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The export of Antarctic Bottom Water (AABW) supplies the bottom cell of the global overturning circulation and plays a key role in regulating climate. This AABW outflow must cross, and is therefore mediated by, the Antarctic Circumpolar Current (ACC). Previous studies present widely varying conceptions of the role of the ACC in directing AABW across the Southern Ocean, suggesting either that AABW may be zonally recirculated by the ACC, or that AABW may flow northward within deep western boundary currents (DWBC) against bathymetry. In this study the authors investigate how the forcing and geometry of the ACC influences the transport and transformation of AABWusing a suite of process-oriented model simulations. The model exhibits a strong dependence on the elevation of bathymetry relative to AABWlayer thickness: higher meridional ridges suppress zonal AABWexchange, increase the strength of flow in the DWBC, and reduce the meridional variation in AABWdensity across the ACC. Furthermore, the transport and transformation vary with density within the AABW layer, with denser varieties of AABW being less efficiently transported between basins. These findings indicate that changes in the thickness of the AABW layer, for example, due to changes in Antarctic shelf processes, and tectonic changes in the sea floor shape may alter the pathways and transformation of AABW across the ACC. [ABSTRACT FROM AUTHOR]

The oceanic Meridional Overturning Circulation (MOC) plays a key role in the climate system, and monitoring its evolution is a scientific priority. Monitoring arrays have been established at several latitudes in the Atlantic Ocean, but other latitudes and oceans remain unmonitored for logistical reasons. This study explores the possibility of inferring the MOC from globally‐available satellite measurements via machine learning (ML) techniques, using the ECCOV4 state estimate as a test bed. The methodological advantages of the present approach include the use purely of available satellite data, its applicability to multiple basins within a single ML framework, and the ML model simplicity (a feed‐forward fully connected neural network (NN) with small number of neurons). The ML model exhibits high skill in MOC reconstruction in the Atlantic, Indo‐Pacific, and Southern Oceans. The approach achieves a higher skill in predicting the model Southern Ocean abyssal MOC than has previously been achieved via a dynamically‐based approach. The skill of the model is quantified as a function of latitude in each ocean basin, and of the time scale of MOC variability. We find that ocean bottom pressure generally has the highest reconstruction skill potential, followed by zonal wind stress. We additionally test which combinations of variables are optimal. Furthermore, ML interpretability techniques are used to show that high reconstruction skill in the Southern Ocean is mainly due to (NN processing of) bottom pressure variability at a few prominent bathymetric ridges. Finally, the potential for reconstructing MOC strength estimates from real satellite measurements is discussed. Plain Language Summary: The Meridional Overturning Circulation (MOC) plays a key role in the exchange of heat and chemical constituents between oceans globally, and between the atmosphere and the deep ocean. However, it is currently directly monitored only at a handful of different latitudes in the Atlantic Ocean. Using an ocean simulation constrained by available measurements, we examine the feasibility of monitoring the MOC indirectly using measurements made by satellites. We train a so‐called "neural network" to learn relations between the MOC and satellite‐observable ocean properties, such as pressure at the sea floor. These relations are used to produce reconstructions of the simulated MOC derived from the satellite‐observable ocean properties alone, and to test which satellite‐observable ocean properties and regions may be most impactful in MOC reconstruction. We find that this approach yields high skill in reconstructing the MOC over much of the ocean, suggesting that this approach could be transferred to indirectly monitor the MOC in nature. Key Points: The Meridional Overturning Circulation (MOC) is a key ocean circulation pattern, which is under‐observedWe use an ocean state estimate to test the feasibility of inferring the MOC based solely on satellite observationsA unified machine learning methodology shows high skill in MOC reconstruction as a function of latitude and basin [ABSTRACT FROM AUTHOR]

