Abstract. We introduce ACCESS-OM2, a new version of the ocean–sea ice model of the Australian Community Climate and Earth System Simulator. ACCESS-OM2 is driven by a prescribed atmosphere (JRA55-do) but has been designed to form the ocean–sea ice component of the fully coupled (atmosphere–land–ocean–sea ice) ACCESS-CM2 model. Importantly, the model is available at three different horizontal resolutions: a coarse resolution (nominally 1∘ horizontal grid spacing), an eddy-permitting resolution (nominally 0.25∘), and an eddy-rich resolution (0.1∘ with 75 vertical levels); the eddy-rich model is designed to be incorporated into the Bluelink operational ocean prediction and reanalysis system. The different resolutions have been developed simultaneously, both to allow for testing at lower resolutions and to permit comparison across resolutions. In this paper, the model is introduced and the individual components are documented. The model performance is evaluated across the three different resolutions, highlighting the relative advantages and disadvantages of running ocean–sea ice models at higher resolution. We find that higher resolution is an advantage in resolving flow through small straits, the structure of western boundary currents, and the abyssal overturning cell but that there is scope for improvements in sub-grid-scale parameterizations at the highest resolution.

The oceans are traversed by a large-scale overturning circulation, essential for the climate system as it sets the rate at which the deep ocean interacts with the atmosphere. The main region where deep waters reach the surface is in the Southern Ocean, where they are transformed by interactions with the atmosphere and sea-ice. Here, we present an observation-based estimate of the rate of overturning sustained by surface buoyancy fluxes in the Southern Ocean sea-ice sector. In this region, the seasonal growth and melt of sea-ice dominate water-mass transformations. Both sea-ice freezing and melting act as a pump, removing freshwater from high latitudes and transporting it to lower latitudes, driving a large-scale circulation that upwells 27 ± 7 Sv of deep water to the surface. The upwelled water is then transformed into 22 ± 4 Sv of lighter water and 5 ± 5 Sv into denser layers that feed an upper and lower overturning cell, respectively.

The Southern Ocean is a critical component of the global climate system and an important ecoregion that contains a diverse range of interdependent flora and fauna. The Southern Ocean also hosts numerous fronts: sharp boundaries between waters with different characteristics. As they strongly influence exchanges between the ocean, atmosphere and cryosphere, fronts are of fundamental importance to the climate system. However, rapid advances in physical oceanography over the past 20 years have challenged previous definitions of fronts and their response to anthropogenic climate change. Here, we review the implications of this recent research for the study of climate, ecology and biology in the Southern Ocean. We include a frontal definition "user guide" to clarify the current debate and facilitate future research. The Southern Ocean, generally defined as the global ocean south of about 35 • S that encircles the Antarctic continent, is unique oceanographic environment due to the lack of continental barriers blocking its flow and the strong winds that blow over its surface 1. At large scales, the Southern Ocean is characterised by both the intense eastward flowing Antarctic Circumpolar Current (ACC), one of the most powerful current systems on Earth, and strongly tilted isopycnals (lines of constant density) that shallow to the south. Observations of the Southern Ocean dating back to the Discovery expedition in the 1920s revealed that the transition from warmer subtropical waters to colder Antarctic waters does not occur smoothly, but is concentrated into a series of sharp transition zones, aligned generally east-west, that have come to be called fronts 2. Further observations revealed that salinity, oxygen, nutrients and various other tracers showed similar behaviour, and that between the fronts, water properties are relatively homogeneous. As such, fronts delimit the boundaries between different water-masses with distinct environmental characteristics 3. These fronts also tend to coincide with the location of narrow yet very intense currents known as "jets" 4 that dominate the ACC's flow 5. The Southern Ocean is divided by fronts into a number of distinct biophysical zones, and hence a number of distinct habitats, which in turn support distinct biota 6, 7. Numerous studies have shown that seabirds and marine mammals tend to congregate and forage in and around fronts 7. As the Earth continues to warm due to anthropogenic climate change, it is vital that we understand how these fronts and jets will respond to changes in the global climate system, and what influence that might have on associated ecosystems 8-10. Due to its remoteness and harsh climate, undertaking field studies in the Southern Ocean is both difficult and expensive. As a result, the Southern Ocean is amongst the most data-sparse of all major ocean basins, which has hindered progress on key questions regarding its dynamics and ecological communities 10. In recent decades however, a deluge of new data from satellites and Argo profiling floats, along with e...

The frontal structure of the Southern Ocean is investigated using a sophisticated frontal detection methodology, the Wavelet/Higher Order Statistics Enhancement (WHOSE) method, introduced in Chapman [2014]. This methodology is applied to 21 years of daily gridded sea-surface height (SSH) data to obtain daily maps of the locations of the fronts. By forming 'heat-maps' of the frontal occurrence frequency and then approximating these heat-maps by a superposition of simple functions, the time-mean locations of the fronts, as well as a measure of their capacity to meander, are obtained and related to the frontal locations found by previous studies.The spatial and temporal variability of the frontal structure is then considered. The number of fronts is found to be highly variable throughout the Southern Ocean, increasing ('splitting') downstream of large bathymetric features and decreasing ('merging') in regions where the fronts are tightly controlled by the underlying topography. In contrast, frontal meandering remains relatively constant. Contrary to many previous studies, little no southward migration of the fronts over the 1993-2014 time period is found, and there is only weak sensitivity to atmospheric forcing related to SAM or ENSO. The reasons for the discrepancy between this study and previous studies using contour methods are investigated and it is shown that the spatial variability of the frontal structure is not tied to the underlying sea-surface height. It is argued that the results of studies using sea-surface height contours to define front must be interpreted with care.