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This article is a perspective on the recently discovered realm of submesoscale currents in the ocean. They are intermediate-scale flow structures in the form of density fronts and filaments, topographic wakes and persistent coherent vortices at the surface and throughout the interior. They are created from mesoscale eddies and strong currents, and they provide a dynamical conduit for energy transfer towards microscale dissipation and diapycnal mixing. Consideration is given to their generation mechanisms, instabilities, life cycles, disruption of approximately diagnostic force balance (e.g. geostrophy), turbulent cascades, internal-wave interactions, and transport and dispersion of materials. At a fundamental level, more questions remain than answers, implicating a programme for further research.

The elevations of water surfaces hold important information on the earth's oceans and land surface waters. Ocean sea surface height is related to the internal change of the ocean's density and mass associated with ocean circulation and its response to climate change. The flow rates of rivers and volume changes of lakes are crucial to freshwater supplies and the hazards of floods and drought resulting from extreme weather and climate events. The Surface Water and Ocean Topography (SWOT) Mission is a new satellite using advanced radar technology to make headway in observing the variability of the elevation of water surfaces globally, providing fundamentally new information previously not available to the study of earth's waters. Here, we provide the first results of SWOT over oceans, rivers, and lakes. We demonstrate the potential of the mission to address science questions in oceanography and hydrology. Plain Language Summary Earth is a water planet. The vast amount of ocean water has stored most of the heat released to the atmosphere since the Industrial Revolution through burning fossil fuels. Climate change is thus moderated by the ocean. Over land the freshwater in lakes, rivers, and reservoirs, a critical natural resource, is affected by the warming climate and direct human modifications. Processes of oceanic uptake of heat and carbon from the atmosphere and cycling of freshwater on land take place at spatial scales too small to have been adequately quantified from space. A new satellite, the Surface Water and Ocean Topography (SWOT) mission, was launched in December 2022. Using advanced radar technology, SWOT provides unprecedented global observations for understanding the ocean's role in climate change and how freshwater resources respond to human influence. SWOT observations near coasts will also advance understanding of how rising sea levels impact those coasts. Key Points The first space observations of submesoscale ocean surface topography for understanding ocean's role in heat uptake from the atmosphere The first space observations of the change of water storage of lakes and flow rates of rivers for understanding the freshwater cycle The first space observations of the details of the change of coastal water levels to assess the impact of local sea level rise

Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km² converged into a 60 × 60 m region within a week, a factor of more than 10⁵ decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material.

The overturning circulation of the global ocean is critically shaped by deep-ocean mixing, which transforms cold waters sinking at high latitudes into warmer, shallower waters. The effectiveness of mixing in driving this transformation is jointly set by two factors: the intensity of turbulence near topography and the rate at which well-mixed boundary waters are exchanged with the stratified ocean interior. Here, we use innovative observations of a major branch of the overturning circulation—an abyssal boundary current in the Southern Ocean—to identify a previously undocumented mixing mechanism, by which deep-ocean waters are efficiently laundered through intensified near-boundary turbulence and boundary–interior exchange. The linchpin of the mechanism is the generation of submesoscale dynamical instabilities by the flow of deepocean waters along a steep topographic boundary. As the conditions conducive to this mode of mixing are common to many abyssal boundary currents, our findings highlight an imperative for its representation in models of oceanic overturning.

