Mesoscale eddies are widely regarded as coherent structures that transport heat, salt, nutrients, and biogeochemical tracers over long distances while largely retaining their water-mass properties. However, the extent to which submesoscale turbulence modifies eddy coherence and facilitates cross-eddy exchange remains poorly known. Here, we present unprecedented Lagrangian observations of submesoscale turbulence within a cyclonic mesoscale eddy in the South China Sea obtained from a high-resolution cluster of drifters. The drifter observations resolve energetic, intermittent ageostrophic motions that are not captured by traditional geostrophic descriptions of mesoscale eddy dynamics. Analysis of relative dispersion and tracer exchange reveals vigorous submesoscale stirring throughout the eddy. These motions promote substantial water-mass exchange between the eddy core and surrounding waters in both the surface mixed layer and the subsurface ocean.
   By estimating the relative dispersion and diffusivity, we obtained effective horizontal diffusivities associated with submesoscale turbulence on the order of 10–100 m² s⁻¹. These diffusivities are sufficiently large to erode water-mass anomalies and substantially reduce eddy retention timescales. The results demonstrate that submesoscale processes can fundamentally alter the transport pathways within mesoscale eddies, providing an efficient mechanism for cross-eddy exchange that is absent from purely mesoscale or geostrophic perspectives. Our findings 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.