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 Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC. Key Points:• Observations and high-resolution models have changed view on the AMOC pathways • High-resolution models suggest the presence of previously unknown high-frequency AMOC variability • High-resolution models allow to estimate the intrinsic/chaotic component of the AMOCCorrespondence to:

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Oceanic eddies play a profound role in mixing tracers such as heat, carbon, and nutrients, thereby regulating regional and global climate. Yet, it remains unclear how global oceanic eddy kinetic energy has evolved over the past few decades. Furthermore, coupled climate model predictions generally fail to resolve oceanic mesoscale dynamics, which could limit their accuracy in simulating future climate change. Here we show a global statistically significant increase of the eddy activity using two independent observational datasets of mesoscale variability, one directly measuring currents and the other from sea surface temperature.Regions characterized by different dynamical processes show distinct evolution in the eddy field. For example, eddy-rich regions such as boundary current extensions and the Antarctic Circumpolar Current show a significant increase of 2% and 5% per decade in eddy activity, respectively. In contrast, most of the regions of observed decrease are found in the tropical oceans. Because eddies play a fundamental role in the ocean transport of heat, momentum, 1 and carbon, our results have far-reaching implications for ocean circulation and climate, and the modelling platforms we use to study future climate change.Changes in the climate system over recent decades have warmed the upper ocean and modified the wind stress, heat and freshwater fluxes that drive ocean circulation 1, 2 . These changes have the capacity to modify the ocean circulation at all scales, including the overturning circulation 3, 4 , basin-scale gyres 5,6 , boundary currents 7,8 , and the mesoscale 9 . The ocean's mesoscale incorporates motions that occur at spatial scales from ∼10 to ∼100 km. These motions include both steady flows, such as jets and re-circulations, and time-varying flows, generally referred to as eddies. Mesoscale eddies are ubiquitous in the global ocean and feed back onto all scales, from regional processes 10 up to the meridional overturning circulation 3 . Moreover, these eddies act to transport and mix tracers such as heat, salt, and nutrients 11,12 . Thus, understanding the evolution of the mesoscale circulation is crucial to better predict our changing oceans. Kinetic energy (KE) quantifies the magnitude of ocean currents 9,[13][14][15] . Kinetic energy is proportional to the square of the velocity, and is commonly separated into the mean KE (MKE; computed from the time-mean velocity field) and the KE of the time-varying velocity (known as the Eddy Kinetic Energy; EKE). The EKE is dominated by mesoscale variability and is a significant fraction of the total KE 16,17 . A recent study has inferred a global increase of KE anomaly from ocean reanalyses and ARGO floats 15 . However, these reanalyses and observations do not have the spatial resolution required to resolve the mesoscale field. Satellite observations, which can resolve the mesoscale, suggest that EKE in the Southern Ocean has a robust increasing trend 9,18,19 . How-

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1] We provide a general framework for identifying the constituents responsible for asymmetry in any tidal time series, by extending and generalizing the skewness-based approach of Nidzieko (2010) to include any number of tidal constituents. We show that this statistic has two features which greatly simplify the attribution of asymmetry to particular constituents: (1) only combinations of two or three constituents can contribute to skewness, regardless of how many constituents are significant in the time series and (2) of those combinations, only the few meeting the frequency conditions 2w 1 = w 2 or w 1 + w 2 = w 3 will give rise to long-term mean asymmetry. It is therefore relatively easy to identify every such combination, even when many constituents are present. We then go on to show how the relative contribution of each such combination can be measured and compared, based on the amplitudes, frequencies and relative phases of the constituents. We also show that there is an upper bound to the skewness generated by any such combination. The metrics are applied to data from 335 worldwide sea level stations and from a global ocean tidal model based on TOPEX/POSEIDON altimetry. Global maps are made of the patterns of tidal skewness. We identify the combinations of astronomical tides that dominate skewness in different tidal regimes and geographic locations, and explain the dependence of skewness on tidal form number.

Topography influences the circulation in the Southern Ocean, generating stationary meanders in the lee of topographic features, triggering hot spots of mesoscale eddy kinetic energy, and modifying jets and fronts. However, the relationship between topography and submesoscale flows (with length scales of order 1-10 km) has not yet been explored. The first submesoscale-resolving (1/80• resolution) ocean model, with realistic topography of the Indian sector of the Southern Ocean, is used to investigate this interaction.The results show spatial inhomogeneity in submesoscale activity that is correlated with topography. Topographic influence is primarily indirect: topography controls mesoscale flows, which in turn generate submesoscale activity. Mesoscale eddy kinetic energy and strain rate can be used, to first order, to infer submesoscale vertical velocity, implying a possible route to parameterise submesoscale activity in coarser resolution models.

The mechanisms that initiate and maintain oceanic ''storm tracks'' (regions of anomalously high eddy kinetic energy) are studied in a wind-driven, isopycnal, primitive equation model with idealized bottom topography. Storm tracks are found downstream of the topography in regions strongly influenced by a largescale stationary meander that is generated by the interaction between the background mean flow and the topography. In oceanic storm tracks the length scale of the stationary meander differs from that of the transient eddies, a point of distinction from the atmospheric storm tracks. When the zonal length and height of the topography are varied, the storm-track intensity is largely unchanged and the downstream storm-track length varies only weakly. The dynamics of the storm track in this idealized configuration are investigated using a wave activity flux (related to the Eliassen-Palm flux and eddy energy budgets). It is found that vertical fluxes of wave activity (which correspond to eddy growth by baroclinic conversion) are localized to the region influenced by the standing meander. Farther downstream, organized horizontal wave activity fluxes (which indicate eddy energy fluxes) are found. A mechanism for the development of oceanic storm tracks is proposed: the standing meander initiates localized conversion of energy from the mean field to the eddy field, while the storm track develops downstream of the initial baroclinic growth through the ageostrophic flux of Montgomery potential. Finally, the implications of this analysis for the parameterization and prediction of storm tracks in ocean models are discussed.