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Abstract The Southern Ocean's eddy response to changing climate remains unclear, with observations suggesting non‐monotonic changes in eddy kinetic energy (EKE) across scales. Here simulations reappear that smaller‐mesoscale EKE is suppressed while larger‐mesoscale EKE increases with strengthened winds. This change was linked to scale‐wise changes in the kinetic energy cycle, where a sensitive balance between the dominant mesoscale energy sinks—inverse KE cascade, and source—baroclinic energization. Such balance induced a strong (weak) mesoscale suppression in the flat (ridge) channel. Mechanistically, this mesoscale suppression is attributed to stronger zonal jets weakening smaller mesoscale eddies and promoting larger‐scale waves. These EKE multiscale changes lead to multiscale changes in meridional and vertical eddy transport, which can be parameterized using a scale‐dependent diffusivity linked to the EKE spectrum. This multiscale eddy response may have significant implications for understanding and modeling the Southern Ocean eddy activity and transport under a changing climate.

The article discusses a workshop funded by the NSF's Office of Polar Programs to address research priorities for observing the dynamics of the Southern Ocean. The Southern Ocean plays a crucial role in global climate, but its dynamics remain uncertain, impacting climate projections. The workshop identified research priorities, such as understanding ice shelf melt rates, sea ice variability, and ocean circulation, and emphasized the need for observational strategies to address these challenges. Additionally, the article highlights the importance of equity, diversity, and inclusion in fieldwork, data management, and international collaboration within the Southern Ocean research community. [Extracted from the article]

The Southern Ocean's eddy response to changing climate remains unclear, with observations suggesting non‐monotonic changes in eddy kinetic energy (EKE) across scales. Here simulations reappear that smaller‐mesoscale EKE is suppressed while larger‐mesoscale EKE increases with strengthened winds. This change was linked to scale‐wise changes in the kinetic energy cycle, where a sensitive balance between the dominant mesoscale energy sinks—inverse KE cascade, and source—baroclinic energization. Such balance induced a strong (weak) mesoscale suppression in the flat (ridge) channel. Mechanistically, this mesoscale suppression is attributed to stronger zonal jets weakening smaller mesoscale eddies and promoting larger‐scale waves. These EKE multiscale changes lead to multiscale changes in meridional and vertical eddy transport, which can be parameterized using a scale‐dependent diffusivity linked to the EKE spectrum. This multiscale eddy response may have significant implications for understanding and modeling the Southern Ocean eddy activity and transport under a changing climate. Plain Language Summary: The response of eddies in the Southern Ocean to climate change is not well understood. In this study, we used a channel model that simulates the effects of wind on eddies. We found that smaller eddies have less kinetic energy (KE) when the winds are stronger. On the other hand, larger‐scale eddies have more KE with stronger winds. Similar phenomena are also observed in the observations. By analyzing the eddy's KE budget, the interaction between different scales of eddies and the interaction between the eddies and mean flow are strengthened when the winds get stronger. This leads to a reduction of eddy KE at smaller mesoscale scales and an increase at larger scales. From the observational view, stronger winds weaken smaller eddies and promote larger waves. This change in eddy KE also affects how eddies meridionally transport materials and how eddy diffusivity varies at different scales. Smaller eddies transport materials less when their KE is weakened, while larger eddies become stronger in transporting materials. These findings determine how eddy diffusivity responds to the changed eddy KE at different scales. The multi‐scale response of eddies to wind has important implications for understanding the behavior of Southern Ocean eddies in a changing climate. Key Points: Larger eddies got stronger and smaller eddies got weaker as Southern Ocean westerlies strengthenedBoth flat and ridge channel simulations suggest that these changes may be linked to changes in the inverse energy cascadeThe corresponding changes in scale‐wise meridional and vertical transport are also non‐monotonic [ABSTRACT FROM AUTHOR]

It has been hypothesized that submesoscale flows play an important role in the vertical transport of climatically important tracers, due to their strong associated vertical velocities. However, the multiscale, nonlinear, and Lagrangian nature of transport makes it challenging to attribute proportions of the tracer fluxes to certain processes, scales, regions, or features. Here we show that criteria based on the surface vorticity and strain joint probability distribution function (JPDF) effectively decompose the surface velocity field into distinguishable flow regions, and different flow features, like fronts or eddies, are contained in different flow regions. The JPDF has a distinct shape and approximately parses the flow into different scales, as stronger velocity gradients are usually associated with smaller scales. Conditioning the vertical tracer transport on the vorticity–strain JPDF can therefore help to attribute the transport to different types of flows and scales. Applied to a set of idealized Antarctic Circumpolar Current simulations that vary only in horizontal resolution, this diagnostic approach demonstrates that small-scale strain-dominated regions that are generally associated with submesoscale fronts, despite their minuscule spatial footprint, play an outsized role in exchanging tracers across the mixed layer base and are an important contributor to the large-scale tracer budgets. Resolving these flows not only adds extra flux at the small scales, but also enhances the flux due to the larger-scale flows. [ABSTRACT FROM AUTHOR]

