Baylor Fox-Kemper

Understanding oceanic transport and material fate has long relied on observations that primarily resolve mesoscale, largely geostrophic flows, leaving the role of submesoscale turbulence comparatively unobserved and poorly understood. Here, using new high-resolution drifter-cluster experiments with closest initial spacings below 1 km, we provide direct Lagrangian observations of how submesoscale turbulence modifies mesoscale eddy coherence and transport pathways. The clustered drifters resolve intense, transient ageostrophic motions that are fundamentally absent from geostrophic descriptions of eddy behavior. Analyzed in an eddy-centered reference frame, the observations reveal pronounced submesoscale-driven radial dispersion that facilitates substantial water-mass exchange between the eddy core and its periphery in both the mixed layer and the subsurface. Quantitative estimates indicate submesoscale horizontal diffusivity of O (10 to 100) m ² /s and a nonlocal dispersion regime across submesoscales. These results challenge the traditional view of mesoscale eddies as coherent structures containing a persistent water mass, with broad implications for interpreting and predicting the transport and fate of tracers in the ocean.

Marine heatwaves (MHWs) are devastating extreme ocean temperature events whose frequency and severity have surged under global warming, threatening marine ecosystems and fisheries. Yet, the fundamental mechanisms that control their intensity and persistence remain incompletely understood. Here, using a submesoscale-permitting global model simulation validated by long-term observational and reanalysis data, this study reveals a previously overlooked submesoscale feedback that exerts a critical control on MHW severity. It is demonstrated that the submesoscale-induced vertical heat flux suppresses the intensity and persistence of atmosphere-driven MHWs in winter via a negative feedback mechanism. Enhanced air-sea net heat flux strengthens stratification and shoals the mixed layer, weakening submesoscale vertical heat transport by 19.4%, reducing MHW intensification by 18.1%, and accelerating decay by 21.7%. This missing regulatory mechanism indicates that models ignoring the submesoscale feedback likely overestimate MHW severity, with critical implications for future climate projections.

Vertical exchange between the surface mixed layer (ML) and the interior is important for biogeochemistry and air–sea interaction. Focused observations have consistently shown that submesoscale processes (horizontal scales of 0.1–10 km) generate substantial exchange of heat, freshwater, and biogeochemical tracers across the ML base, particularly near fronts. However, the role of finescale turbulence in this exchange remains poorly understood. Here we investigate this multiscale problem by analyzing the transport of a surface‐fluxed passive tracer in large‐eddy simulations of baroclinic mixed‐layer instabilities (MLIs) under surface cooling. Budgets for the tracer below the evolving ML show that finescale turbulence contributes to tracer subduction only at fronts sharpened to a few hundred meters wide by eddy straining and horizontal convergence. Although submesoscale vertical flux dominates the exchange across the ML base, finescale turbulent flux is comparable to the indirect contribution from MLI‐induced restratification. Analysis of the finescale energetics shows that the turbulence at sharp fronts is primarily driven by shear production associated with frontogenesis, with large contributions from horizontal shear, rather than by convective forcing. In contrast, outside the frontal zone turbulence is driven solely by surface forcing, and the tracer is largely confined to the ML. This suggests a multiscale picture of vertical exchange at submesoscale fronts that is not represented in existing parameterizations. Submesoscale‐permitting ocean models may partially capture the frontogenetic process, but the associated forward energy flux is likely dissipated implicitly or explicitly, rather than contributing to vertical mixing and tracer transport.

Ocean and atmosphere models are coupled through near-surface exchanges of heat, momentum, and salinity. How the turbulent energy cascades of the two boundary layers interact across scales remains vague, limiting km-scale model evaluation, near-surface remote sensing, and improved flux parameterizations. Here, we use structure functions (SF) to derive a scale-dependent analysis of near-surface kinetic energy budgets and apply it to km-scale COAS simulations of the California Current System. We identify two routes of energy transfer: (1) Synoptic-scale events dominate mesoscale wind work on the ocean, producing forward cascades in both fluids. (2) Ocean submesoscale instabilities energize inverse cascades to the mesoscale and induce wind- and pressure-work responses. These processes coexist, yielding competing up- and downscale cascades in the ocean boundary layer. SF-based comparison with S-MODE airborne Doppler scatterometer data supports this variability and demonstrates a new route to km-scale model validation with remote sensing.

