Max-Planck-Institute for Meteorology: New ICON Configuration Provides Realistic View of Small-Scale Ocean Eddies

For the first time, scientists were able to study the behavior of eddies measuring just a few kilometers in size in a realistic simulation of the North Atlantic using an innovative computational grid. It enabled them to determine how efficiently these sub-mesoscale eddies transported heat, salt, and trace gases between the ocean surface and the underlying water masses. This quantitative assessment within a realistic ocean model with unprecedented resolution allowed for a better understanding of small-scale ocean phenomena and their role within the climate system.

ICON simulation of the North Atlantic with a telescope grid. Credit: M. Epke/ N. Brüggemann

The exchange of heat and greenhouse gases between the atmosphere and the ocean depends on various factors, including small-scale processes. For instance, ocean eddies ranging in diameter from one to ten kilometers strongly influence this exchange. These sub-mesoscale eddies, which are clearly visible on satellite images, form where water masses with different temperatures and salinities meet. They re-stratify the water column, bringing new water masses into contact with the atmosphere while cutting others off. However, due to their small size, they elude the coarse resolution of traditional climate models. Researchers use approximate equations to still describe the statistical effect of these processes (parameterizations).

Scientists at the Max Planck Institute for Meteorology (MPI-M) have demonstrated that sub-mesoscale eddies can be explicitly and realistically simulated with an innovative configuration of the ICON ocean model. They also showed that this simulation can be used to understand small-scale climate processes and to test their common parameterizations under realistic conditions.

Telescope grid enables resolution as fine as 500 meters

The computational grid of the ICON model consists of triangles, which allows for a more flexible refinement than grids based on rectangles. Like the mesh of an elastic net over a ball, the telescope function allows the cells to be pulled into the region of interest on the globe. This type of grid refinement provides a high resolution in specific regions while avoiding undesirable effects occurring at their edges.

“By uniformly refining our telescope grid, we achieve high resolution not only in the region under investigation, but also in the adjacent areas,” said the study’s first author, MPI-M researcher Moritz Epke. “This eliminates the need to select boundary conditions, which would be required if refinement were restricted to a limited area.”

Map showing turbulent structures at the ocean surface. On the left, a broad field of eddies and narrow fronts; red dashed lines mark ocean fronts, and a red box indicates the enlarged area. On the right, a zoom into a front downstream of an island. The purple color scale represents intensity. — 50 ocean fronts highlighted in (a) were analyzed, a close-up is shown in (b). CC BY 4.0 Epke et al. 2025, DOI: 10.1175/JPO-D-25-0015.1

Cross‑section of the upper ocean showing density surfaces (isopycnals, thin gray) and the mixed‑layer depth (green dashed), which deepens slightly toward the right. Colors from green (downwelling) through white to magenta (upwelling) represent vertical motion; a coherent, strong upwelling core occupies the central region, flanked by weaker downwelling. Two black vertical lines divide the section into three parts; a color bar appears on the right. — Streamlines of the eddying circulation indicated by the color shading show in which direction the eddies transport water masses. Density is indicated by grey lines (isopycnals). The eddies move light water from one side of the ocean front on top of heavier water at the other side of the front. CC BY 4.0 Epke et al. 2025, DOI: 10.1175/JPO-D-25-0015.1

In the recently published study, which also involved MPI-M researchers Nils Brüggemann, Peter Korn, and Leonidas Linardakis, the focus of the telescope grid was on the North Atlantic, where pronounced ocean fronts with sub-mesoscale eddies occur in winter. The researchers pulled the cells of a simulation with a global horizontal grid-spacing of approximately four kilometers together to achieve a grid-spacing well below one kilometer in the North Atlantic.

“Such resolution over such a large area is extraordinary and pushes the limits of what is computationally possible with global models,” says co-author Nils Brüggemann.

Realistic variability

The simulation contains many sub-mesoscale eddies. Its temporal variability in the ocean exceeds that of comparable simulations by other teams. In addition, the spatial variability corresponded well with measurements from the Surface Water and Ocean Topography (SWOT) satellite mission. Using 50 identified ocean fronts, the researchers were able to study the behavior of the sub-mesoscale eddies in more detail and, among other things, determine that theoretical scaling arguments can roughly predict the overturning strength of those eddies. However, the authors also revealed that the theories come to a limit e.g. when larger mesoscale eddies are considered. Those eddies appeared to be much more stable compared to ocean fronts and compared to the theoretical expectations.

According to the authors, configuring the ICON ocean model with a telescope grid has proven very useful for investigating sub-mesoscale eddies. The team plans to use it next to examine other high-frequency processes, such as internal waves in the South Atlantic.

Original publication

Epke, M., Linardakis, L., Korn, P., Brüggemann, N. (2025). Overturning of mixed layer eddies in a submesoscale resolving simulation of the North Atlantic. Journal of Physical Oceanography, DOI: 10.1175/JPO-D-25-0015.1

Contact

Dr. Moritz Epke
Max Planck Institute for Meteorology
moritz.epke@we dont want spammpimet.mpg.de

Dr. Nils Brüggemann
Max Planck Institute for Meteorology
nils.brueggemann@we dont want spammpimet.mpg.de

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