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Air‐sea exchanges across oceanic fronts are critical in powering cloud formation, precipitation, and atmospheric storms. Oceanic submesoscale fronts of scales 1–10 km are characterized by strong sea surface temperature (SST) gradients. However, it remains elusive how submesoscale fronts affect the overlying atmosphere due to a lack of high‐resolution observations or models. Based on rare high‐resolution in situ observations in the Kuroshio Extension region, we quantify the air‐sea exchanges across an oceanic submesoscale front. The cross‐front SST and turbulent heat flux gradients reaches 2.4°C/km and 47 W/m2/km, respectively, far stronger than that typically found in mesoscale‐resolving products. The stronger SST gradient drives substantially stronger air‐sea fluxes and vertical mixing than mesoscale fronts, enhancing cloud formations. The intense air‐sea exchanges across submesoscale fronts are confirmed in idealized model simulations, but not resolved in mesoscale‐resolving climate models. Our finding provides essential knowledge for improving simulations of cloud formation, precipitation, and storms in climate models. Plain Language Summary Oceanic fronts, characterized by large sea surface temperature (SST) gradients, are ubiquitous in the global ocean. Through intense heat and moisture release, these oceanic fronts induce large horizontal gradient of sea level pressure or increasing vertical mixing intensity in the lower atmosphere, are critical in powering cloud formation, precipitation, and atmospheric storms, but are sensitive to SST gradients. Oceanic submesoscale fronts of spatial scales 1–10 km are characterized by strong SST gradients. However, our knowledge of how the submesoscale fronts affect the overlying atmosphere is by and large void, due to a lack of high‐resolution observations or models. Here, based on high‐resolution in situ observations and model simulations, we show that submesoscale fronts drive much stronger air‐sea exchanges and vertical mixing as compared to mesoscale fronts, with significant implications for marine atmosphere boundary layer changes and cloud formations. Limited by the coarse resolution, the intense air‐sea exchanges across submesoscale fronts are not resolved in mesoscale‐resolving climate models. These results highlight the importance of submesoscale air‐sea interactions and call for a proper representation of submesoscale air‐sea exchanges in the next generation of climate models. Key Points Observations show strong gradient in sea surface temperature and turbulent heat flux across a submesoscale oceanic front Submesoscale fronts drive substantially stronger air‐sea fluxes and vertical mixing than mesoscale fronts The intense air‐sea exchanges across submesoscale fronts are not resolved in mesoscale‐resolving climate models

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

A neutrally buoyant float deployed in an atmospherically driven turbulent ocean boundary layer on the dense side of a submesoscale front was repeatedly carried across the boundary layer by the turbulence and then trapped beneath the slumping front. Lagrangian particles in a large‐eddy simulation of a similar baroclinically unstable front forced by surface cooling move along convergent surface filaments toward filament junctions. They are also caught by convective plumes that downwell them at speeds similar to those of the float. Subsequently, some are trapped in the pycnocline by frontal slumping due to ageostrophic secondary frontal circulations. In both observations and simulations, boundary layer turbulence and frontal circulations work together to trap and subduct particles from the mixed layer. The small‐scale boundary layer motions move them vertically within the boundary layer and larger, submesoscale frontal circulations move them laterally out of the boundary layer and under the slumping fronts. Plain Language Summary The present study provides evidence for enhanced tracer transport at ocean fronts in both observation and numerical simulation. The finding suggests a pathway for pollutants (e.g., microplastics) to accumulate in the ocean interior. The transport pathway is driven by both large‐scale eddy dynamics and finescale turbulence, of which the latter plays a larger role in downwelling speed while the spatial sites of subduction are determined by the former. Key Points Lagrangian float observations reveal downwelling across and trapping beneath the turbulent mixed layer at a submesoscale front Similar particle trajectories in convectively forced LES result from joint effects of boundary layer turbulence and submesoscale dynamics Downwelling is due to finescale convective plumes while trapping and restratificationare due to the submesoscale circulation of the front

