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1. Wind- and Wave-driven Ocean Surface Boundary Layer in a Frontal Zone: Roles of Submesoscale Eddies and Ekman-Stokes Transport

   https://doi.org/10.1175/JPO-D-20-0270.1  

   Yuan, Jianguo; Liang, Jun-Hong (June 2021, Journal of Physical Oceanography)

   null (Ed.)

   Abstract Large-eddy simulations are used to investigate the influence of a horizontal frontal zone, represented by a stationary uniform background horizontal temperature gradient, on the wind- and wave-driven ocean surface boundary layers. In a frontal zone, the temperature structure, the ageostrophic mean horizontal current, and the turbulence in the ocean surface boundary layer all change with the relative angle among the wind and the front. The net heating and cooling of the boundary layer could be explained by the depth-integrated horizontal advective buoyancy flux, called the Ekman Buoyancy Flux (or the Ekman-Stokes Buoyancy Flux if wave effects are included). However, the detailed temperature profiles are also modulated by the depth-dependent advective buoyancy flux and submesoscale eddies. The surface current is deflected less (more) to the right of the wind and wave when the depth-integrated advective buoyancy flux cools (warms) the ocean surface boundary layer. Horizontal mixing is greatly enhanced by submesoscale eddies. The eddy-induced horizontal mixing is anisotropic and is stronger to the right of the wind direction. Vertical turbulent mixing depends on the superposition of the geostrophic and ageostrophic current, the depth-dependent advective buoyancy flux, and submesoscale eddies. 

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2. Assessing a Turbulent Mixing Scheme in Diurnal Warm Layers considering Langmuir Turbulence

   https://doi.org/10.1175/JPO-D-24-0164.1  

   Wang, Xingchi; Kukulka, Tobias (July 2025, Journal of Physical Oceanography)

   Abstract This study investigates how Langmuir turbulence (LT) driven by Stokes drift shear affects the heated ocean surface boundary layer (OSBL) based on turbulence-resolving large-eddy simulations (LES) and assesses an analytic vertical mixing parameterization based on a simplified second-moment closure (SMC) approach. Diurnal solar heating forces OSBL shoaling to generate a diurnal warm layer (DWL) in which heat and momentum are trapped. Without LT, relatively weak turbulent mixing results in a near-surface jet that is associated with enhanced turbulent kinetic energy (TKE) production of shear-driven turbulence (ST), which approximately balances TKE dissipation rates. Conversely, LT maintains strong mixing, delaying the DWL formation and preventing the TKE dissipation enhancement by generating a less sheared jet. However, sufficiently strong heating destroys TKE to ultimately reduce mixing and to create more sheared jets, which effectively shifts the LT to an ST-dominated regime. A second-moment turbulence budget analysis suggests that 1) the near-surface OSBL responds rapidly to the surface forcing, 2) Stokes drift impacts heat and momentum budgets in profoundly different ways, and 3) buoyancy terms are to leading order negligible. Building on these findings and introducing a physics-based mixing length, we develop a simplified SMC model that can be solved for near-surface expressions for key turbulent variables and mixing coefficients in terms of known variables. For ST, these expressions are consistent with the Monin–Obukhov similarity theory. For LT, these expressions reveal a fundamental dependence of turbulent variables on Stokes drift shear. 

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3. Controls on Wintertime Ventilation in Southern Drake Passage

   https://doi.org/10.1029/2022GL102550  

   Dove, Lilian A.; Viglione, Giuliana A.; Thompson, Andrew F.; Flexas, M. Mar; Cason, Taylor R.; Sprintall, Janet (February 2023, Geophysical Research Letters)

   Abstract Drake Passage is a key region for transport between the surface and interior ocean, but a mechanistic understanding of this exchange remains immature. Here, we present wintertime, submesoscale‐resolving hydrographic transects spanning the southern boundary of the Antarctic Circumpolar Current and the Polar Front (PF). Despite the strong surface wind and buoyancy forcing, a freshwater lens suppresses surface‐interior exchange south of the PF; ventilation is instead localized to the PF. Multiple lines of the analysis suggest submesoscale processes contribute to ventilation at the PF, including small‐scale, O(10 km), frontal structure in water mass properties below the mixed layer and modulation of a surface eddy diffusivity at sub‐50 km scales. These results show that ventilation is sensitive to both submesoscale properties near fronts and non‐local processes, for example, sea‐ice melt, that set stratification and mixed layer properties. This highlights the need for adaptive observing strategies to constrain Southern Ocean heat and carbon budgets. 

