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.

Surface winds around the Antarctic continent control coupled ocean-ice processes that influence the climate system, including bottom water production, heat transport onto the continental shelf and sea ice coverage. Few studies have examined projected changes in these winds, even though it would aid in the interpretation and understanding of the ocean’s response to climate change. In this work we examine historical changes in the near-Antarctic surface winds using Coupled Model Intercomparison Project Phase 6 models and reanalysis data, and quantify projected changes to the end of the 21st Century. These changes include a significant reduction in both the easterly and meridional wind components, which under the high emission scenario amounts to 23% and 7% respectively, most of which occurs during the summer season. The projected weakening of surface winds are coherent with a trend towards a positive Southern Annular Mode and a reduction of the pole-to-coast meridional pressure gradient.

Identifying internal waves in complex flow fields is a long-standing problem in fluid dynamics, oceanography and atmospheric science, owing to the overlap of internal waves temporal and spatial scales with other flow regimes. Lagrangian filtering — that is, temporal filtering in a frame of reference moving with the flow — is one proposed methodology for performing this separation. Here we (i) describe a new implementation of the Lagrangian filtering methodology and (ii) introduce a freely available, parallelised Python package that applies the method. We show that the package can be used to directly filter output from a variety of common ocean models including MITgcm, ROMS and MOM5 for both regional and global domains at high resolution. The Lagrangian filtering is shown to be a useful tool to both identify (and thereby quantify) internal waves, and to remove internal waves to isolate the ‘balanced’ flow field.

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.

“Eddy saturation” is the regime in which the total time-mean volume transport of an oceanic current is relatively insensitive to the wind stress forcing and is often invoked as a dynamical description of Southern Ocean circulation. We revisit the problem of eddy saturation using a primitive-equations model in an idealized channel setup with bathymetry. We apply only mechanical wind stress forcing; there is no diapycnal mixing or surface buoyancy forcing. Our main aim is to assess the relative importance of two mechanisms for producing eddy saturated states: (i) the commonly invoked baroclinic mechanism that involves the competition of sloping isopycnals and restratification by production of baroclinic eddies, and (ii) the barotropic mechanism, that involves production of eddies through lateral shear instabilities or through the interaction of the barotropic current with bathymetric features. Our results suggest that the barotropic flow-component plays a crucial role in determining the total volume transport.

The meridional variability of the Subtropical Front (STF) and the drivers of variability on interannual time scales in the New Zealand region are analysed using a multi-decadal eddy-resolving ocean hindcast model, in comparison with Argo data. The STF marks the water mass boundary between subtropical waters and subantarctic waters, and is defined as the southern-most location of the 11 degree C isotherm and 34.8 isohaline between 100 m and 500 m. The STF shifts up to 650 km (6 degree) meridionally on seasonal timescales. In addition to seasonal variability, shifts of around 200 km (2 degree) occur on interannual time scales. These shifts are connected to local wind stress curl anomalies in the eastern Tasman Sea, which trigger Ekman convergence/divergence and result in meridional transport of heat and salt into/out of the Tasman Sea. The net transports across the northern boundary of the Tasman Sea show the largest sensitivity to these wind stress curl anomalies. During periods of positive wind stress curl anomalies and Ekman convergence, the heat and salt content increases shifting the position of the STF southward. The opposite tendency occurs during periods of negative wind stress curl anomalies. The migration of the STF does not appear to be directly linked to regional climate oscillations.

The generation of topographic internal waves (IWs) by the sum of an oscillatory and a steady flow is investigated experimentally and with a linear model. The two forcing flows represent the combination of a tidal constituent and a weaker quasi-steady flow interacting with an abyssal hill. The combined forcings cause a coupling between internal tides and lee waves that impacts their dynamics of internal waves as well as the energy carried away. An asymmetry is observed in the structure of upstream and downstream internal wave beams due to a Doppler shift effect. This asymmetry is enhanced for the narrowest ridge on which a super-buoyancy (ω>N) downstream beam and an evanescent upstream beam are measured. Energy fluxes are measured and compared with the linear model, that has been extended to account for the coupling mechanism. The structure and amplitude of energy fluxes match well in most regimes, showing the relevance of the linear prediction for IW wave energy budgets, while the energy flux toward IW beams is limited by the generation of periodic vortices in a particular experiment. The upstream-bias energy flux - and consequently net horizontal momentum - described in Shakespeare [2020] is measured in the experiments. The coupling mechanism plays an important role in the pathway to IW induced mixing, that has previously been quantified independently for lee waves and internal tides. Hence, future parameterizations of IW processes ought to include the coupling mechanism to quantify its impact on the global distribution of mixing.