The Surface Water and Ocean Topography (SWOT) mission provides a good opportunity to study fine‐scale processes in the global ocean but whether it can detect balanced submesoscale eddies is uncertain due to the “contamination” by unbalanced inertial gravity waves. Here, based on concurrent observations from SWOT and a mooring array in the northwestern Pacific, we successfully captured two submesoscale cyclonic eddies with negative sea level anomalies (SLAs) in spring 2023. We find that the SLA amplitude and equivalent radius of the first (second) eddy are 2.5 cm and 16.0 km (2.0 cm and 18.8 km), respectively. For both eddies, their vertical scales are around 150 m and their horizontal velocities and Rossby numbers exceed 15.0 cm/s and 0.4, respectively. Further analysis suggests that similar submesoscale eddies can commonly occur in the northwestern Pacific and that SWOT is capable to detect larger submesoscale eddies with scales greater than ∼10 km. Plain Language Summary The Surface Water and Ocean Topography (SWOT) mission measures sea surface heights with a spatial resolution an order of magnitude higher than the prior nadir altimetry missions. However, whether SWOT can detect oceanic submesoscale eddies that play key roles in oceanic energy cycle and vertical material transports, is uncertain. To investigate this issue, we deployed a mooring array beneath a SWOT orbital swath in the northwestern Pacific. Based on the concurrent SWOT and mooring data, we successfully captured two submesoscale cyclonic eddies in spring 2023. Radii of the submesoscale eddies are found to be between 10 and 20 km and their horizontal velocities exceed 15.0 cm/s. The ratio between their vertical relative vorticity and the planetary vorticity exceeds 0.4. Further analysis suggests that similar submesoscale eddies can commonly occur in the northwestern Pacific. This study demonstrates the capability of SWOT to detect submesoscale eddies in the global ocean. Key Points Two submesoscale cyclonic eddies (SCEs) were observed by SWOT and moorings in the northwestern Pacific Kinematic and dynamic features of the SCEs were revealed It is feasible to use SWOT data to detect larger submesoscale eddies in the ocean

The multiscale energetics and submesoscale instabilities after the eddy shedding of Kuroshio Loop Current (KLC) intrusion into the South China Sea (SCS) remain ambiguous. Here, a typical KLC eddy shedding process is well simulated using a downscaled submesoscale-permitting model. Then, energy and dynamics diagnostics are employed to investigate the cross-scale interactions between mesoscales and submesoscales during and after this process. In energetics, although the forward and inverse energy cascades coexist, the forward cascade of available potential energy (APE) is crucial in energizing submesoscales, while the strength of forward kinetic energy (KE) is relatively weak. The submesoscale KE is primarily charged by strong buoyancy conversion and secondarily by horizontal advection from upstream, which is mainly balanced by turbulence dissipation and vertical pressure work. In dynamics, except for the release of submesoscale APE by baroclinic instability, symmetric instability (SI) can extract KE from geostrophic flows and drive forward KE cascades. Specifically, strain-induced advective frontogenesis can rapidly sharpen submesoscales by enhancing lateral buoyancy gradients, the increased baroclinicity together with atmospheric-forced buoyancy loss causes negative total Ertel potential vorticity and creates favorable conditions for SI. These results highlight the significance of submesoscales in multiscale energetics and dynamical instabilities of the KLC eddy shedding.

Sustained observations are required to determine the marine plastic debris mass balance and to support effective policy for planning remedial action. However, observations currently remain scarce at the global scale. A satellite remote sensing system could make a substantial contribution to tackling this problem. Here, we make initial steps towards the potential design of such a remote sensing system by: (1) identifying the properties of marine plastic debris amenable to remote sensing methods and (2) highlighting the oceanic processes relevant to scientific questions about marine plastic debris. Remote sensing approaches are reviewed and matched to the optical properties of marine plastic debris and the relevant spatio-temporal scales of observation to identify challenges and opportunities in the field. Finally, steps needed to develop marine plastic debris detection by remote sensing platforms are proposed in terms of fundamental science as well as linkages to ongoing planning for satellite systems with similar observation requirements.

After two decades of development, the Surface Water and Ocean Topography (SWOT) mission launched on 16 December 2022, pioneering the use of Ka‐band Radar Interferometry (KaRIn) for measuring water surface elevation and achieving two‐dimensional altimetry on two 50 km swaths separated by a 20 km nadir gap. Rigorous validation against in situ observations in this study demonstrates that KaRIn achieves enough accuracy to resolve sub‐100 km oceanic processes, with measurement errors 2–4 times smaller than anticipated. These results confirm SWOT's transformative capabilities for advancing oceanographic research and establishing a robust foundation for future applications of the swath altimetry. The results also underscore the innovative advancements in mooring system design, driven by the stringent science requirements of the SWOT mission.

