Surface mixed-layer modulation by mesoscale eddies in the Southern Ocean

Deep Southern Ocean (SO) winter mixed layers play a critical role in setting the global ocean uptake of heat and carbon. Yet coupled climate models typically fail to represent these, with prevailing shallow mixed-layer biases that have previously been attributed to excess surface freshwater fluxes. Here we explore the role of mesoscale circulations in setting SO surface mixing by applying eddy tracking to both observations and a high-resolution (1/20°) eddy-resolving model of the SO.

To assess the role of eddies in modulating model mixed-layer depth, we use smoothed model fields in an `offline' simulation of tracer budgets (in an otherwise fixed mean state). Thereby a 50 % suppression of model kinetic energy on spatial scales smaller than Ο(100 km) is found to result in mixed layers shallower by as much as ≈ 200 m in the regions of deep winter mixing along and equatorward of the ACC.

An observational estimate of SO mesoscale mixed-layer signatures is provided by analyzing the relationship between multi-altimeter sea surface height and mixed-layer depth derived from individual Argo temperature-salinity profiles. By following eddy tracks with origins along the ACC path we isolate a systematic mixed-layer signal of ACC eddies: warm-top anticyclones are characterized by deeper, cold-top cyclones by shallower mixed-layers, in both model and observations. The track-following analysis moreover reveals an asymmetry between enhanced surface mixing and deeper mixed layers in anticyclones that outweighs the suppression of mixing in cyclones. Mesoscale features are thereby identified to be at the origin of the SO mixed-layer shallowing found in association to EKE suppression in the model. A missing representation of mesoscale-driven enhancement of surface mixing may thus be a major factor in current climate model biases, highlighting the need to further develop existing parameterizations.