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. However, 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. Using Coupled Model Intercomparison Project Phase 6 and reanalysis data, we show a significant reduction in the near‐Antarctic surface winds throughout the historical period that continues until the end of the twenty‐first century, amounting to 23% and 7% for the easterly and southerly wind components respectively under the high emission scenario. The most intense weakening happens during the summer season. We find that the weakening is coherent with the trend toward a positive Southern Annular Mode and a reduction of the pole‐to‐coast meridional pressure gradient, which we term Antarctic Annular Index. Plain Language Summary: Surface winds over the ocean around the Antarctic continent influence several aspects of the oceanic circulation and sea ice in the region that become relevant in the context of climate change. For example, Antarctic coastal surface winds have been found to drive the warming experienced in some regions that subsequently triggers increased ice shelf melt. However, there is little understanding regarding how this wind regime is expected to change in the future, with most research focusing on the mid‐latitude westerlies. In this work, we use Coupled Model Intercomparison Project Phase 6 models to quantify projected changes in these winds to the end of the twenty‐first century, hoping that it will aid in the interpretation of the ocean's response to climate change. Under the high emission scenario, we find a significant weakening of 23% for the easterly wind component and 7% for the meridional wind component. This weakening can be partly explained by a large‐scale pattern of change in sea level pressure that reflects in an increase of the atmospheric mode of variability known as the Southern Annular Mode, and a decrease of the pole‐to‐coast surface pressure gradient, which we term Antarctic Annular Index. Key Points: Coupled Model Intercomparison Project Phase 6 models show a weakening of the near‐Antarctic surface winds during the period 1979–2015Future projections in CMIP6 models show that the weakening trend continues until the end of the twenty‐first centuryWeakened winds are associated with a more positive Southern Annular Mode and a reduction in the pole‐to‐coast meridional pressure gradient [ABSTRACT FROM AUTHOR]

Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee-wave parameterization that includes both lee-wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee-wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation. [ABSTRACT FROM AUTHOR]

Ocean circulation and mixing regulate Earth's climate by moving heat vertically within the ocean. We present a new formalism to diagnose the role of ocean circulation and diabatic processes in setting vertical heat transport in ocean models. In this formalism we use temperature tendencies, rather than explicit vertical velocities, to diagnose circulation. Using quasi-steady-state simulations from the Australian Community Climate and Earth-System Simulator Ocean Model (ACCESS-OM2), we diagnose a diathermal overturning circulation in temperature–depth space. Furthermore, projection of tendencies due to diabatic processes onto this coordinate permits us to represent these as apparent overturning circulations. Our framework permits us to extend the concept of "Super Residual Transport," which combines mean and eddy advection terms with subgridscale isopycnal mixing due to mesoscale eddies but excludes small-scale three-dimensional turbulent mixing effect, to construct a new overturning circulation—the "Super Residual Circulation" (SRC). We find that in the coarse-resolution version of ACCESS-OM2 (nominally 1° horizontal resolution) the SRC is dominated by an ~11-Sv (1 Sv ≡ 106 m3 s−1) circulation that transports heat upward. The SRC's upward heat transport is ~2 times as large in a finer-horizontal-resolution (0.1°) version of ACCESS, suggesting that a differing balance of super-residual and parameterized small-scale processes may emerge as eddies are resolved. Our analysis adds new insight into superresidual processes, because the SRC elucidates the pathways in temperature and depth space along which water mass transformation occurs. [ABSTRACT FROM AUTHOR]

Internal waves generated at the seafloor propagate through the interior of the ocean, driving mixing where they break and dissipate. However, existing theories only describe these waves in two limiting cases. In one limit, the presence of an upper boundary permits bottom-generated waves to reflect from the ocean surface back to the seafloor, and all the energy flux is at discrete wavenumbers corresponding to resonant modes. In the other limit, waves are strongly dissipated such that they do not interact with the upper boundary and the energy flux is continuous over wavenumber. Here, a novel linear theory is developed for internal tides and lee waves that spans the parameter space in between these two limits. The linear theory is compared with a set of numerical simulations of internal tide and lee wave generation at realistic abyssal hill topography. The linear theory is able to replicate the spatially averaged kinetic energy and dissipation of even highly nonlinear wave fields in the numerical simulations via an appropriate choice of the linear dissipation operator, which represents turbulent wave breaking processes. [ABSTRACT FROM AUTHOR]