A common dynamical paradigm is that turbulence in the upper ocean is dominated by three classes of motion: mesoscale geostrophic eddies, internal waves and microscale three‐dimensional turbulence. Close to the ocean surface, however, a fourth class of turbulent motion is important: submesoscale frontal dynamics. These have a horizontal scale of O(1–10) km, a vertical scale of O(100) m, and a time scale of O(1) day. Here we review the physical‐chemical‐biological dynamics of submesoscale features, and discuss strategies for sampling them. Submesoscale fronts arise dynamically through nonlinear instabilities of the mesoscale currents. They are ephemeral, lasting only a few days after they are formed. Strong submesoscale vertical velocities can drive episodic nutrient pulses to the euphotic zone, and subduct organic carbon into the ocean's interior. The reduction of vertical mixing at submesoscale fronts can locally increase the mean time that photosynthetic organisms spend in the well‐lit euphotic layer and promote primary production. Horizontal stirring can create intense patchiness in planktonic species. Submesoscale dynamics therefore can change not only primary and export production, but also the structure and the functioning of the planktonic ecosystem. Because of their short time and space scales, sampling of submesoscale features requires new technologies and approaches. This paper presents a critical overview of current knowledge to focus attention and hopefully interest on the pressing scientific questions concerning these dynamics. Key Points Submesoscale physics control ecology locally, but also feedback to basin scales Strong gradients in community structure are created at the submesoscale Despite recent innovations, sampling the submesoscale remains a major challenge

Oceanic submesoscale processes (submesoscales) with O(1–10) km horizontal scale can generate strong vertical buoyancy flux (VBF) that significantly modulate upper‐ocean stratification. Because submesoscales cannot be resolved by the prevailing ocean models, their VBFs have to be properly parameterized in order to improve model performance. Here, based on theoretical scaling analysis, we propose a new parameterization of submesoscale VBF by simultaneously considering mixed‐layer baroclinic instability (MLI) and strain‐induced frontogenesis, which are two leading generation mechanisms of submesoscales that typically co‐occur in open ocean. Compared with the parameterization of Fox‐Kemper et al. (2008, https://doi.org/10.1175/2007jpo3792.1; F08) that only considers the MLI, the new parameterization includes mesoscale strain rate and improves vertical structure function. Diagnostic validations based on submesoscale permitting simulation outputs suggest that the newly parameterized VBFs are more realistic than F08 in regard to three‐dimension distributions. How to incorporate this new parameterization into coarser‐grid ocean models, however, needs further studies. Plain Language Summary Oceanic submesoscale processes with spatial scale of O(1–10 km) can generate strong vertical buoyancy flux (VBF) in upper ocean and therefore, they significantly modulate the vertical density distribution (i.e., stratification). Because the prevailing ocean circulation models typically have horizontal resolutions of O(10–100 km), they are unable to resolve the submesoscale VBF. In order to accurately simulate the upper‐ocean stratification, the unresolved VBF needs to be expressed using the resolved larger‐scale quantities, which is called parameterization. Here, based on theoretical scaling analysis, we propose a new parameterization of submesoscale VBF by simultaneously considering contributions from mixed‐layer instability (MLI; baroclinic instability occurring in the mixed layer) and front sharpening induced by mesoscale strain, which are two important generation mechanisms of submesoscales. Compared with the previous parameterization by Fox‐Kemper et al. (2008, https://doi.org/10.1175/2007jpo3792.1; F08) that only includes MLI mechanism, the new parameterization has incorporated mesoscale strain rate and improved vertical structure function more realistically. Diagnostic analysis based on high‐resolution simulation outputs demonstrates that the newly parameterized VBFs are more realistic in aspect of both horizontal and vertical distributions than the F08 parameterization. The test and application of this parameterization in ocean models will be a focus of future studies. Key Points A new parameterization of submesoscale vertical buoyancy flux is proposed The parameterization considers simultaneously mixed‐layer baroclinic instability and strain‐induced frontogenesis Performance of the parameterization is diagnostically validated using high‐resolution simulation outputs