Vertical exchange between the surface mixed layer (ML) and the interior is important for biogeochemistry and air–sea interaction. Focused observations have consistently shown that submesoscale processes (horizontal scales of 0.1–10 km) generate substantial exchange of heat, freshwater, and biogeochemical tracers across the ML base, particularly near fronts. However, the role of finescale turbulence in this exchange remains poorly understood. Here we investigate this multiscale problem by analyzing the transport of a surface-fluxed passive tracer in large-eddy simulations of baroclinic mixed-layer instabilities (MLIs) under surface cooling. Budgets for the tracer below the evolving ML show that finescale turbulence contributes to tracer subduction only at fronts sharpened to a few hundred meters wide by eddy straining and horizontal convergence. Although submesoscale vertical flux dominates the surface–interior exchange across the ML base, finescale turbulent flux is comparable to the indirect contribution from MLI-induced restratification. Analysis of the finescale energetics shows that the turbulence at sharp fronts is primarily driven by shear production associated with frontogenesis, with large contributions from horizontal shear, rather than by convective forcing. In contrast, outside the frontal zone turbulence is driven solely by surface forcing, and the tracer is largely confined to the ML. This suggests a multiscale picture of vertical exchange at submesoscale fronts that is not represented in existing parameterizations. Submesoscale-permitting ocean models may partially capture the frontogenetic process, but the associated forward energy flux is likely dissipated implicitly or explicitly, rather than contributing to vertical mixing and tracer transport.

The ocean and sea ice are central to Earth's climate system, influencing global heat and carbon cycles, weather patterns, and sea level rise. Recent decades have seen rapid advances in Earth System Models (ESMs), but limitations remain in simulating and comparing key oceanic and cryospheric processes across models. A recurring challenge in model intercomparison efforts like the Coupled Model Intercomparison Project (CMIP) is determining the output variables that best represent essential mechanisms while remaining manageable in volume and complexity. Here we present the CMIP7 ocean and sea ice data request, developed through an international, community-based process to prioritize variables for model output. We identify seven opportunities – science-based use cases spanning ocean and cryosphere drivers and responses, paleoclimate, polar amplification, extremes, wind waves, and rapid model evaluation – to guide variable selection and temporal resolution. To address these opportunities, we request new high-frequency and depth-integrated variables, support improved diagnostics of ocean heat uptake, sea ice processes, and model-observation comparison, and build on lessons from CMIP6. Our approach enables targeted, efficient, and transparent data curation to support a wide range of users, from model developers to policymakers. This effort reflects a growing need for more sophisticated, integrative model outputs that address pressing climate questions, including regional extremes and tipping points, while laying the groundwork for future modeling developments.

