Upper-ocean submesoscale fronts, with their associated strong vertical velocities, are often claimed to play a significant role in subducting tracers into the interior. The role of these submesoscale processes in restratifying the mixed layer is now well recognized, but whether they simultaneously flux tracers through the base of the boundary layer remains an open question. We vary the resolution in a semirealistic channel model to control turbulent processes at various scales and study their influence on tracers. It is found that the submesoscale-permitting simulations flux far more tracer downward than the lower-resolution simulations: The 1-km simulation takes up 50% more tracer compared to the 20-km simulation, despite the increased restratifying influence of the resolved submesoscale processes. A full frequency-wave number cross-spectra of the vertical velocity and vertical tracer flux show that the high-frequency inertia-gravity waves that appear in the highest-resolution simulation play no role in irreversible downward tracer transport. Plain Language SummaryThe oceanic uptake and storage of anthropogenic carbon plays a central role in the global carbon budget, and a significant fraction of this uptake occurs in the Southern Ocean. It is well established that the eddies and fronts, turbulent fluctuations of the flow, are important for this uptake, but the relative influence of different scales is less understood. The eddies and fronts associated with the submesoscale (1-50 km) have been recognized to play a leading role in shallowing the surface boundary layer depths, but the role of the strong vertical velocities, which are generally associated with these features, in transporting fluid below the mixed layer remains unknown. Here we investigate these questions using an idealized Southern Ocean model with an imposed tracer source at the surface. The model resolution is varied as a means to include or omit turbulent processes at various scales. We find that the submesoscale-permitting simulations flux far more tracer downward than the lower-resolution simulations, despite the reduction in the depths of the vigorously mixed boundary layers. We also found that inertia-gravity waves, which are ubiquitous in the ocean and are generally associated with very strong vertical velocities, had no impact on the net tracer flux.

As part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES), 210 subsurface floats were deployed west of the Drake Passage on two targeted density surfaces. Absolute (single particle) diffusivities are calculated for the floats. The focus is on the meridional component, which is less affected by the mean shear. The diffusivities are estimated in several ways, including a novel method based on the probability density function of the meridional displacements. This allows the determination of the range of possible lateral diffusivities, as well as the period over which the spreading can be said to be diffusive. The method is applied to the float data and to synthetic trajectories generated with the Massachusetts Institute of Technology General Circulation Model (MITgcm). Because of ballasting problems, many of the floats did not remain on their targeted density surface. However, the float temperature records suggest that most occupied a small range of densities, so the floats were grouped together for the analysis. The latter focuses on a subset of 109 of the floats, launched near 105°W. The different methods yield a consistent estimate for the diffusivity of 800 ± 200 m2 s−1. The same calculations were made with model particles deployed on 20 different density surfaces and the result for the particles deployed on the neutral density surface γ = 27.7 surface was the same within the errors. The model was then used to map the variation of the diffusivity in the vertical, near the core of the Antarctic Circumpolar Current (ACC). The results suggest mixing is intensified at middepths, between 1500 and 2000 m, consistent with several previous studies.

Biological productivity in the Southern Ocean is limited by iron availability. Previous studies of iron supply have focused on mixed-layer entrainment and diapycnal fluxes. However, the Southern Ocean is a region highly energetic mesoscale and submesoscale turbulence. Here we investigate the role of eddies in supplying iron to the euphotic zone, using a flat-bottom zonally re-entrant model, configured to represent the Antarctic Circumpolar Current region, that is coupled to a biogeochemical model with a realistic seasonal cycle. Eddies are admitted or suppressed by changing the model's horizontal resolution. We utilize cross spectral analysis and the generalized Omega equation to temporally and spatially decompose the vertical transport attributable to mesoscale and submesoscale motions. Our results suggest that the mesoscale vertical fluxes provide a first-order pathway for transporting iron across the mixing-layer base, where diapycnal mixing is weak, and must be included in modeling the open-Southern Ocean iron budget. Plain Language SummaryOcean currents at the surface on the spatial scales of 1-200 km are energetic due to heating by the sun and stirring by the winds. These currents contribute to the climate system by transporting heat and carbon horizontally towards the poles and vertically into the deep ocean. By running a numerical simulation at very high spatial resolution that resolve these currents, we show that these currents are also responsible for transporting iron from the ocean interior to the surface in the Southern Ocean, where phytoplankton growth is limited by the lack of iron-a key nutrient for most living organisms on Earth. Our results highlight the importance of accurately representing these ocean currents and associated iron transport, in order to understand the Southern Ocean ecosystem and its impact on the climate via photosynthesis, the process in which carbon dioxide is converted to organic carbon and oxygen is produced as a bi-product.

Mixing by mesoscale eddies in the ocean plays a major role in setting the distribution of oceanic tracers, with important implications for physical and biochemical systems at local to global scales. Roach et al. (2016; https://doi.org/10.1002/2015JC011440) demonstrated that a two-particle analysis of Argo trajectories produces robust estimates of horizontal mixing in the Southern Ocean. Here we extend this analysis to produce global 1°× 1°maps of eddy diffusivity at the nominal Argo parking depth of 1,000 m. We also applied this methodology to estimate surface eddy diffusivities from Global Drifter Program (GDP) surface drifters. The global mean eddy diffusivity was 543 ± 155 m 2 /s at 1,000 m and 2637 ± 311 m 2 /s at the surface, with elevated diffusivities in regions of enhanced eddy kinetic energy, such as western boundary currents and along the Antarctic Circumpolar Current. The eddy kinetic energy at the equator is high at both the surface and depth, but the eddy diffusivity is only enhanced near the surface. At depth the eddy diffusivity is strongly suppressed due to the presence of mean flow. We used our observational estimates to test the validity of an eddy diffusivity parameterization that accounts for mixing suppression in the presence of zonal mean flows. Our results indicated that this parameterization generally agrees with the directly observed eddy diffusivities in the midlatitude and high-latitude oceans.