The Southern Ocean has undergone significant climate-related changes over recent decades, including intensified westerly winds and increased radiative heating. The interplay between wind-driven cooling and radiative warming of the ocean is complex and remains unresolved. In this study, idealized wind and thermal perturbations are analyzed in a global ocean–sea ice model at two horizontal resolutions: nominally, 1° and 0.1°. The sea surface temperature (SST) response shows a clear transition from a wind-driven cooling phase to a warming phase. This warming transition is largely attributed to meridional and vertical Ekman heat advection, which are both sensitive to model resolution due to the model-dependent components of temperature gradients. At higher model resolution, due to a more accurate representation of near-surface vertical temperature inversion and upward Ekman heat advection around Antarctica, the anomalous SST warming is stronger and develops earlier. The mixed layer depth at midlatitudes initially increases due to a wind-driven increase in Ekman transport of cold dense surface water northward, but then decreases when the thermal forcing drives enhanced surface stratification; both responses are more sensitive at lower model resolution. With the wind intensification, the residual overturning circulation increases less in the 0.1° case because of the adequately resolved eddy compensation. Ocean heat subduction penetrates along more tilted isopycnals in the 1° case, but it orients to follow isopycnal layers in the 0.1° case. These findings have implications for understanding the ocean response to the combined effects of Southern Hemisphere westerly wind changes and anthropogenic warming. [ABSTRACT FROM AUTHOR]

The flow of tides over rough bathymetry in the deep ocean generates baroclinic motion including internal waves and bottom‐trapped tides. The stresses generated by this motion feedback on the amplitude and phase of the large‐scale tide. Quantifying the stresses associated with tidal flow over abyssal hills is especially important, as this scale of bathymetry is often unresolved in global baroclinic tide models, and the stresses must therefore be parameterized. Here, we extend the previous theoretical work of the authors to determine the amplitude, phasing, and vertical location of the stresses exerted on a flow driven by a time‐periodic body force when it encounters rough bathymetry. The theory compares favorably with a suite of fully nonlinear numerical simulations. It is shown that all topographic stresses are applied directly above the bathymetry, leading to a two‐layer baroclinic flow, with the near‐bottom spatial‐mean flow (the benthic tide) strongly modified by topographic stresses, and the flow at height unperturbed by the presence of topography. Our results provide a framework to improve baroclinic tide models by (i) providing a simple parameterization for the subinertial stress which is currently not included in any models, (ii) establishing that parameterized stresses should be applied in the diffusive boundary layer directly above the topography, independent of where internal tides may dissipate, and (iii) identifying a minimum resolution of ∼10 km for baroclinic tidal models to adequately capture wave resonance effects that can significantly impact the magnitude of the benthic tide. Plain Language Summary: The ocean tide is a periodic motion of the water column at the scale of ocean basins. The amplitude of the ocean tide is determined by a balance between the gravitational attraction of the moon and the forces exerted when the tide flows over the rough seafloor. Much of the roughness in the deep ocean is due to mountains on the seafloor, 10–50 km in width, known as "abyssal hills." In this study, mathematical descriptions are developed for the strength and distribution of the forces exerted on the ocean tide due to its interaction with these hills. These forces always act below the crest of the hills, leading to a sub‐crest or "benthic tide" that differs from the tide above the crest of the hills. Our results provide a framework for improving the representation of tides in numerical ocean models. Key Points: The interaction of the spatial‐mean tide with rough bathymetry generates baroclinic motion including internal tidesThese motions exert stresses on the spatial‐mean tide near the seafloor which modifies the benthic tidal amplitudeThe benthic tidal amplitude may be decreased or increased depending on the nature of the interaction [ABSTRACT FROM AUTHOR]