Vertical exchange between the surface mixed layer (ML) and the interior is important for biogeochemistry and air–sea interaction. Focused observations have consistently shown that submesoscale processes (horizontal scales of 0.1–10 km) generate substantial exchange of heat, freshwater, and biogeochemical tracers across the ML base, particularly near fronts. However, the role of finescale turbulence in this exchange remains poorly understood. Here we investigate this multiscale problem by analyzing the transport of a surface-fluxed passive tracer in meter-scale large-eddy simulations of baroclinic mixed-layer instabilities (MLIs) under surface cooling. Budgets for the tracer below the evolving ML show that finescale turbulence contributes to tracer subduction only at fronts sharpened to large Rossby number by eddy straining and horizontal convergence. Although submesoscale vertical flux dominates the surface–interior exchange across the ML base, finescale turbulent flux is comparable to the indirect contribution from MLI-induced restratification. Analysis of the finescale energetics shows that the turbulence at sharp fronts is primarily driven by horizontal shear production associated with frontogenesis, rather than by convective forcing. In contrast, outside the frontal zone turbulence is driven solely by surface forcing, and the tracer is largely confined to the ML. This suggests a multiscale picture of vertical exchange at submesoscale fronts that is not represented in existing parameterizations. Submesoscale-permitting ocean models may partially capture the frontogenetic process, but the associated forward energy flux is likely dissipated implicitly or explicitly, rather than contributing to vertical mixing and tracer transport. Plain Language Summary Exchange of tracers such as heat, nutrients, and carbon between the surface ocean and the deeper interior affects climate and marine ecosystems. Observations and models have shown that small ocean fronts (0.1–10 km wide) can greatly enhance this vertical exchange. However, the role of even smaller (less than 100 m), but faster, turbulence is not well understood. Here we use turbulence-resolving simulations of ocean fronts to analyze the evolution of a tracer injected at the surface (like a dye). We find that turbulent motions are strongly intensified at sharp fronts, providing an additional, significant contribution to tracer transport into the interior. This turbulence is mainly driven by strong horizontal gradients of currents as the front sharpens, rather than by surface forcing. These results reveal a complex, multiscale process that is not fully captured in ocean circulation models, underscoring the need for model improvements.

The Eddy-Diffusivity Mass-Flux (EDMF) parameterization provides a unified framework to represent both local (diffusive) and non-local (convective) vertical mixing in the ocean. While EDMF has been evaluated in idealized and regional configurations, its performance in global ocean models has not yet been assessed. We present the first global-scale evaluation of the EDMF scheme implemented in NEMO and compare it with the widely used Enhanced Vertical Diffusion (EVD) and Mixed Layer Penetration (MLP) parameterizations. Three global 1/4° simulations for 1999–2020 are analyzed, focusing on convective regimes, upper-ocean temperature and mixed-layer depth. EDMF successfully reproduces both shallow diurnal convection in the tropics and deep wintertime convection in subpolar regions, with modelled plume timing, penetration depth and vertical velocities consistent with available observations. In the tropics, EDMF captures the observed penetrative cooling and depth variability of nocturnal cold plumes, in contrast to the instantaneous adjustment produced by EVD. In deep-convection regions such as the Labrador Sea, EDMF simulates realistic interannual variability in convective depth and intensity. The improved representation of convective regimes leads to a ~30 % reduction in global upper-ocean (0–700 m) temperature biases in EDMF compared to EVD-based simulations. Changes in latent heat loss, driven by SST differences, dominate the associated changes in net surface heat flux between EDMF and EVD simulations, corresponding to ~6.5 % of the global average net climatological surface heat balance. These findings demonstrate that EDMF provides a more physically consistent representation of oceanic convection and offers an alternative to traditional and empirical EVD- and MLP- based approaches.

Surface mixed‐layer dynamics play a crucial role in modulating the climate as it is the oceanic layer that directly communicates with the atmosphere. The resolution of global ocean models is, however, often restricted to ; this is too coarse to adequately resolve mixed‐layer processes, and we depend on parametrizations. One of such parametrizations is the mixed‐layer eddy (MLE) parametrization. Here, we compare the performance of two MLE parametrizations [Fox‐Kemper et al. (2011, https://doi.org/10.1016/j.ocemod.2010.09.002 hereon BFF11) and Bodner et al. (2023, https://doi.org/10.1175/jpo-d-21-0297.1 hereon BOD23)], and document their impact in three global ocean simulations. Upon tuning, and diagnosing submesoscale‐permitting truth simulations, the MLE efficiency coefficient in BOD23 ranges between the values of 0.003–0.038, while 0.06 to 0.07 for BFF11. We find that the spatial distribution of mixed‐layer depth and ventilation of the abyssal ocean, using the ideal‐age tracer and Atlantic Meridional Overturning Circulation as its proxy, are sensitive to the interaction between MLE parametrizations and ocean surface boundary‐layer mixing schemes.