L4: Multiscale Ocean-Atmosphere Coupling

Principal investigators: Dr. Nils Brüggemann (Max Planck Institute for Meteorology), Dr. Cathy Hohenegger (Max Planck Institute for Meteorology), Prof. Stephan Juricke (GEOMAR Helmholtz Centre for Ocean Research Kiel), Dr. Lars Umlauf (Leibniz Institute for Baltic Sea Research Warnemünde)

Objectives

Recent studies highlight the importance of small-scale coherent structures associated with atmospheric convection and (sub-)mesoscale ocean dynamics on atmosphere-ocean feedbacks in key regions of the climate system. In state-of-the-art coupled climate models, such small-scale atmosphere-ocean feedbacks are either ignored or rely on parameterizations, which may lead to model biases and energetic inconsistencies. In this subproject, we investigate these small-scale coupling mechanisms using coupled simulations on a global scale, based on resolutions allowing us, for the first time, to directly simulate the underlying key processes. More specifically, we focus on the following topics:

(i) With the help of storm-resolving coupled simulations and data from a large-scale field experiment conducted in the tropical Atlantic, we investigate the interaction of shallow atmospheric convection and near-surface processes in the ocean. A special focus here is on the dynamics of thin (meter-scale) "diurnal warm layers" and "rain layers" at the ocean surface, their role in mediating air-sea fluxes, and their feedback with atmospheric convection (see figure 1).

(ii) We also study the effect of parameterized submesoscale dynamics like baroclinic instability and symmetric instability on atmosphere-ocean coupling (see figure 2). To this end, the role of submesoscale dynamics on atmosphere-ocean feedbacks in selected key regions of the climate system is investigated with the help of coupled global simulations at unprecedented resolution (see figure 3). These simulations are also used to develop and test parameterizations of these effects in more coarsely resolved global ocean-atmosphere models. Our final goal is to obtain an integral estimate of the role of submesoscale dynamics on the coupling between the atmosphere and the ocean.

(iii) Finally, we aim to clarify the effect of resolved mesoscale eddies on air-sea coupling (see figure 2), develop a new stochastic parameterization that represents such effects in surface fluxes of heat and momentum, and investigate the global and regional impact of the parameterization in coupled atmosphere-ocean simulations that cannot use high enough resolution to represent those processes explicitly.

Reports

Report Stay in Toulouse, France by Henning Franke (Jan 26)

My name is Henning Franke, and I am a Postdoctoral Researcher in subproject L4, „Multiscale Ocean-Atmosphere Coupling“, at the Max Planck Institute for Meteorology (MPI-M). My scientific work focuses on better understanding the role of small-scale heterogeneity of the sea surface temperature in driving convective precipitation over tropical oceans in km-scale global climate simulations. For this task, it is important to reliably identify deep convective systems - commonly referred to as mesoscale convective systems (MCS) - in the simulations, which is usually done by MCS tracking algorithms. One of the world’s most sophisticated MCS tracking algorithm is the TOOCAN algorithm, which is developed by Dr. Thomas Fiolleau and Dr. Rémy Roca. In order to learn about the application of TOOCAN on my own climate simulations with the km-scale model ICON, I visited Thomas and Rémy at their lab in Toulouse at the end of January 2026.

The week started with getting to know the members of the research group led by Rémy and by presenting my ongoing and planned research in their weekly group meeting. Afterwards, I received a comprehensive overview of TOOCAN from Thomas, which proved to be very helpful to understand how TOOCAN actually works and what distinguishes it from other MCS tracking algorithms. With the extensive support of Thomas, I was able to get TOOCAN running on my ICON simulations within just two days. Therefore, there was even some time for an initial analysis of the TOOCAN output. I discussed these results with Rémy in detail, and given Rémy’s extensive expertise on deep convection, these discussions were really helpful for me and stimulated some new ideas for my research that I plan to test in the future.

Additionally, I liked experiencing a different research culture in another institute and working group, and I was impressed by the level of engagement and scientific discussions within the working group. The working group made me feel welcome immediately, which was not only due to the bar night and the joint lunches, which I really enjoyed.

In addition to the academic experience, my time in Toulouse was very pleasant and flew by far too quickly. Toulouse itself is a beautiful and lively city, and I particularly liked its historic old town with its many nice restaurants and cafés. Those offered plenty of possibilities to enjoy the great French cuisine, which I miss a lot. I also miss my runs along the Canal du Midi, a historic waterway that stretches from Toulouse to the Mediterranean Sea, winding through the city.

I would like to express my gratitude to Rémy and Thomas for organizing a very pleasant and interesting stay in Toulouse. My special thanks go to Thomas for his extensive support with TOOCAN - I don’t think that I would have been able to set up TOOCAN that quickly without the possibility to ask him countless questions whenever I needed to. Overall, my stay in Toulouse was very valuable, and in my opinion it would have been very hard to get TOOCAN running if I would have stayed in Hamburg. Therefore, I would like to also thank the TRR181 for providing the funding for this research trip.

