Patterns of Thermohaline Interleaving in the Cape Basin

Abstract The Cape Basin off the western coast of South Africa is abundant in mesoscale and submesoscale variability. Here, the Agulhas retroflection sheds eddies and filaments that bring warm, salty Indian Ocean water into contact with the cooler, fresher South Atlantic Ocean. Thermohaline interleaving is prevalent in the Cape Basin, presumably due to strong lateral water-mass gradients and eddy stirring. 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. This work presents the first observational study that tracks interleaving features at high vertical and horizontal resolution over distances of O (100) km in the Cape Basin using the Wire Flyer towed profiling vehicle and rapid profiling floats. Interleaving features are present throughout mode, thermocline, and intermediate waters ( σ = 26.0–27.5 kg m −3 ), but their scales and slopes vary significantly. Wire Flyer sections highlight strong differences in interleaving characteristics and generation mechanisms over distances of tens of kilometers. Several transects exhibit signatures of internal waves, which appear to modulate the interleaving structure. Profiling floats provide an alternative, semi-Lagrangian sampling perspective and show the persistence of interleaving over time scales of 1–20 days. Sections from both the Wire Flyer and profiling floats reveal that the interleaving strength can change abruptly over short distances of O (10) km. This work demonstrates 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. Significance Statement This study reveals novel observations of interleaving in the unique and highly energetic environment of the Cape Basin. The Wire Flyer’s unparalleled capability to sample across strong currents and resolve interleaving characteristics in thermocline waters at unprecedented horizontal and vertical scales provides an objective snapshot of the ocean’s fine-scale structure at the periphery of mesoscale eddies. These observations should prompt a renewed discussion about the mechanisms driving the formation of interleaving, as well as future work on the complex interactions between the physical processes that contribute to interleaving, which is integral to the forward cascade of temperature and salinity variance in the ocean.