Mesoscale eddies play a major role in the transport of tracers in the ocean. Focusing on a sector in the east Pacific, the authors present estimates of eddy diffusivities derived from kinematic tracer simulations using satellite-observed velocity fields. Meridional diffusivities are diagnosed, and how they are related to eddy properties through the mixing length formulation of Ferrari and Nikurashin, which accounts for the suppression of diffusivity due to eddy propagation relative to the mean flow, is shown. The uniqueness of this study is that, through systematically varying the zonal-mean flow, a hypothetical “unsuppressed” diffusivity is diagnosed. At a given latitude, the unsuppressed diffusivity occurs when the zonal-mean flow equals the eddy phase speed. This provides an independent estimate of eddy phase propagation, which agrees well with theoretical arguments. It is also shown that the unsuppressed diffusivity is predicted very well by classical mixing length theory, that is, that it is proportional to the rms eddy velocity times the observed eddy size, with a spatially constant mixing efficiency of 0.35. Then, the suppression factor is estimated and it is shown that it too can be understood quantitatively in terms of easily observed mean flow properties. The authors then extrapolate from these sector experiments to the global scale, making predictions for the global surface eddy diffusivity. Together with a prognostic equation for eddy kinetic energy and a theory explaining observed eddy sizes, these concepts could potentially be used in a closure for eddy diffusivities in coarse-resolution ocean climate models.

Almost all heat reaching the bases of Antarctica's ice shelves originates from warm Circumpolar Deep Water in the open Southern Ocean. This study quantifies the roles of mean and transient flows in transporting heat across almost the entire Antarctic continental slope and shelf using an ocean/sea ice model run at eddy‐ and tide‐resolving (1/48°) horizontal resolution. Heat transfer by transient flows is approximately attributed to eddies and tides via a decomposition into time scales shorter than and longer than 1 day, respectively. It is shown that eddies transfer heat across the continental slope (ocean depths greater than 1,500 m), but tides produce a stronger shoreward heat flux across the shelf break (ocean depths between 500 m and 1,000 m). However, the tidal heat fluxes are approximately compensated by mean flows, leaving the eddy heat flux to balance the net shoreward heat transport. The eddy‐driven cross‐slope overturning circulation is too weak to account for the eddy heat flux. This suggests that isopycnal eddy stirring is the principal mechanism of shoreward heat transport around Antarctica, though likely modulated by tides and surface forcing.

Lateral diffusivities are computed from synthetic particles and tracers advected by a velocity field derived from sea-surface height measurements from the South Pacific, in a region west of Drake Passage. Three different estimates are compared: (1) the tracer-based "effective diffusivity" of Nakamura (1996), (2) the growth of the second moment of a cloud of tracer and (3) the single-and two-particle Lagrangian diffusivities. The effective diffusivity measures the cross-stream component of eddy mixing, so this article focuses on the meridional diffusivities for the others, as the mean flow (the ACC) is zonally oriented in the region. After an initial transient of a few weeks, the effective diffusivity agrees well with the meridional diffusivity estimated both from the tracer cloud and from the particles. This proves that particleand tracer-based estimates of eddy diffusivities are equivalent, despite recent claims to the contrary. Convergence among the three estimates requires that the Lagrangian diffusivities be estimated using their asymptotic values, not their maximum values. The former are generally much lower than the latter in the presence of a mean flow. Sampling the long-time asymptotic behavior of Lagrangian diffusivities requires very large numbers of floats in field campaigns. For example, it is shown that hundreds of floats would be necessary to estimate the vertical and horizontal variations in eddy diffusivity in a sector of the Pacific Southern Ocean.

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.

Particle-and tracer-based estimates of lateral diffusivities are used to estimate the suppression of eddy mixing across strong currents. Particles and tracers are advected using a velocity field derived from sea surface height measurements from the South Pacific, in a region west of Drake Passage. This velocity field has been used in a companion paper to show that both particle-and tracer-based estimates of eddy diffusivities are equivalent, despite recent claims to the contrary. These estimates of eddy diffusivities are here analyzed to show 1) that the degree of suppression of mixing across the strong Antarctic Circumpolar Current is correctly predicted by mixing length theory modified to include eddy propagation along the mean flow and 2) that the suppression can be inferred from particle trajectories by studying the structure of the autocorrelation function of the particle velocities beyond the first zero crossing. These results are then used to discuss how to compute lateral and vertical variations in eddy diffusivities using floats and drifters in the real ocean.

Mesoscale eddies mix tracers along isopycnals and horizontally at the sea surface. This paper compares different methods of diagnosing eddy mixing rates in an idealized, eddy-resolving model of a channel flow meant to resemble the Antarctic Circumpolar Current. The first set of methods, the ''perfect'' diagnostics, are techniques suitable only to numerical models, in which detailed synoptic data is available. The perfect diagnostic include flux-gradient diffusivities of buoyancy, QGPV, and Ertel PV; Nakamura effective diffusivity; and the four-element diffusivity tensor calculated from an ensemble of passive tracers. These diagnostics reveal a consistent picture of isopycnal mixing by eddies, with a pronounced maximum near 1000 m depth. The isopycnal diffusivity differs from the buoyancy diffusivity, a.k.a. the Gent-McWilliams transfer coefficient, which is weaker and peaks near the surface and bottom. The second set of methods are observationally ''practical'' diagnostics. They involve monitoring the spreading of tracers or Lagrangian particles in ways that are plausible in the field. We show how, with sufficient ensemble size, the practical diagnostics agree with the perfect diagnostics in an average sense. Some implications for eddy parameterization are discussed.

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Declining atmospheric CO2 concentrations are considered the primary driver for the Cenozoic Greenhouse-Icehouse transition, ~34 million years ago. A role for tectonically opening Southern Ocean gateways, initiating the onset of a thermally isolating Antarctic Circumpolar Current, has been disputed as ocean models have not reproduced expected heat transport to the Antarctic coast. Here we use high-resolution ocean simulations with detailed paleobathymetry to demonstrate that tectonics did play a fundamental role in reorganising Southern Ocean circulation patterns and heat transport, consistent with available proxy data. When at least one gateway (Tasmanian or Drake) is shallow (300 m), gyres transport warm waters towards Antarctica. When the second gateway subsides below 300 m, these gyres weaken and cause a dramatic cooling (average of 2–4 °C, up to 5 °C) of Antarctic surface waters whilst the ACC remains weak. Our results demonstrate that tectonic changes are crucial for Southern Ocean climate change and should be carefully considered in constraining long-term climate sensitivity to CO2.