Finite volume ocean models are key tools in oceanography and climate science. However, confidence in their fidelity can be undermined by spurious numerical mixing of tracers such as heat and carbon. Substantial efforts have been made in recent years to remedy this issue, for example the use of high-order advection schemes and modifications to model vertical coordinates, though existing tools to quantify rates of numerical mixing are cumbersome. Simple and efficient ways of diagnosing how much a model is numerically mixing are needed to guide model development, especially for accurate representation of the Southern Ocean where significant spurious numerical mixing can occur. We present a new formula for the variance production in a model grid cell due to numerical mixing computed from available prognostic and diagnostic model information. The formula is derived using the water mass transformation framework within grid cells during a model’s tracer advection step. We use the formula to determine how vertical coordinate choice impacts rates of numerical mixing in an idealised configuration of the Modular Ocean Model version 6 with a focus on how much numerical mixing occurs along the idealised ''Antarctic slope''.

Co-presenter:
Dr Jan Zika, Associate Professor UNSW

The Antarctic Circumpolar Current (ACC) is a critical component of the climate system due to its role in the global ocean. This study focuses on a meander of the ACC South of Tasmania, a hotspot for small-scale ocean motions ( < 100 km) participating actively in cross-frontal and vertical transport. These scales were only partially observed by in situ observations and are now globally observed by the Surface Water and Ocean Topography (SWOT) satellite from space. By analysing drifter, SWOT swath observations and gridded altimetry products, we characterise the observed fine-scale dynamics to give insights about their role in the region South of Tasmania.
While gridded products successfully capture the bulk of the kinetic energy associated with large-scale mesoscale features, critical differences emerge at finer scales. In the spectral domain, scale-dependent analysis from SWOT and gridded products differ significantly between the 100 km and 10 km scales, illustrating the enhanced finer scale energy uniquely resolved by SWOT. This change of regime at small scales is corroborated by drifter-based metrics. Scale-dependent drifter-pair analysis reveals elevated dispersion rates at these scales, marked by a transition in the dispersion regime. By linking drifter and altimetry-based metrics through a spatial scale decomposition approach, we bridge the observational gap at fine scales establishing a robust framework to accurately quantify ocean transport and exploit SWOT observations more widely.

Surface cooling and brine rejection during sea-ice formation over the Antarctic continental shelf generate dense water masses that descend through the water column, cross the shelf break, and propagate downslope to form Antarctic Bottom Water (AABW). Observations of these dense overflows frequently exhibit low-frequency oscillations and coherent eddies, particularly in the Weddell Sea, although their dynamics remain poorly understood. Previous studies have linked these oscillations to topographic Rossby waves (TRWs) generated through baroclinic instability of the dense overflow. As the overflow descends along the continental slope, its evolution is governed by the combined effects of buoyancy forcing, rotation, bottom topography, and potential-vorticity conservation, providing favourable conditions for the generation of TRWs.

In this study, we investigate the dispersion characteristics of TRWs over a range of Rossby numbers using idealised simulations of the Antarctic continental shelf conducted using Direct Numerical Simulations (DNS). The dispersion relation is also derived analytically and systematically compared with results from the simulations, demonstrating consistent dynamical behaviour with the analytical solution. In addition, an empirical orthogonal function (EOF) analysis is employed to quantify the energetic structure of the vertical modes of the wave field. This analysis reveals a clear transition in the vertical energy distribution with changes in Rossby number, highlighting a shift from predominantly baroclinic structures at higher Rossby numbers toward increasingly barotropic dominance as rotational effects become more dominant.