Standing meanders are a key component of the Antarctic Circumpolar Current (ACC) circulation system, and numerical studies have shown that these features may locally enhance subduction, upwelling, as well as lateral and vertical tracer transport. Yet, observational data from these regions remain sparse. Here, we present results based on measurements made by a group of autonomous platforms sampling an ACC standing meander formed due to the interaction of the Polar Front with the Southwest Indian Ridge. Two Seagliders were deployed alongside a Biogeochemical‐Argo float that was advected through the standing meander. In the high eddy kinetic energy region of the standing meander, the glider observations reveal enhanced submesoscale frontal gradients as well as heightened tracer variability at depth, as compared to the more quiescent region further downstream. Vertical gradients in spice and apparent oxygen utilization are reduced in the standing meander despite similarities in the large‐scale vertical stratification, suggesting greater ventilation of the surface ocean. These observations are consistent with numerical studies that highlight standing meanders as hotspots for ventilation and subduction due to enhanced mesoscale stirring and submesoscale vertical velocities. Our results emphasize the need to account for spatial heterogeneity in processes influencing air‐sea exchange, carbon export, and biogeochemical cycling in the Southern Ocean. Plain Language Summary: The Southern Ocean plays a vital role in taking up carbon dioxide and heat from the Earth's atmosphere. Yet, historically, this region has suffered from a lack of direct observations, making it difficult to quantify rates of exchange. In regions where the strongest currents of the Southern Ocean interact with underwater topography turbulence and mixing may be locally enhanced. Data collected from two different types of robotic vehicles, including steerable ocean gliders, were used to study one of these mixing "hotspots." The instrumented platforms provided expanded data as compared to typical Southern Ocean observational techniques. The results show that these turbulent regions allow water properties, such as temperature and oxygen, to mix vigorously. In particular, at these hotspots the exchange of waters between the ocean surface and interior increases. It is important to document the magnitude and regional patterns of this exchange because only surface waters interact with the atmosphere to take up or release carbon and heat, while interior waters store these properties for centuries or longer. Numerical models of the ocean have indicated that these turbulent regions of the Southern Ocean are particularly influential, and we provide some of the first observational evidence confirming these hotspots' importance. Key Points: Observations with high spatial and temporal resolution were collected from two ocean gliders following a Biogeochemical‐Argo floatPhysical and biogeochemical tracer variance is enhanced within a standing meander of the Antarctic Circumpolar CurrentThe standing meander shows evidence of heightened exchange of water properties between the ocean surface boundary layer and the interior [ABSTRACT FROM AUTHOR]

The spring bloom in the Southern Ocean is the rapid‐growth phase of the seasonal cycle in phytoplankton. Many previous studies have characterized the spring bloom using chlorophyll estimates from satellite ocean color observations. Assumptions regarding the chlorophyll‐to‐carbon ratio within phytoplankton and vertical structure of biogeochemical variables lead to uncertainty in satellite‐based estimates of phytoplankton carbon biomass. Here, we revisit the characterizations of the bloom using optical backscatter from biogeochemical floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling and Southern Ocean and Climate Field Studies with Innovative Tools projects. In particular, by providing a three‐dimensional view of the seasonal cycle, we are able to identify basin‐wide bloom characteristics corresponding to physical features; biomass is low in Ekman downwelling regions north of the Antarctic Circumpolar Current region and high within and south of the Antarctic Circumpolar Current. Plain Language Summary: The advent of satellites has allowed us to observe the ocean surface on unprecedented scale. One of the major biogeochemical findings from these observations was that phytoplankton in the Southern Ocean repeatedly went through a dramatic phase of growth every spring basin wide. Phytoplankton, however, exist not only at the ocean surface but also in the interior, generally in the top 100 m. In our study, we revisit the characterization of this spring growth using autonomous floats that vertically profile biogeochemical properties including phytoplankton concentration, providing three‐dimensional estimates of phytoplankton. Our results support the conventional knowledge of there being a robust seasonal cycle in phytoplankton within and south of the Antarctic Circumpolar Current region, an energetic eastward current that wraps around Antarctica. Key Points: In situ estimates of phytoplankton biomass and its seasonal cycle are derived based on optical backscatter from biogeochemical Argo floatsDepth‐integrated biomass peaks after mixed layers start shoaling, but accumulation rates turn positive, while mixed layers are deepeningBiomass is low in Ekman downwelling regions and high in the Antarctic Circumpolar Current and seasonal sea ice zone [ABSTRACT FROM AUTHOR]

