Submesoscale flows (0.1 - 10 km) are often associated with large vertical velocities, which can have a significant impact on the transport of surface tracers, such as carbon. However, global models do not adequately account for these small-scale effects, which still require a proper parameterization. In this study, we introduced a passive tracer into the mixed layer of the northern Atlantic Ocean using a CROCO simulation with a high horizontal resolution of Δx = 800 m, aiming to investigate the seasonal submesoscale effects on vertical transport. Using surface vorticity and strain criteria, we identified regions with submesoscale fronts and quantified the associated subduction, that is the export of tracer below the mixed layer depth. The results suggest that the tracer vertical distribution and the contribution of frontal subduction can be estimated from surface strain and vorticity. Notably, we observed significant seasonal variations. In winter, the submesoscale fronts contribute up to 40% of the vertical advective transport of tracer below the mixed layer, while representing only 5% of the domain. Conversely, in summer, fronts account for less than 1% of the domain and do not contribute significantly to the transport below the mixed layer. The findings of this study contribute to a better understanding of the seasonal water subduction due to fronts in the region.

Ocean eddies play an important role in the distribution of heat, salt, and other tracers in the global ocean. But while surface eddies have been studied extensively, deeper eddies are less well understood. Here we study deep coherent vortices (DCVs) in the Northeast Atlantic Ocean using a high resolution numerical simulation. We perform a census of the DCVs on the $27.60$ kg/m$^3$ isopycnal, at the depth of $700-1500$ m, where DCVs of Mediterranean water (meddies) propagate. We detect a large number of DCVs, with maxima around continental shelves, and islands, dominated by small and short-lived cyclones. However, the large and long-lived DCVs are mostly anticyclonic. Among the long-lived DCVs, anticyclonic meddies, stand out. They grow in size by merging with other anticyclonic meddies. Cyclonic meddies are also regularly formed, but most of them are destroyed near their formation sites due to the presence of the energetic anticyclonic meddies, which destroy cyclones by straining and wrapping the positive vorticity around their core. During their life cycle, as they propagate to the southwest, anticyclonic meddies can interact with other DCVs, including anticyclones containing Antarctic Intermediate Water generated near the Moroccan coast, Canary anticyclonic DCVs and cyclonic DCVs generated south of $30^\circ$N along the African continental shelf. With these latter, they can form dipoles, and with the former, they co-rotate pro tempore. Thus, a more detailed view of the life cycle of anticyclonic meddies is proposed: they grow by merging, undergo multiple interactions along their path, and they decay at low latitudes.

Western boundaries have been suggested as mesoscale eddy graveyards, using a diagnostic of the eddy kinetic energy (EKE) flux divergence based on sea surface height (η). The graveyard’s paradigm relies on the approximation of geostrophy — required by the use of η — and other approximations that support long baroclinic Rossby waves as the dominant contribution to the EKE flux divergence. However, a recent study showed an opposite paradigm in the Agulhas Current region using an unapproximated EKE flux divergence. Here, we assess the validity of the approximations used to derive the η-based EKE flux divergence using a regional numerical simulation of the Agulhas Current. The EKE flux divergence consists of the eddy pressure work (EPW) and the EKE advection (AEKE). We show that geostrophy is valid for inferring AEKE, but that all approximations are invalid for inferring EPW. A scale analysis shows that at mesoscale (L > O(30)km), EPW is dominated by coupled geostrophic-ageostrophic EKE flux and that Rossby waves effect is weak. There is also a hitherto neglected topographic contribution, which can be locally dominant. AEKE is dominated by the geostrophic EKE flux, which makes a substantial contribution (54%) to the net regional mesoscale EKE source represented by the EKE flux divergence. Other contributions, including topographic and ageostrophic effects, are also significant. Our results support the use of η to infer a qualitative estimate of the EKE flux divergence in the Agulhas Current region. However, they invalidate the approximations on mesoscale eddy dynamics that underlie the graveyard’s paradigm.

The propagation characteristics of near-inertial waves (NIWs) and how mesoscale and submesoscale processes affect the waves’ vertical penetration (i.e., the chimney effect) are investigated using observations from a mooring array located in the northeast Atlantic. The year-long observations show that near-inertial motions are mainly generated by local wind forcing and that they radiate predominantly downward following several strong wind events. Once below the mixed layer, NIWs preferentially propagate equatorward primarily in the form of low modes. High-mode NIWs, however, are most likely dissipated locally near the base of the mixed layer. Enhanced near-inertial kinetic energy and vertical shear are found only in mesoscale anticyclones with Rossby number of O(0.1). By contrast, submesoscale motions with order one Rossby number have little effect on the trapping and vertical penetration of NIWs, due to their smaller horizontal scales and confined vertical extent compared to mesoscale eddies.

