Submesoscale processes reduce water mass retention in Agulhas Rings

Abstract. Mesoscale and submesoscale ocean processes (1–100 km) are central to global ocean circulation, water mass transformation, and both the horizontal and vertical transport of tracers such as heat and carbon. The Agulhas Current system and the Cape Basin are hotspots for such dynamics, where mesoscale and submesoscale flows interact strongly and impact the large-scale flow. Among the most prominent features in this region are anticyclonic Agulhas Rings that are shed from the Agulhas retroflection as well as cyclonic eddies, which both decay while transporting relatively warm and saline waters into the South Atlantic. In this study, we investigate the retention potential of these eddies, representing the loss of water masses into the surrounding ocean. To isolate the role of the submesoscale, we compare two global ocean model configurations with grid-refinements in the focus region: one with 1/20 ° resolution that is strongly eddying but does not resolve submesoscale flows, and another with 1/60 ° resolution and modified tracer and momentum advection, allowing for the simulation of realistic submesoscale motions throughout the Agulhas region. We use Lagrangian simulations within the Parcels framework, with virtual particles representing small amounts of water masses. Coherent eddies, both anticyclonic and cyclonic, are identified horizontally and vertically using automated eddy-detection and tracking algorithms. Over a 6-year period, large numbers of particles (O(10⁵–10⁷) per eddy) are seeded in 3D within eddies intersecting the GoodHope line and tracked to assess how long the particles remain in the eddies and the spatiotemporal patterns of their exchange with the surrounding ocean. Our results reveal that cyclones lose their water masses much faster than anticyclones. Submesoscale processes reduce the retention timescale of anticyclonic eddies by a factor of about 2. The enhanced water parcel loss in anticyclones is associated with lateral stirring driven by submesoscale fronts, filaments, and mixed-layer baroclinic instability as shown by a case study, thereby accelerating the transport of heat and salt into the South Atlantic. For cyclonic eddies, the results point in the same direction, albeit more weakly, but the smaller sample size and the weaker representation of strong cyclones in the coarser configuration prevent a robust quantification.