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L2: Quantifying Dynamical Regimes in the Ocean and the Atmosphere

Principal investigators: Prof. Marcel Oliver (Constructor University/Catholic University of Eichstätt-Ingolstadt), Prof. Jin-Song von Storch (Max Planck Institute for Meteorology/Universität Hamburg), Prof. Nedjeljka Žagar (Universität Hamburg)

Dynamical regimes are flows that share common and uniform dynamical or statistical properties, often within a well-defined region of space or time. More generally, we speak of dynamical regimes in the sense of a flow decomposition into constituents with uniform properties, most prominently Rossby waves or “balanced flow” on the one hand and inertia-gravity waves (or, for short, gravity waves) on the other.  In order to understand couplings between these processes and the two wave regimes, this subproject aims at their separation and quantification.

The need to quantify the individual constituents of different dynamical regimes arises, first, from the wish to understand their overall dynamics and interactions and their roles in the energy pathways in the atmosphere and ocean. Second, dynamical regimes give rise to bulk properties of the flow, such as vertical momentum fluxes in the atmosphere and spectral characteristics of mesoscale and submesoscale currents in the ocean, which can be learned from high-resolution models but need to be parameterized in relatively low-resolution simulations as are used in climate models. Third, we need to develop tools to detect the remnants of unresolved dynamical regimes from properties of the large scale flow, for example to couple gravity wave parameterizations to the dynamical core.

We approach the regime quantification problem using sophisticated mathematical tools applied to high-resolution primitive equation models, atmospheric analyses and climate simulations.  We  shall

(a) apply the so-called optimal balance algorithm to realistic primitive equation models, and
(b) systematically evaluate the vertical momentum fluxes associated with Rossby and inertia-gravity waves using 3D linear wave theory with a focus on the tropics.

The results will provide, among others, a novel scale-dependent quantification of the vertical momentum fluxes associated with different atmospheric regimes in analyses and climate models, as a guide for their improved parameterization in climate models. In addition, we will not only provide new and improved diagnostics but also obtain a deeper understanding of the concept of balance itself, especially in the context of global and complex models. For this,  a Telescope-ocean-simulation, which has  the focus in the Walvis Ridge and simulates not only sub-mesoscale and mesoscale eddies but also internal tides,  will be carried out in a joint effort with other subprojects, especially with L4 and W2.

Horizontal velocity divergence (top) and geopotential at 130 mb on day 11 of the T126 model simulation. Vectors show the horizontal wind. Inertia-gravity waves are described by the velocity divergence. The quasi-balanced flows that emit the waves are described by the geopotential contours and wind vectors. From O'Sullivan and Dunkerton (1995).

Both the large-scale currents and the meso-scale eddies in the ocean are essentially balanced, with geostrophic or gradient-wind balances in the horizontal and hydrostatic balance in the vertical. Moreover, meso-scale eddies tend to transfer energy upward towards larger scales. This situation points to the challenging question of what are the ocean's routes to dissipation that are needed for the interior general circulation, including its related eddy fi eld, to reach an equilibrium state in the presence of the constant atmospheric forcing. This subproject contributes to answering this question by exploring the route to dissipation via spontaneous wave generation: Eddying flows spontaneously emit internal waves. The waves, once generated, are refracted by the eddying flow, and may be captured later. Wave capture is characterised by an exponential increase in wavenumber and wave amplitude and an exponential decrease in the intrinsic group velocity. While capture is well understood theoretically for various types of flows, it is still open whether it occurs in the real ocean. By spontaneous emission and subsequent interaction with the emitted waves, energy is transferred from meso-scale eddies to smaller scales where instabilities and smallscale turbulence complete the downscale cascade to dissipation.

Quantifying this route to dissipation is central for the present CRC. Speci cally, this subproject aims to address the following questions:

  • How important is the route to dissipation via spontaneous wave generation and wave capture?
  • What are the key factors that control the internal wave emission by quasi-balanced and
    eddying flows and the subsequent interaction between waves and flows?
  • Is it possible to formally characterise the internal waves emitted by a turbulent geostrophic flow?

We will address these questions using a combination of theory, conceptual models with idealised configurations based on rotating shallow water equations, and existing and specially designed numerical experiments with an ocean general circulation model based on the primitive equations.