Report Stay in Japan by Henning Franke (Nov 25)

My name is Henning Franke and I am a Postdoctoral Researcher in subproject L4 „Multiscale Ocean-Atmosphere Coupling“ at the Max Planck Institute for Meteorology (MPI-M). My scientific work focuses on better understanding the role of small-scale heterogeneity of the sea surface temperature in driving convective precipitation over tropical oceans. My main tool for addressing this research topic are numerical simulations with global km-scale coupled ocean-atmosphere models, in particular the ICON model from MPI-M. Since km-scale models are still a fairly new type of climate model, an extensive exchange with other modeling groups that are active in this field is essential for making progress. This is the reason why in November 2025 I joined a group of scientists from the MPI-M on a research trip to Japan.

The Japanese atmospheric research community under the leadership of Prof. Masaki Satoh from the Atmosphere and Ocean Research Institute (AORI) of the University of Tōkyō is a global pioneer in developing and using km-scale global climate models. In order to deepen the collaboration between AORI and MPI-M in developing and applying km-scale climate models, Prof. Satoh and Prof. Bjorn Stevens from MPI-M organized a two-day workshop at AORI. The workshop was further attended by researchers from the National Taiwan University (NTU) and Monash University in Melbourne. During the workshop, the main task for me and other participants was to develop ideas for the scope, the format, and the topics for a more in-depth collaboration between the participating institutions. As a starting point for these discussions, early-career researchers gave a short scientific presentation. These presentations gave an impressive overview of the breadth of research topics related to km-scale climate models that are addressed by the participating institutions. To deepen the connections made at the workshop, the workshop also left time for a Ping Pong tournament, a football game, and a hike to Mount Tsukuba. Especially the hike to Mount Tsukuba was a great experience and I really enjoyed the beautiful Japanese nature.

After three nice days at AORI, me and my colleagues from MPI-M moved on to Morioka, a small town in the north of Japan’s main island Honshu. Here, the 7th International Workshop on Nonhydrostatic
modelling (WS-NHM) took place, which was followed by a one-day meeting on coordinated experiments for km-scale climate model intercomparison. The WS-NHM is one of the
major conferences for the km-scale climate modeling community and as such a great possibility to present some of my research results. It further offered plenty of possibilities for exchange with
colleagues and gave a great overview of the latest progress in the field. This valuable input stimulated some new ideas for my research that I plan to test when I return to Hamburg.

However, my stay in Japan was a great experience also beyond scientific input and exchange. Daily dinners offered plenty of possibilities to delve into the amazing Japanese cuisine, which is much more than just Sushi and Ramen. My personal favorite is Okonomiyaki, a savory pancake which blends cabbage and other choosable toppings, and I highly recommend everyone to try it once in Japan! Furthermore, I learned a lot about the Japanese culture and the Japanese way of life in general. For example, I was not aware of the rich Japanese bath culture before my trip, but after my first visit to a traditional onsen (a natural hot-spring with a water temperature of around 40°C) I immediately understood why most Japanese love this tradition since centuries. By combining my research trip with some days of vacation, I was also able to visit Kyōto, often considered as Japan’s cultural center, as well as Tōkyō, Japan’s capital with its countless skyscrapers and modern overstimulation. Overall, I had a great time in Japan and I hope that I will be able to visit this amazing country again at some point in the future.

At this point, I would like to thank Prof. Dr. Masaki Satoh for organizing a very pleasant and interesting stay at AORI, as well as the organizers of WS-NHM for organizing such a great conference in Morioka. Furthermore, I would like to thank the TRR181 for providing the funding for this trip. I am very grateful for this research trip to Japan and would encourage other early-career researchers to seize similar opportunities.