The Southern Ocean drives global ocean circulation and carbon cycling, yet remains critically under-observed. Three coupled observational challenges prevent a complete picture: (1) sparse, asynchronous sampling cannot resolve mesoscale features evolving on 10–100 day timescales; (2) Antarctic Circumpolar Current (ACC) fronts and eddies at 1–100 km scales drive most exchange but sit below observing resolution; (3) vertical velocities O(10 m/day) are unmeasurable directly, and cross-frontal flow is ageostrophic and unresolved without synoptic mesoscale context.
We propose a diagnostic pathway with an example region characterised by strong vertical and cross-frontal fluxes south of Tasmania and New Zealand. The framework combines: (i) SATGEM converts sparse CTD profiles into continuous 4D hydrography (SSH, temperature, salinity) with thermal wind balance; (ii) gradient wind balance diagnoses full-depth ageostrophic flow, capturing convergence and divergence in high-curvature ACC jets; (iii) quasi-geostrophic omega equation diagnoses full-depth vertical velocity from buoyancy and gradient wind velocities.
The result provides daily estimates of heat and tracers across ACC fronts, identifying upwelling and downwelling regions critical for nutrient transport, primary production and carbon sequestration. We outline validation with EM-APEX floats (Argo floats with velocity sensors) and demonstrate utility for field campaign targeting and model validation.

At the continental shelf in Antarctica, cross-shelf exchange is a major source of heat for ice-shelf melting, particularly near the grounding line. This heat supply to ice-shelf cavities occurs primarily at depth, along troughs, and is associated with ocean dynamics on scales as small as 1–5 km. Given the remote and icy conditions, the Antarctic continental margins remain extremely poorly observed, particularly for ocean dynamics at these scales and depths. The recent launch of the Surface Water and Ocean Topography (SWOT) satellite enables us to observe ocean dynamics at unprecedented horizontal resolution, but sea ice makes the interpretation of these observations challenging. Here, we use a novel method to interpret SWOT observations in the presence of sea ice and provide a first assessment of small-scale ocean dynamics in Vincennes Bay, East Antarctica. Further, we use a high-resolution model to link the surface velocities to cross-shelf heat exchange at depth.

The Southern Ocean and its continental shelves play critical roles in regulating Earth’s climate through their influence on heat and energy transport. Accurate representation of these processes is hindered by large uncertainties in regional bathymetry. We present a comprehensive, animal-borne bathymetric dataset derived from 1,629 southern elephant, Weddell and crabeater seals equipped with satellite relay data loggers across the Southern Ocean. The data set contains 4,492,767 individually georeferenced dives, with a mean location uncertainty of 1.5 km. Approximately 10.3 % of dives recorded depths at least 20 m deeper than corresponding values in IBCSO V2 (3.3% were 200 m deeper). These discrepancies were most common on the Antarctic continental shelf and mid-ocean plateaus. The seal-derived bathymetry identified numerous areas of uncharted deep water, ocean channels and troughs. These new bathymetric observations will improve oceanographic models and global climate forecasts. The new dataset is complementary to the global SEABED 2030 initiative to map the ocean floor by 2030.

The Antarctic Circumpolar Current (ACC) and its main Polar Front (PF) form a strong barrier between warm northern waters and cold polar waters, limiting heat exchange across the Southern Ocean. Yet fine-scale ocean features (< 50km), such as eddies and filaments, can act as efficient routes transporting water-mass properties across major ACC fronts. These fine-scale features were largely unobserved until the recent Surface Water and Ocean Topography (SWOT) mission, which provides unprecedented high-resolution observations of the ocean surface.
The FOCUS voyage targeted an energetic meander of the ACC south of Tasmania, during the formation of an eddy dipole associated with the flexing of the Polar Front. Here, we combine the high-resolution hydrographic sections collected during FOCUS with SWOT observations to investigate how fine-scale dynamics drive cross-frontal exchanges at the surface and in the ocean interior.
SWOT observations reveal intense strain and frontal structures around the eddy dipole, where the flow flexes and pinches the Polar front. Hydrographic sections show that this eddy dipole stretches water-mass anomalies and promotes active cross-frontal exchange. Cyclonic stirring is associated with northward cold intrusions below the mixed layer, while pinching of the Polar Front induces an anticyclonic recirculation that advects warm/light surface filaments southward. The resulting stirring, filamentation, and mixing highlight the important role of submesoscale processes in driving net poleward heat transport across the Polar Front.