Jan Jaap Meijer: A 4D Diagnostic Framework for Heat and Tracer Fluxes Across the Antarctic Circumpolar Current
The Southern Ocean drives global ocean circulation and carbon cycling, yet remains critically under-observed. Three coupled
observational challenges prevent a complete picture: (1) sparse, asynchronous sampling cannot resolve mesoscale features evolving on 10–100 day timescales; (2) Antarctic Circumpolar Current (ACC) fronts and eddies at 1–100 km scales drive most exchange but sit below observing resolution; (3) vertical velocities O(10 m/day) are unmeasurable directly, and cross-frontal flow is ageostrophic and unresolved without synoptic mesoscale context. We propose a diagnostic pathway with an example region characterised by strong vertical and cross-frontal fluxes south of Tasmania and New Zealand. The framework combines: (i) the Gravest Empirical Mode that converts sparse CTD profiles into continuous 4D hydrography (SSH, temperature, salinity) with thermal wind balance; (ii) gradient wind balance diagnoses full-depth ageostrophic flow, capturing convergence and divergence in high-curvature ACC jets; (iii) quasi geostrophic omega equation diagnoses full-depth vertical velocity from buoyancy and gradient wind velocities. The result provides daily estimates of heat and tracers across ACC fronts, identifying upwelling and downwelling regions critical for nutrient transport, primary production and carbon sequestration. We outline validation with EM-APEX floats (Argo floats with velocity sensors) and demonstrate utility for field campaign targeting and model validation.