Publications

Schmitt, M., Klingbeil, K., Shevchenko, R. & Umlauf, L. (2025). Three‐Dimensional Ocean Surface Layer Response to Atmospheric Cold Pools and Diurnal Heating in the Trade Wind Regime. Journal Of Geophysical Research Oceans, 130(8). https://doi.org/10.1029/2024jc022129
Peng, J., Jones, N. L., Rayson, M. D., Schmitt, M., Umlauf, L., Whitwell, C., Keating, S. R., Shakespeare, C. J. & Ivey, G. N. (2025). Interactions Between Diurnal Warm Layers and Surface‐Layer Fronts. Journal Of Geophysical Research Oceans, 130(1). https://doi.org/10.1029/2024jc021380
Brüggemann, N., Losch, M., Scholz, P., Pollmann, F., Danilov, S., Gutjahr, O., Jungclaus, J., Koldunov, N., Korn, P., Olbers, D., Eden, C. (2024). Parameterized Internal Wave Mixing in Three Ocean General Circulation Models. Journal of Advances in Modeling Earth Systems, 16, e2023MS003768. doi: https://doi.org/10.1029/2023MS003768
Schmitt, M., Pham, H.T., Sarkar, S., Klingbeil, K. & Umlauf, L. (2024). Diurnal warm layers in the ocean: energetics, non-dimensional scaling, and parameterization. J. Phys. Oceanogr. 54(4), 1037-1055, doi: https://doi.org/10.1175/JPO-D-23-0129.1.
Shevchenko, R., Hohenegger, C., & Schmitt, M. (2023). Impact of diurnal warm layers on atmospheric convection. J. Geophys. Res. - Atmospheres 128(14), e2022JD038473, doi: https://doi.org/10.1029/2022JD038473.
Umlauf, L., Klingbeil K., Radke, H., Schwefel, R., Bruggeman, J. & Holtermann, P.L. (2023). Hydrodynamic control of sediment-water fluxes: Consistent parameterization and impact in coupled benthic-pelagic models. J. Geophys. Res. - Oceans 128, e2023JC019651, doi: https://doi.org/10.1029/2023JC019651.
Shi, J., Stepanek, C., Sein, D., Streffing, J., & Lohmann, G. (2023). East Asian summer precipitation in AWI-CM3: Comparison with observations and CMIP6 models. International Journal of Climatology, 1– 16, doi: https://doi.org/10.1002/joc.8075.
Pithan, F., Athanase, M., Dahlke, S., Sánchez-Benítez, A., Shupe, M. D., Sledd, A., Streffing, J., Svensson, G., & Jung, T. (2023). Nudging allows direct evaluation of coupled climate models with in situ observations: a case study from the MOSAiC expedition. Geosci. Model Dev. 16(7), 1857–1873, doi: https://doi.org/10.5194/gmd-16-1857-2023.
Hohenegger, C., Korn, P., Brüggemann, N., Gutjahr, O., Jungclaus, J., Shevchenko, R., von Storch, J.S. et al. (2023). ICON-Sapphire: simulating the components of the Earth system and their interactions at kilometer and subkilometer scales. Geosci. Model Dev. 16, 779–811, doi: https://doi.org/10.5194/gmd-16-779-2023.
Chrysagi, E., Basdurak, N.B., Umlauf, L., Gräwe, U. & Burchard, H. (2022). Thermocline Salinity Minima Due To Wind-Driven Differential Advection. J. Geophys. Res.- Oceans 127(11), doi: https://doi.org/10.1029/2022JC018904.
Streffing, J., Scholz, P., Koldunov, N., Danilov, S., Juricke, S., Jung, T. et al. (2022). AWI-CM3 coupled climate model: description and evaluation experiments for a prototype post-CMIP6 model. Geosci. Model Dev. 15, 6399–6427, doi: https://doi.org/10.5194/gmd-15-6399-2022.
Strommen, K., Juricke, S. & Cooper, F. (2022). Improved teleconnection between Arctic sea ice and the North Atlantic Oscillation through stochastic process representation. Weather Clim. Dynam. 3(3), 951–975, doi: https://doi.org/10.5194/wcd-3-951-2022.
Franzke, C.L.E., Gugole, F. & Juricke, S. (2022). Systematic multi-scale decomposition of ocean variability using machine learning. Chaos: An Interdisciplinary Journal of Nonlinear Science 32(7), 073122, doi: https://doi.org/10.1063/5.0090064.
Kumar, A., Brüggemann, N., Smith, R., & Marotzke, J. (2022). Response of a tropical cyclone to a subsurface ocean eddy and the role of boundary layer dynamics. Q.J.R. Meteorol. Soc. 148, 378-402, doi: https://doi.org/10.1002/qj.4210.
Peng, J.-P., Dräger-Dietel, J., North, R. P., & Umlauf, L. (2021). Diurnal Variability of Frontal Dynamics, Instability, and Turbulence in a Submesoscale Upwelling Filament, J. Phys.Oceanogr. 51(9), 2825-2843, doi: https://doi.org/10.1175/JPO-D-21-0033.1.
Sidorenko, D., Danilov, S., Streffing, J., Juricke, S., Jung, T., Koldunov, N. et al. (2021). AMOC variability and watermass transformations in the AWI climate model. J. Adv. Model Earth Sy. 13, e2021MS002582, doi: https://doi.org/10.1029/2021MS002582.
Li, Q., Bruggemann, J., Burchard, H., Klingbeil, K., Umlauf, L. & Bolding, K. (2021). Integrating CVMix into GOTM (v6.0): a consistent framework for testing, comparing, and applying ocean mixing schemes. Geosci. Model Dev., doi: https://doi.org/10.5194/gmd-14-4261-2021.
Carpenter, J. R. , Rodrigues, A., Schultze, L. K. P., Merckelbach, L. M., Suzuki, N., Baschek, B. & Umlauf, L. (2020). Shear Instability and Turbulence Within a Submesoscale Front Following a Storm. Geophys. Res. Lett., doi: https://doi.org/10.1029/2020GL090365.
Rackow, T., & Juricke, S (2019). Flow‐dependent stochastic coupling for climate models with high ocean‐to‐atmosphere resolution ratio. Q. J. Roy. Meteor. Soc., 1-17, https://doi.org/10.1002/qj.3674.