The baroclinic component of the sea surface height, referred to as steric height, is governed by geostrophically balanced motions and unbalanced internal waves, and thus is an essential indicator of ocean interior dynamics. Using yearlong measurements from a mooring array, we assess the distribution of upper‐ocean steric height across frequencies and spatial scales of O (1–20 km) in the northeast Atlantic. Temporal decomposition indicates that the two largest contributors to steric height variance are large‐scale atmospheric forcing (32.8%) and mesoscale eddies (34.1%), followed by submesoscale motions (15.2%), semidiurnal internal tides (8%), super‐tidal variability (6.1%) and near‐inertial motions (3.8%). Structure function diagnostics further reveal the seasonality and scale dependence of steric height variance. In winter, steric height is dominated by balanced motions across all resolved scales, whereas in summer, unbalanced internal waves become the leading‐order contributor to steric height at scales of O (1 km). Plain Language Summary Steric height is the sea surface height component associated with changes in water‐column density, and is typically contributed by ocean dynamic processes across a wide range of scales, from the large‐scale ocean circulation to the small‐scale wave motion. In this study, the effects of balanced motions (e.g., eddies and ocean fronts) and unbalanced wave motions (e.g., internal waves) on steric height are quantified based on yearlong moored observations at a mid‐latitude ocean site of the northeast Atlantic. Overall, balanced motions and unbalanced wave motions account for approximately 67% and 33% of the upper‐ocean steric height variance, respectively. Steric height variance also show notable seasonal variations and scale dependence. At spatial scales of O (10 km), the steric height is predominately determined by balanced motions throughout the year. By contrast, at spatial scales of O (1 km), unbalanced wave motions are the major contributor to steric height in summer whereas balanced motions still dominate in winter. Together, our findings provide insights for the exploration of next‐generation high‐resolution altimetry data and highlight the non‐negligible role of unbalanced wave motions in forming an energy sink for the balanced flow. Key Points The distribution of steric height variance across frequencies and spatial scales of O (1–20 km) is revealed by yearlong mooring measurements Balanced motions dominate the upper‐ocean steric height variance, and account for ∼67% of the total variance Internal waves become increasingly important in summer, and are able to dominate over balanced motions at spatial scales of O (1 km)

The challenges associated with meso- and submesoscale variability (between 1-100 km) require high-resolution observations and integrated approaches. Here we describe a major oceanographic experiment designed to capture the intense but transient vertical motions associated with mesoscale and submesoscale features in an area characterized by strong fronts. Finescale processes were studied in the eastern Alboran Sea (Western Mediterranean) about 400 km east of the Strait of Gibraltar, a relatively sparsely sampled area. In-situ systems were coordinated with satellite data to provide a full description of the physical and biogeochemical variability. Hydrographic data confirmed the presence of an intense salinity front formed by the confluence of Atlantic Waters, entering from Gibraltar, with the local Mediterranean waters. The drifters coherently followed the northeastern limb of an anticyclonic gyre. Near real time data from acoustic current meter data profiler showed consistent patterns with currents of up to 1m/s in the southern part of the sampled domain. High-resolution glider data revealed submesoscale structures with tongues of chlorophyll-a and oxygen associated with the frontal zone. Numerical results show large vertical excursions of tracers that could explain the subducted tongues and filaments captured by ocean gliders. A unique aspect of AlborEx is the combination of high-resolution synoptic measurements of vessel-based measurements, autonomous sampling, remote sensing and modeling, enabling the evaluation of the underlying mechanisms responsible for the observed distributions and biogeochemical patchiness. The main findings point to the importance of fine-scale processes enhancing the vertical exchanges between the upper ocean and the ocean interior.

Ocean surface fronts and filaments have a strong impact on the global ocean circulation and biogeochemistry. Surface Lagrangian advection with time-evolving altimetric geostrophic velocities can be used to simulate the submesoscale front and filament structures in large-scale tracer fields. We study this technique in the Southern Ocean region south of Tasmania, a domain marked by strong meso- to submesoscale features such as the fronts of the Antarctic Circumpolar Current (ACC). Starting with large-scale surface tracer fields that we stir with altimetric velocities, we determine ‘advected’ fields which compare well with high-resolution in situ or satellite tracer data. We find that fine scales are best represented in a statistical sense after an optimal advection time of ∼2 weeks, with enhanced signatures of the ACC fronts and better spectral energy. The technique works best in moderate to high EKE regions where lateral advection dominates. This technique may be used to infer the distribution of unresolved small scales in any physical or biogeochemical surface tracer that is dominated by lateral advection. Submesoscale dynamics also impact the subsurface of the ocean, and the Lagrangian advection at depth shows promising results. Finally, we show that climatological tracer fields computed from the advected large-scale fields display improved fine-scale mean features, such as the ACC fronts, which can be useful in the context of ocean modelling.