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4. Submesoscale Mixing Across the Mixed Layer in the Gulf of Mexico

   https://doi.org/10.3389/fmars.2021.615066  

   Liu, Guangpeng; Bracco, Annalisa; Sitar, Alexandra (March 2021, Frontiers in Marine Science)

   null (Ed.)

   Submesoscale circulations influence momentum, buoyancy and transport of biological tracers and pollutants within the upper turbulent layer. How much and how far into the water column this influence extends remain open questions in most of the global ocean. This work evaluates the behavior of neutrally buoyant particles advected in simulations of the northern Gulf of Mexico by analyzing the trajectories of Lagrangian particles released multiple times at the ocean surface and below the mixed layer. The relative role of meso- and submesoscale dynamics is quantified by comparing results in submesoscale permitting and mesoscale resolving simulations. Submesoscale circulations are responsible for greater vertical transport across fixed depth ranges and also across the mixed layer, both into it and away from it, in all seasons. The significance of the submesoscale-induced transport, however, is far greater in winter. In this season, a kernel density estimation and a detailed vertical mixing analysis are performed. It is found that in the large mesoscale Loop Current eddy, upwelling into the mixed layer is the major contributor to the vertical fluxes, despite its clockwise circulation. This is opposite to the behavior simulated in the mesoscale resolving case. In the “submesoscale soup,” away from the large mesoscale structures such as the Loop Current and its detached eddies, upwelling into the mixed layer is distributed more uniformly than downwelling motions from the surface across the base of the mixed layer. Maps of vertical diffusivity indicate that there is an order of magnitude difference among simulations. In the submesoscale permitting case values are distributed around 10 –3 m 2 s –1 in the upper water column in winter, in agreement with recent indirect estimates off the Chilean coast. Diffusivities are greater in the eastern portion of the Gulf, where the submesoscale circulations are more intense due to sustained density gradients supplied by the warmer and saltier Loop Current. 

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5. Submesoscale Eddies in the Upper Ocean of the Kuroshio Extension from High-resolution Simulation: Energy Budget

   https://doi.org/10.1175/JPO-D-20-0267.1  

   Cao, Haijin; Fox-Kemper, Baylor; Jing, Zhiyou (April 2021, Journal of Physical Oceanography)

   Abstract The submesoscale energy budget is complex and remains understood only in region-by-region analyses. Based on a series of nested numerical simulations, this study investigated the submesoscale energy budget and flux in the upper ocean of the Kuroshio Extension, including some innovations for examining submesoscale energy budgets in general. The highest-resolution simulation on a ~500 m grid resolves a variety of submesoscale instabilities allowing an energetic analysis in the submesoscale range. The frequency–wavenumber spectra of vertical vorticity variance (i.e., enstrophy) and horizontal divergence variance were used to identify the scales of submesoscale flows as distinct from those of inertia-gravity waves but dominating horizontal divergence variance. Next, the energy transfers between the background scales and the submesoscale were examined. The submesoscale kinetic and potential energy (SMKE and SMPE) were mainly contained in the mixed layer and energized through both barotropic (shear production) and baroclinic (buoyancy production) routes. Averaged over the upper 50 m of ROMS2, the baroclinic transfers amounted to approximately 75% of the sources for the SMKE (3.42 × 10 −9 W/kg) versus the remaining 25% (1.12 × 10 −9 W/kg) via barotropic downscale KE transfers. The KE field was greatly strengthened by energy sources through the boundary—this flux is larger than the mesoscale-to-submesoscale transfers in this region. Spectral energy production, importantly, reveals upscale KE transfers at larger submesoscales and downscale KE transfers at smaller submesoscales (i.e., a transition from inverse to forward KE cascade). This study seeks to extend our understanding of the energy cycle to the submesoscale and highlight the forward KE cascade induced by upper-ocean submesoscale activities in the research domain. 

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