SQG-based Reconstruction of Mesoscale-to-Submesoscale Dynamics: Applicability of Different Methods in the Core Agulhas System

High-resolution satellite observations increasingly enable characterization of mesoscale and submesoscale ocean variability, but their ability to inform reconstruction of subsurface circulation remains uncertain, particularly in regions where intense multi-scale interactions challenge geostrophic constraints. The Surface Quasi-Geostrophic (SQG) framework offers potential in reconstructing three-dimensional upper-ocean dynamics from surface fields, yet its performance across methods and dynamical regimes has not been systematically quantified. Using two parallel physically consistent mesoscale-resolving (1/20°) and submesoscale-permitting (1/60°) model simulations, we investigate four established SQG-based reconstruction methods for their applicability to reconstruct three-dimensional subsurface velocity and density anomalies from surface information in the core Agulhas region. The extended “interior + surface quasigeostrophic” numerical solution-based method (L19), which refines the representation of higher baroclinic modes following the “effective” SQG framework, emerges as the most skillful. L19 effectively reconstructs mesoscale structures (>100 km) and maintains strong spectral agreement with model simulations down to ~50 km near the surface. Its skill varies with seasonal mixed‐layer depth and regional eddy activity, improving under shallow, stable mixed layers and in energetic areas along the Agulhas Retroflection, ring pathways, and the Agulhas Return Current. While density reconstructions remain robust across dynamical regimes, velocity reconstructions deteriorate when submesoscale (<50 km) surface variability dominates, reflecting unresolved ageostrophic motions and rapid vertical decorrelation at submesoscales. These results delineate the effective operating range of SQG-based methods and provide a benchmark for applying submesoscale-resolving satellite observations (e.g., Surface Water and Ocean Topography) to investigate upper-ocean circulation within the Agulhas Current system and other dynamically active regions.