Fine-scale processes (1–100 km) are the most energetic features of the ocean, influencing basin-scale and global circulation. They have a major impact on the transport of heat and salt, water mass distribution, regulate climate-critical air-sea fluxes, and shape the biogeochemical environment. Within the ERC Synergy Grant project WHIRLS, we aim to define the structure, evolution and phenomenology of fine-scale physical and biogeochemical processes in the upper 1000 m of the water column in our focus area, the Agulhas Current system around South Africa and the Cape Basin, a global hotspot for mesoscale and submesoscale activity, where eddy dynamics, heat exchange, and marine productivity strongly interact. The approach involves combining a variety of in-situ observations with model simulations. We develop a dedicated hierarchy of global NEMO-based configurations, including high-resolution nests with horizontal grid spacing below the km-scale. Results from a first reference simulation with eINALT100, a double-nested configuration with grid spacings down to 900 m in the core Agulhas region, are presented. Validation against SWOT and other observations shows that mesoscale features and the full inverse kinetic energy cascade are resolved. After further validation of the general circulation, e.g., circulation and transport along and across the observational ASCA array, ongoing analyses focus on assessing the representation of submesoscale processes across a series of sensitivity experiments that vary in the numerical representation of advection, tides, mixing, and wind forcing. Analyses include the distribution of kinetic energy across scales, the structure and statistics of eddies, fronts and filaments, and their impact on vertical mixing. These metrics, compared where possible to observations from satellites, Argo floats, and high-frequency moored current-meter measurements, will guide the set-up of an improved reference configuration which will be used for a decadal-scale simulation.