Circulation in the Southern Ocean is unique. The strong wind stress forcing and buoyancy fluxes, in concert with the lack of continental boundaries, conspire to drive the Antarctic Circumpolar Current replete with an intense eddy field. The effect of Southern Ocean eddies on the ocean circulation is significant – they modulate the momentum balance of the zonal flow, and the meridional transport of tracers and mass. The strength of the eddy field is controlled by a combination of forcing (primarily thought to be wind stress) and intrinsic, chaotic, variability associated with the turbulent flow field itself. Here, we present results from an eddy-permitting ensemble of ocean model simulations to investigate the relative contribution of forced and intrinsic processes in governing the variability of Southern Ocean eddy kinetic energy. We find that variations of the eddy field are mostly random, even on longer (interannual) timescales. Where correlations between the wind stress forcing and the eddy field exist, these interactions are dominated by two distinct timescales – a fast baroclinic instability response; and a multi-year process owing to feedback between bathymetry and the mean flow. These results suggest that understanding Southern Ocean eddy dynamics and its larger-scale impacts requires an ensemble approach to eliminate intrinsic variability, and therefore may not yield robust conclusions from observations alone.

Ocean eddies influence regional and global climate through mixing and transport of heat and properties. One of the most recognizable and ubiquitous feature of oceanic eddies are coherent vortices with spatial scales of tens to hundreds of kilometers, frequently referred as “mesoscale eddies”. Coherent mesoscale eddies are known to transport properties across the ocean and to locally affect near-surface wind, cloud properties, and rainfall patterns. Although coherent eddies are ubiquitous, their climatology, seasonality, and long-term temporal evolution remains poorly understood. Here, we examine the kinetic energy contained by coherent eddies and present the seasonal, interannual and long-term variability using satellite observations between 1993 to 2019. A total of $\sim$37 million coherent eddies are detected in this analysis. Around 50% of the kinetic energy contained by ocean eddies corresponds to coherent eddies. Additionally, a strong seasonal cycle is observed, with a 3-6 months lag between the wind forcing and the response of the coherent eddy field. The seasonality of the number of coherent eddies and their amplitude reveals that the number of coherent eddies responds faster to the forcing ($\sim$3 months), than the coherent eddy amplitude (which lags by $\sim$6 months). This seasonal cycle is spatially variable, so we also analyze their climatology in key oceanic regions. Our analysis highlights the relative importance of the coherent eddy field in the ocean kinetic energy budget, implies a strong response of the eddy number and eddy amplitude to forcing at different time-scales, and showcases the seasonality, and multidecadal trends of coherent eddy properties.