First application of regional coupled ocean-atmosphere components in the ICON Earth System Model: the case study of medicane IANOS

The ICON-Ocean (ICON-O) model (Korn et al., 2022), a component of the ICON Earth System Model, has been extensively utilized for global ocean modeling. Within the project “Earth System Modelling at the Weather Scale" (ESM-W) by DWD in collaboration with GeoInfoDienst BW, the development of a coupled ocean-atmosphere forecasting system is underway, combining ICON-O for the ocean component and ICON-NWP for the atmosphere. To advance this effort, the regional version of ICON-O is being implemented in Limited Area Mode (LAM), enabling targeted, high-resolution studies of regional ocean dynamics.

This work presents the first high-resolution application (2.5km) of ICON-O-LAM coupled with its atmospheric counterpart ICON-NWP, focusing on the simulation of the Medicane IANOS (September 2020). Medicanes, Mediterranean tropical-like cyclones, are intense, small-scale systems whose development is strongly influenced by air-sea interactions. Accurate simulation of these phenomena requires dynamic coupling between the ocean and atmosphere to capture feedback processes such as SST cooling, latent heat fluxes, and evolving surface wind patterns. Static SSTs, often used in uncoupled atmospheric simulations, such as those produced by the operational ICON atmospheric component, cannot capture these interactions and may lead to significant biases in storm intensity, structure, and track.

The coupled ICON-O-LAM/ICON-NWP simulations of Medicane IANOS reveal clear improvements in the representation of key storm characteristics compared to uncoupled simulations. The coupled system reproduces more realistic wind intensities and sea level pressure deepening, highlighting the role of air-sea coupling in modulating the storm's lifecycle. These results emphasize the added value of using a regional coupled modeling approach for high-impact weather events in semi-enclosed basins like the Mediterranean, where fine-scale ocean-atmosphere feedbacks play a critical role in storm dynamics.