sensing data (right). Shown in the left panel is only the high-resolution area of the central Baltic Sea as described in Holtermann et al. (2014). Note that the colour shading is similar but not identical in these panels. The right panel is based on data from the German Federal Maritime and Hydrographic Agency (BSH); courtesy of H. Siegel (IOW).

The energy pathways of the submesoscales, which exist in the range between the mesoscale (on the order of 100 km) and the largest turbulent eddies (order of 1 m), will be quantified and parameterised so they can be incorporated into global climate models. This will be done using a numerical approach consisting of two different model studies specialised in both turbulent flows and regional ocean processes, as well as a dedicated field program using the Baltic Sea as a "natural laboratory" for the measurement of submesoscale energy pathways.

The surface mixing layer (SML) is the ocean side of the air-sea interface through which the fluxes of energy, momentum and tracers have to pass in a coupled atmosphere-ocean system. Pathways and transformations of energy, momentum and tracers in the SML are complex, highly variable, and not sufficiently understood. Even in high-resolution ocean models, energy and momentum budgets are energetically inconsistent because the additional energy reservoirs and transformation processes due to unresolved processes (e.g.,mesoscale/submesoscale motions, surface waves) are either ignored or not correctly taken into account. In coarse-resolution climate models, the situation is even worse. The goal of this subproject is therefore to investigate energy transport and transformation processes in the SML that are relevant for the ocean-atmosphere coupling in climate models.

Our major efforts to understand the energy budget of the SML will be conducted through the use of idealised Large Eddy Simulations (LES), high-resolution ocean modelling, and coordinated field surveys including high-resolution turbulence observations. The results from the LES and field work will be used in a realistic model to understand the energy pathways associated with submesoscale motions in the SML, and to test the developed parameterisations.
 

Main findings of phase 1


In WP2, we analyzed a realistic high-resolution numerical simulation focusing on the central basin of the Baltic Sea, an area where available observations from a field campaign confirm that features persistent lateral density gradients and rich submesoscale activity, hence forming an ideal natural laboratory for this study. The simulation revealed a strong thermal frontal structure that persists during autumn and had not been reported previously. Cold submesoscale filaments with sharp lateral buoyancy gradients, strong surface convergence and high vertical velocities arise from this front. Highly heterogeneous Mixed Layer Depth (MLD) patterns appear, with the shallowest MLDs found in the vicinity of submesoscale features. As it turned out, submesoscales are able to maintain shallow MLDs during storms and induce vigorous and rapid restratification when the wind subsides, creating significant temporal MLD variability. The interaction of strong near-surface turbulence and submesoscale restratification results in highly efficient mixing inside submesoscale fronts.

Sketch of the submesoscale frontal structure and frontal instabilities at the edge of a dense upwelling filament. Figure taken from the T2 PhD thesis of Peng (2020).


In WP3, we investigated submesoscale frontal dynamics, instabilities, and mixing processes inside dense upwelling filaments, based on data from the central CRC cruise M132 with R/V Meteor in the south-east Atlantic Ocean (Benguela upwelling system). With the help of specialized intrumentation, including a towed research catamaran and high-resolution turbulence microstructure measurements, we were able to obtained a detailed view of the structure of all dynamically relevant parameters (e.g., vertical shear and vorticity, vertical and cross-front stratification, energy dissipation) inside narrow submesoscale fronts and filaments at unprecedented resolution.  This data set allowed us to test the real-ocean relevance of recent theoertical and numerical ideas regarding the submesoscale dynamics of surface-layer fronts.

As summarized in the figure, our analysis showed that the effect of the front is reflected in a gradual transition from purely wind-driven turbulence towards mixing energized by forced symmetric instability (FSI). Our microstructure data were in excellent agreement with previous numerical studies, suggesting that the energy dissipation due to FSI scales with the Ekman buoyancy flux, i.e. with the rate at which the cross-front Ekman transport moves dense water on top of light water. Our high-resolution catamaran-based velocity measurements also allowed us, for the first time, to demonstrate that the vorticity in the cyclonic flank of the frontal jet may be strong enough to fully suppress FSI. In this case, turbulence is fueled by marginal shear instability. Our data therefore provide the first direct and conclusive evidence for the combined relevance of FSI, inertial instability, and marginal shear instability for overall kinetic energy dissipation in real-ocean submesoscale fronts and filaments (see Peng et al., 2020). 
 

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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.