Report 2026 Annual Meeting of the European Meterology Society (EMS) in Utrecht by Justian Chen (September 26)

The 2026 Annual Meeting of the European Meteorological Society (EMS) took place in Utrecht, from 6 to 11 September. The theme is “Delta Climate in Europe: weather, water, and warnings”.

This was my first EMS meeting, and what struck me most was the strong emphasis on applied meteorology. Many presentations focus more on how scientific knowledge can help society. My research focuses on global, large-scale atmospheric dynamics, and I usually approach scientific questions with the aim of understanding the underlying physical processes. Before this meeting, I had given much less thought to how this understanding translates into practical applications, or how my own research might matter beyond the scientific community. Some presentations made this contrast particularly clear. Studies of urban wind fields considered wind around individual buildings, down to the conditions over a particular rooftop. Other work explored how wind farms disturb low-level jets and generate gravity waves, with consequences for wind-energy production. This attention to local detail was quite different from the large-scale perspective I am used to. Such processes often lie outside the main questions we investigate, yet here they were central to addressing practical problems. Listening to these presentations broadened my perspective.

The meeting also prompted me to think about how we communicate our work. Compared with major conferences such as EGU and AGU, EMS felt much smaller. Larger meetings offer a wider pool of potential listeners, including researchers with closely overlapping interests. At a much smaller conference with a strong applied focus, a specialized contribution on atmospheric dynamics may reach fewer people. This makes the way we introduce and explain our research particularly important. While I was explaining my work, a listener told me that I was using too much mathematics for him to follow. Equations that help me explain an idea precisely may not provide the same clarity to someone from another field. How could I convey the physical question and the significance of the results without relying on a shared mathematical background? Making specialized research understandable across different scientific disciplines is an ability that needs continual practice. Simplifying a presentation may make it easier to follow, yet removing too much detail can also remove the basis for a substantive scientific discussion. An accessible introduction can open a conversation, but it does not by itself create the conditions for an in-depth exchange. Finding the balance requires careful thought and practice.

Overall, I am grateful to TRR for supporting this trip, which gave me the opportunity to share my research and gain new perspectives beyond my own field

Research Stay in Brest by Mariana Lage (Dec 22)

One of the best parts of being a scientist in my opinion is to go abroad, meet new researchers and discuss ideas. It is amazing to see what other scientists are doing and how different the institutes are. Last year I had the opportunity to go to Ifremer (Institut français de recherche pour l'exploitation de la mer), in Brest, Brittany, France, and all started with a simple email to Claire Ménesguen introducing myself and asking whether I could visit the institute.

Claire is one of the team leaders of the Ocean Scale Interactions group at the Laboratory for Ocean Physics and Satellite remote sensing (LOPS) together with Jonathan Gula. The main focus of the group is to study ocean dynamics with a particular interest in small horizontal and temporal scales. Once the collaboration was settled and I arrived in Brest, we had several meetings to start planning the structure of the upcoming work. The infrastructure at Ifremer is great, and I met many PhD students and posdocs. As the time and work progressed, we decide to slightly modify our initial plan. Science is highly non-linear, so we had to adapt given the results we obtained with some of the analyses. The good part about that is that I was able to constantly discuss not only with Claire (and Jeff), but also with a lot of people from both Ifremer and LOPS. Because people have different backgrounds, we were able to approach my research topic from many different angles, which led to many nice ideas.

Apart from work (because it would be a shame not to enjoy Brittany’s landscape), I enjoyed the weekends hiking and traveling to small cities around Brest, and, of course, eating! Brittany is very well-known for crêpes, sea food (oysters!) and caramel, which are musts to try when you are there. Brest is on the west coast of France and the landscape is just stunning! The color of the water, the lighthouses and the shape of the coast make this city quite unique. There is also an Aquarium which is really worth visiting. One curiosity from there is that they have their own language (Breton, or Brezhoneg), although nowadays French is the main language spoken. Another curiosity is that it rains a lot, and the weather can easily change from heavy storm to shining sun in a matter of hours.

After my return, Claire and I are still in close contact and we are already planning the next steps regarding our collaborative research. My time there was really pleasant and fruitful and a second research stay is planned in October 2023. I really recommend sometime abroad for everyone especially because the TRR provides the most difficult thing to get: money. This is a unique opportunity to gather different opinions about one’s research topic and to get people to know you too. I left behind many open doors and I am really excited to continue working with all the people I met!