On the role of air-sea interaction in developing destructive tropical-like cyclone Daniel

Between 4 and 12 September 2023, a cyclogenesis event developed near the Greek coast in the Ionian Sea and evolved through two distinct phases: an initial baroclinic stage associated with intense orographic precipitation over Greece, followed by a barotropic phase characterized by the formation of an intense tropical-like cyclone (TLC), named Daniel, which later made landfall in Libya. This study examines the role of air-sea interaction in both the intensification and the structural transition of Daniel by means of numerical experiments performed with the WRF limited-area atmospheric model and with its two-way coupled atmosphere-ocean configuration. A set of sensitivity experiments was designed using different sea surface temperature (SST) datasets and ocean model parameterizations in order to assess how oceanic initial conditions and coupling processes influence cyclone evolution. The results show that SST exerts a primary control on the intensification and tropicalization of the system, not only along the cyclone track but also in the surrounding marine areas, where enhanced surface heat fluxes and high precipable water content contribute to sustaining convection and deepening the cyclone. In particular, warmer SST patterns favor stronger air-sea fluxes, a more symmetric warm-core structure, and a more pronounced tropical-like behavior. The coupled modeling framework, interpreted here as a “digital twin” of the atmosphere-ocean system, improves the representation of the cyclone structure, thermodynamic fields, and feedback mechanisms between the sea and the lower atmosphere. Compared with uncoupled simulations, the coupled experiments provide a more realistic depiction of the spatial distribution of latent and sensible heat fluxes, boundary-layer moistening, and the evolution of the storm core. At the same time, the experiments reveal a marked sensitivity of the simulated cyclone to the specific ocean model setup. Relatively small modifications in vertical mixing schemes, surface drag formulations, or SST initialization lead to substantial differences in both ocean response and atmospheric development, including storm intensity, organization, and transition timing. These findings confirm the central role of air-sea coupling in the development of Mediterranean TLCs and stress the importance of accurately representing upper-ocean processes in high-impact weather simulations. More broadly, they highlight the need for robust and carefully calibrated coupled modeling systems to improve the predictability of extreme Mediterranean cyclones in a warming climate.