Direct observations of submesoscale turbulence driving mesoscale eddy leakage

Understanding oceanic transport and material fate has long relied on observations that primarily resolve mesoscale, largely geostrophic flows, leaving the role of submesoscale turbulence comparatively unobserved and poorly understood. Here, using new high-resolution drifter-cluster experiments with closest initial spacings below 1 km, we provide direct Lagrangian observations of how submesoscale turbulence modifies mesoscale eddy coherence and transport pathways. The clustered drifters resolve intense, transient ageostrophic motions that are fundamentally absent from geostrophic descriptions of eddy behavior. Analyzed in an eddy-centered reference frame, the observations reveal pronounced submesoscale-driven radial dispersion that facilitates substantial water-mass exchange between the eddy core and its periphery in both the mixed layer and the subsurface. Quantitative estimates indicate submesoscale horizontal diffusivity of O(10 to 100) m²/s and a nonlocal dispersion regime across submesoscales. These results challenge the traditional view of mesoscale eddies as coherent structures containing a persistent water mass, with broad implications for interpreting and predicting the transport and fate of tracers in the ocean.