Meridional diffusivity is assessed for a baroclinically unstable jet in a high-latitude idealized circumpolar current (ICC) using the Model for Prediction across Scales Ocean (MPAS-O) and the online Lagrangian in Situ Global High-Performance Particle Tracking (LIGHT) diagnostic via space-time dispersion of particle clusters over 120 monthly realizations of O(106) particles on 11 potential density surfaces. Diffusivity in the jet reaches values of O(6000) m2 s−1 and is largest near the critical layer supporting mixing suppression and critical layer theory. Values in the vicinity of the shelf break are suppressed to O(100) m2 s−1 because of the presence of westward slope front currents. Diffusivity attenuates less rapidly with depth in the jet than both eddy velocity and kinetic energy scalings would suggest. Removal of the mean flow via high-pass filtering shifts the nonlinear parameter (ratio of the eddy velocity to eddy phase speed) into the linear wave regime by increasing the eddy phase speed via the depth-mean flow. Low-pass filtering, in contrast, quantifies the effect of mean shear. Diffusivity is decomposed into mean flow shear, linear waves, and the residual nonhomogeneous turbulence components, where turbulence dominates and eddy-produced filamentation strained by background mean shear enhances mixing, accounting for ≥80% of the total diffusivity relative to mean shear [ O(100) m2 s−1], linear waves [ O(1000) m2 s−1], and undecomposed full diffusivity [ O(6000) m2 s−1]. Diffusivity parameterizations accounting for both the nonhomogeneous turbulence residual and depth variability are needed. [ABSTRACT FROM AUTHOR]

The meridional overturning circulation (MOC) in the Southern Ocean is investigated using hydrographic observations combined with satellite measurements of sea surface height. A three-dimensional (spatial and vertical) estimate of the isopycnal eddy diffusivity in the Southern Ocean is obtained using the theory of Ferrari and Nikurashin that includes the influence of suppression of the diffusivity by the strong, time-mean flows. It is found that the eddy diffusivity is enhanced at depth, reaching a maximum at the critical layer near 1000 m. The estimate of diffusivity is used with a simple diffusive parameterization to estimate the meridional eddy volume flux. This estimate of eddy volume flux is combined with an estimate of the Ekman transport to reconstruct the time-mean overturning circulation. By comparing the reconstruction with, and without, suppression of the eddy diffusivity by the mean flow, the influence of the suppression on the overturning is illuminated. It is shown that the suppression of the eddy diffusivity results in a large reduction of interior eddy transports and a more realistic eddy-induced overturning circulation. Finally, a simple conceptual model is used to show that the MOC is influenced not only by the existence of enhanced diffusivity at depth but also by the details of the vertical structure of the eddy diffusivity, such as the depth of the critical layer. [ABSTRACT FROM AUTHOR]

The large-scale middepth circulation and eddy diffusivities in the southeast Pacific Ocean and Scotia Sea sectors between 110° and 45°W of the Antarctic Circumpolar Current (ACC) are described based on a subsurface quasi-isobaric RAFOS-float-based Lagrangian dataset. These RAFOS float data were collected during the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The mean flow, adjusted to a common 1400-m depth, shows the presence of jets in the time-averaged sense with speeds of 6 cm s−1 in the southeast Pacific Ocean and upward of 13 cm s−1 in the Scotia Sea. These jets appear to be locked to topography in the Scotia Sea but, aside from negotiating a seamount chain, are mostly free of local topographic constraints in the southeast Pacific Ocean. The eddy kinetic energy (EKE) is higher than the mean kinetic energy everywhere in the sampled domain by about 50%. The magnitude of the EKE increases drastically (by a factor of 2 or more) as the current crosses over the Hero and Shackleton fracture zones into the Scotia Sea. The meridional isopycnal stirring shows lateral and vertical variations with local eddy diffusivities as high as 2800 ± 600 m2 s−1 at 700 m decreasing to 990 ± 200 m2 s−1 at 1800 m in the southeast Pacific Ocean. However, the cross-ACC diffusivity in the southeast Pacific Ocean is significantly lower, with values of 690 ± 150 and 1000 ± 200 m2 s−1 at shallow and deep levels, respectively, due to the action of jets. The cross-ACC diffusivity in the Scotia Sea is about 1200 ± 500 m2 s−1. [ABSTRACT FROM AUTHOR]

A laboratory experiment of multiple baroclinic zonal jets is described, thought to be dynamically similar to flow observed in the Antarctic Circumpolar Current. Differential heating sets the overall temperature difference and drives unstable baroclinic flow, but the circulation is free to determine its own structure and local stratification; experiments were run to a stationary state and extend the dynamical regime of previous experiments. A topographic analog to the planetary β effect is imposed by the gradient of fluid depth with radius supplied by a sloping bottom and a parabolic free surface. New regimes of a low thermal Rossby number (Ro T ~ 10−3) and high Taylor number (Ta ~ 1011) are explored such that the deformation radius L ρ is much smaller than the annulus gap width L and similar to the Rhines length. Multiple jets emerge in rough proportion to the smallness of the Rhines scale, relatively insensitive to the Taylor number; a regime diagram taking the β effect into account better reflects the emergence of the jets. Eddy momentum fluxes are consistent with an active role in maintaining the jets, and jet development appears to follow the Vallis and Maltrud phenomenology of anisotropic wave-turbulence interaction on a β plane. Intermittency and episodes of coherent meridional jet migration occur, especially during spinup. [ABSTRACT FROM AUTHOR]