Changes in ventilation of the Southern Hemisphere oceans in response to changes in midlatitude westerly winds are examined by analyzing the ideal age tracer from global eddy-permitting ocean–ice model simulations in which there is an abrupt increase and/or a meridional shift in the winds. The age response in mode and intermediate waters is found to be close to linear; the response of a combined increase and shift of peak winds is similar to the sum of the individual responses to an increase and a shift. Further, a barotropic response, following Sverdrup balance, can explain much of the age response to the changes in wind stress. There are similar peak decreases (of around 50 years) in the ideal age for a 40% increase or 2.5° poleward shift in the wind stress. However, while the age decreases throughout the thermocline for an increase in the winds, for a poleward shift in the winds the age increases in the north part of the thermocline and there are decreases in age only south of 35°S. As a consequence, the change in the volume of young water differs, with a 15% increase in the volume of water with ages younger than 50 years for a 40% increase in the winds but essentially no change in this volume for a 2.5° shift. As ventilation plays a critical role in the uptake of carbon and heat, these results suggest that the storage of anthropogenic carbon and heat in mode and intermediate waters will likely increase with a strengthening of the winds, but will be much less sensitive to a meridional shift in the peak wind stress. [ABSTRACT FROM AUTHOR]

The action of the barotropic tide over seafloor topography is the major source of internal waves at the bottom of the ocean. This internal tide has long been recognized to play an important role in ocean mixing. Here it is shown that the internal tide is also associated with a net (domain integrated) momentum flux. The net flux occurs as a result of the Doppler shifting of the internal tide at the point of generation by near-bottom mean flows. Linear theory is presented that predicts the amplitude of the wave momentum flux. The net flux scales with the bottom flow speed and the topographic wavenumber to the fourth power and is directed opposite to the bottom flow. For realistic topography, the predicted peak momentum flux occurs at scales of order 10 km and smaller, with magnitudes of order 10−3–10−2 N m−2. The theory is verified by comparison with a suite of idealized internal wave-resolving simulations. The simulations show that, for the topography considered, the wave momentum flux radiates away from the bottom and enhances mean and eddying flow when the tidal waves dissipate in the upper ocean. Our results suggest that internal tides may play an important role in forcing the upper ocean. [ABSTRACT FROM AUTHOR]

In the Southern Ocean, strong eastward ocean jets interact with large topographic features, generating eddies that feed back onto the mean flow. Deep-reaching eddies interact with topography, where turbulent dissipation and generation of internal lee waves play an important role in the ocean's energy budget. However, eddy effects in the deep ocean are difficult to observe and poorly characterized. This study investigates the energy contained in eddies at depth, when an ocean jet encounters topography. This study uses a two-layer ocean model in which an imposed unstable jet encounters a topographic obstacle (either a seamount or a meridional ridge) in a configuration relevant to an Antarctic Circumpolar Current frontal jet. The authors find that the presence of topography increases the eddy kinetic energy (EKE) at depth but that the dominant processes generating this deep EKE depend on the shape and height of the obstacle as well as on the baroclinicity of the jet before it encounters topography. In cases with high topography, horizontal shear instability is the dominant source of deep EKE, while a flat bottom or a strongly sheared inflow leads to deep EKE being generated primarily through baroclinic instability. These results suggest that the deep EKE is set by an interplay between the inflowing jet properties and topography and imply that the response of deep EKE to changes in the Southern Ocean circulation is likely to vary across locations depending on the topography characteristics. [ABSTRACT FROM AUTHOR]

An overturning circulation, driven by prescribed buoyancy forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and buoyancy forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing. The zonal momentum budget integrated over each layer is considered in the buoyancy-forced, wind-forced, and combined forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the buoyancy-forced case, the forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom friction to act as the initial sources of eastward momentum, with bottom friction acting over a longer time frame. In the wind-forced and combined forcing cases, the surface wind stress dominates the initial momentum budget, and the time to reach equilibration is shorter in the combined forcing simulation. These results imply that future changes in the rate of formation of Antarctic Bottom Water may alter the volume transport of the Antarctic Circumpolar Current. [ABSTRACT FROM AUTHOR]

