Generation of finescale thermohaline variability in the Antarctic Circumpolar Current

Vertical profiles of temperature (T) and salinity (S) in the ocean commonly exhibit finescale variability in the form of interleaving, with cold and fresh layers in between warm and salty. These features are widespread in the Southern Ocean, particularly in the Polar Front Zone of the Antarctic Circumpolar Current (ACC) and in energetic regions downstream of complex topography. Interleaving is a signature of cross-frontal exchange – with potentially significant contributions to poleward heat transport, mixing and water mass transformation in the frontal regions. With typical timescales on the order of days and vertical scales of O (10 m) in the upper 1000 m of the ACC, interleaving features are not captured in global climate models and their generation mechanism remains unclear. Motivated by in-situ observations of thermohaline interleaving in the Southern Ocean, we use a Lagrangian particle tracking approach to demonstrate the generation of interleaving features by isopycnal stirring and chaotic advection. Particles are advected backward-in-time along isopycnal surfaces, using realistic gradient wind velocities from SatGEM-2 – a full-depth time-evolving representation of Southern Ocean T-S and velocity fields, constructed from observations. With the addition of small-scale perturbations to the velocity field, accounting for unresolved motions, interleaving features develop on timescales of <10 days in the energetic meander regions of the ACC – highlighting the importance of small-scale processes in reproducing the observed variability. While characteristics of the large-scale thermohaline and velocity field are important, we show how transient features of the flow field cause particle dispersion and mediate cross-frontal exchange processes within the energetic meanders.