Thermohaline Interleaving in the Antarctic Circumpolar Current

Abstract Interleaving layers are commonly observed in the subsurface ocean near thermohaline fronts. They imply a lateral exchange of water mass properties and have been shown to influence both lateral and vertical mixing rates. We examine a set of in situ observations, from electromagnetic autonomous profiling explorer (EM-APEX) floats and Triaxus transects, in an energetic meandering region of the Antarctic Circumpolar Current. The observations reveal strong interleaving features that are ubiquitous on the northern side of the Polar Front. Interleaving layers are of O (10) m thick and persist for up to 100-km downstream, over time scales of <1–6 days. The features have cross-stream slopes that are much shallower than expected from mesoscale stirring. The strength of the interleaving layers is partly governed by internal wave–driven diapycnal mixing rates that vary spatially and temporally. Generation of the interleaving layers is complex and could be impacted by several processes, including eddy stirring, shear, ageostrophic dynamics, and double diffusion. The ubiquitous nature and magnitude of these features on the warm side of the front attests to their likely role in cross-frontal exchange, mixing, and water mass transformation. Depth-averaged lateral heat flux estimates associated with the intrusions are of O (1) kW m −2 , while individual values reach O (10) kW m −2 , comparable to eddy heat flux estimates from previous studies. A circumpolar observational dataset reveals hotspots of thermohaline interleaving in the Southern Ocean, associated with major topographic features, confluence regions, and areas of elevated eddy activity. Understanding the processes that generate thermohaline variability, influence mixing rates, and drive cross-frontal exchange are key to understanding how the Southern Ocean regulates the global climate.