Patterns of thermohaline interleaving in the Cape Basin

The Cape Basin off the western coast of South Africa is characterized by rich mesoscale and submesoscale variability generated by the shedding of eddies and filaments from the Agulhas Retroflection. These features carry warm, salty water into the cooler, fresher South Atlantic Ocean. Thermohaline interleaving is common in this region due to strong lateral water-mass gradients and the presence of stirring processes. These intrusions are an important pathway to water-mass transformation because they result in an increased surface area over which diapycnal mixing can work to homogenize water property contrasts. We present the first observational study that can track interleaving features at high vertical and horizontal resolution over distances of O(100 km) in the Cape Basin by using diapycnal spiciness curvature to detect intrusions within data collected by the Wire Flyer towed profiling vehicle and EM-APEX profiling floats. These datasets show interleaving features present throughout thermocline waters (σ = 26.0 – 27.5), but their scales and slopes vary significantly. Wire Flyer sections highlight strong differences in interleaving characteristics and generation mechanisms over distances of 10s of km. Several of the transects exhibit signatures of internal waves, which appear to modulate the interleaving structure. EM-APEX floats provide an alternative, semi-Lagrangian sampling perspective and show the persistence of interleaving features over time scales of 1-15 days. We conclude that the thermohaline variability in this region is primarily driven by mesoscale stirring, although double diffusion may be acting to grow the features after their formation. This study showcases the variety of physical processes existing at different length and time scales that contribute to the formation and structure of interleaving in the Cape Basin.