Oscillatory behavior of the Atlantic meridional overturning circulation (MOC) is thought to underlie Atlantic multidecadal climate variability. While the energy sources and sinks driving the mean MOC have received intense scrutiny over the last decade, the governing energetics of the modes of variability of the MOC have not been addressed to the same degree. This paper examines the energy conversion processes associated with this variability in an idealized North Atlantic Ocean model. In this model, the multidecadal variability arises through an instability associated with a so-called thermal Rossby mode, which involves westward propagation of temperature anomalies. Applying the available potential energy (APE) framework from stratified turbulence to the idealized ocean model simulations, the authors study the multidecadal variability from an energetics viewpoint. The analysis explains how the propagation of the temperature anomalies leads to changes in APE, which are subsequently converted into the kinetic energy changes associated with variations in the MOC. Thus, changes in the rate of generation of APE by surface buoyancy forcing provide the kinetic energy to sustain the multidecadal mode of variability. [ABSTRACT FROM AUTHOR]

The authors study intrinsic variability in the position of jets in a β-plane channel ocean with simple topography using a quasigeostrophic numerical model. This study links the variability in jet position with abyssal anticyclones that form as a result of interaction of mesoscale eddies and subsurface topography, reminiscent of such flows as the Zapiola anticyclone. A simple dynamical framework explaining this behavior is developed. In this framework, this study shows that the topographic anticyclones form closed regions of homogenized yet time-varying potential vorticity. Neighboring topographic anticyclones are coupled by eddy fluxes. Interaction of a baroclinic jet with these two (or more) anticyclones can drive variability in local jet strength. Predictions of the dynamical framework are then compared with the results of the numerical model, and it is demonstrated that this model has merit in explaining the observed model variability. This study argues that this simple mode of variability has relevance for the ocean. [ABSTRACT FROM AUTHOR]

The eddy field in the Southern Ocean offsets the impact of strengthening winds on the meridional overturning circulation and Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. Here, an idealized numerical model is employed to investigate the response of the large-scale circulation in the Southern Ocean to wind stress perturbations at eddy-permitting to eddy-resolving scales. Significant differences are observed between the sensitivities and the resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modeled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesized that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports. [ABSTRACT FROM AUTHOR]

An analytical model of the full-depth ocean stratification and meridional overturning circulation for an idealized Atlantic basin with a circumpolar channel is presented. The model explicitly describes the ocean response to both Southern Ocean winds and the global pattern and strength of prescribed surface buoyancy fluxes. The construction of three layers, defined by the two isopycnals of overturning extrema, allows the description of circulation and stratification in both the upper and abyssal ocean. The system is fully solved in the adiabatic limit to yield scales for the surface layer thickness, buoyancies of each layer, and overturning magnitudes. The analytical model also allows scaling of the Antarctic Circumpolar Current (ACC) transport. The veracity of the three-layer framework and derived scales is confirmed by applying the analytical model to an idealized geometry, eddy-permitting ocean general circulation model. Consistent with previous results, the abyssal overturning is found to scale inversely with wind stress, whereas the North Atlantic overturning and surface-layer thickness scale linearly with wind stress. In terms of the prescribed surface buoyancy fluxes, increased negative fluxes (buoyancy removal) in the North Atlantic increase the North Atlantic overturning and surface-layer thickness, whereas increased positive fluxes in the middle and low latitudes lead to a decrease in both parameters. Increased negative surface buoyancy fluxes to the south of Drake Passage increase the abyssal overturning and reduce the abyssal buoyancy. The ACC transport scales to first order with the sum of the Ekman transport and the abyssal overturning and thus increases with both wind stress and southern surface buoyancy flux magnitude. [ABSTRACT FROM AUTHOR]