Understanding processes associated with eddy-mean flow interactions helps our interpretation of ocean energetics, and guides the development of parameterizations. Here, we focus on the non-local nature of Kinetic Energy (KE) transfers between mean and turbulent reservoirs. Transfers are interpreted as non-local when the energy extraction from the mean flow does not locally sustain energy production of the turbulent flow, or vice versa. The novelty of our approach is to use ensemble statistics to define the mean and the turbulent flow. Based on KE budget considerations, we first rationalize the eddy-mean separation in the ensemble framework, and discuss the interpretation of a mean flow driven by the prescribed (surface and boundary) forcing and a turbulent flow u’ driven by non-linear dynamics sensitive to initial conditions. We then analyze 120-day long, 20-member ensemble simulations of the Western Mediterranean basin run at 1/60 resolution. Our main contribution is to recognize the prominent contribution of the cross energy term .u_h’ to explain non-local energy transfers. This provides a strong constraint on the horizontal organization of eddy-mean flow KE transfers since this term vanishes identically for perturbations (u_h’) orthogonal to the mean flow (). We also highlight the prominent contribution of vertical turbulent fluxes for energy transfers within the surface mixed layer. Analyzing the scale dependence of these non-local energy transfers supports the local approximation usually made in the development of meso-scale, energy-aware parameterizations for non-eddying models, but points out to the necessity of accounting for these non-local effects in the meso-to-submeso scale range.

The processes leading to the depletion of oceanic mesoscale kinetic energy (KE) and the energization of near-inertial internal waves are investigated using a suite of realistically forced regional ocean simulations. By carefully modifying the forcing fields we show that solutions where internal waves are forced have ~25% less mesoscale KE compared with solutions where they are not. We apply a coarse-graining method to quantify the KE fluxes across time scales and demonstrate that the decrease in mesoscale KE is a result of an internal wave-induced reduction of the inverse energy cascade and an enhancement of the forward energy cascade from sub- to super-inertial frequencies. The integrated KE forward transfer rate in the upper ocean is equivalent to half and a quarter of the regionally averaged near-inertial wind work in winter and summer, respectively, with the strongest fluxes localized at surface submesoscale fronts and filaments.

In this study, we revisit the role of curvature in modifying frontal stability. We first consider the statement “fq < 0 implies potential for instability”, where f is the Coriolis parameter and q is the Ertel potential vorticity (PV). This is true for any inviscid baroclinic flow. It is also evident in the transition of a governing equation for circulation within a front from elliptic to hyperbolic form as the discriminant changes sign. However, for curved fronts, an additional scale factor enters the discriminant owing to conservation of absolute angular momentum, L, leading to Solberg’s (1936) generalization of the Rayleigh criterion. In non-dimensional form, this expression also generalizes the classical instability criterion of Hoskins (1974) by accounting for centrifugal forces: modification of the front’s vertical shear and stratification owing to curvature tilts the absolute vorticity vector away from its thermal wind state and, in an effort to conserve the product of non-dimensional PV (q’) and absolute angular momentum (L’), this alters Rossby and Richardson numbers permitted for stable flow. The criterion, Φ’=L’q’ < 0, is then investigated in non-dimensional parameter space representative of low-Richardson-number vortices. An interesting outcome is that, for Richardson numbers near one, anticyclonic flows increase in q’, while cyclonic flows decrease in q’. Though stabilization is muted for anticyclones (owing to multiplication by L’), the de-stabilization of cyclones is robust, and may help to explain an observed asymmetry in the distribution of submesoscale coherent vortices in the global ocean.

Western boundaries (WB) have been suggested to be hotspots of mesoscale eddy decay, using an eddy kinetic energy (EKE) flux divergence based on sea surface height (η). The η-based diagnostic requires approximations, including the use of geostrophic velocities. Here, we assess to what extent mesoscale EKE flux divergence can be inferred from η using a numerical simulation of the Agulhas Current. The EKE flux divergence is composed of two terms: the eddy-pressure work (linear component) and the advection of EKE (nonlinear component). Both are mainly positive in the WB region (net EKE sources). However, it is not reliably accounted by both η-based diagnostics. The η-based eddy-pressure work has a net contribution in the WB region of the opposite sign than the true one. Ageostrophic eddy-pressure work dominates the geostrophic one (corresponding to a β-contribution). It is explained by mesoscale eddies’s scale to fall below the scale of ζ/β (ζ: root mean square of normalized relative vorticity for mesoscale eddies; β: latitudinal variations of Coriolis parameter). The advection done by geostrophic EKE flux dominates the EKE flux divergence in the WB region. It results in the EKE flux divergence to be qualitatively estimable using η (up to 54 % of the net EKE source). Our results in the Agulhas Current show a mesoscale eddy dynamics in contrast with the decay’s paradigm at western boundaries. Further analysis in other western boundaries are required to complete our understanding of mesoscale eddies dynamics.

The dispersal of dissolved iron (DFe) from hydrothermal vents is poorly constrained. Combining field observations and a hierarchy of models, we show that the dispersal of DFe from the Trans-Atlantic-Geotraverse vent site occurs predominantly in the colloidal phase and is controlled by multiple physical processes. Enhanced mixing near the seafloor and transport through fracture zones at fine-scales interacts with the wider ocean circulation to drive predominant westward DFe dispersal away from the Mid-Atlantic ridge at the 100km scale. In contrast, diapycnal mixing predominantly drives northward DFe transport within the ridge axial valley. The observed DFe dispersal is not reproduced by the coarse resolution ocean models typically used to assess ocean iron cycling due to their omission of local topography and mixing. Unless biogeochemical models include high-resolution nested grids, they will inaccurately represent DFe dispersal from axial valley ridge systems, which make up half of the global ocean ridge crest.