The turbulent processes in the Cape Basin, the southeasternmost gate of the Atlantic Ocean, play a key role in the transport and mixing of upper to intermediate water masses entering the area from the Indian Ocean, making them especially relevant for the Indo‐Atlantic transfer of heat and salt. In this paper, two numerical simulations at different horizontal resolutions are used to study mesoscale and submesoscale dynamics, their phenomenology, their evolution, and their impact on the local water masses. Submesoscale processes seasonally affect both, the upper and intermediate layers, but there are clear dynamical differences between the two layers. Several types of instabilities underline this spatial and temporal variability. Near the surface, mixed layer instabilities occur during winter, while mesoscale‐driven instabilities, as the symmetric type, prevail in summer. The connection between these two seasonal regimes is ensured, in anticyclonic eddies and within the mixed layers, by Charney baroclinic instabilities, involved in the local formation and subduction of mode water, that we have dubbed as Agulhas Rings mode water. Intermediate depths are instead characterized by mesoscale mechanisms of density compensation and lateral stirring of the tracer variance, triggering a significant filamentogenesis whose vertical scales are comparable to those mentioned in previous studies. This leads to a particularly efficient mixing of Antarctic Intermediate Waters of Indian and Atlantic origins. Lagrangian estimates highlight the new and significant role of fine scale structures in setting the water masses properties of upper and lower thermocline waters materializing the Indo‐Atlantic exchange and therefore potentially affecting the global ocean circulation.

This study evaluates the capability of eddy‐permitting regional ocean models to reproduce the interocean exchange south of Africa. In this highly turbulent region, we show that the vertical structure of the horizontal flows need to be appropriately resolved to realistically advect thermocline water masses into the South Atlantic. Our results point out that a grid‐spacing of 1/24° on the horizontal and 50 m on the vertical homogeneously distributed are required to account for a correct transport of surface and subsurface water masses properties and their in‐route transformation by mixing. Preliminary Lagrangian analyses highlight the primary role of the upper‐ocean mesoscale eddies on water masses transport and fate, with a particular emphasis on Antarctic Intermediate Waters (AAIWs) dynamics and characteristics. We evaluate the numerical results against observations (AVISO data and Argo floats profiles). Modeled and observed eddies were examined in number, polarity, size, trajectory, and for their contribution to AAIW properties. A clear asymmetry, in number and radius, emerges between cyclones and anticyclones. The high‐resolution simulation was the most energetic, with more abundant and smaller structures than those detected in AVISO. However, eddy statistics compare reasonably well in terms of mean pathways when restricted to Agulhas Rings, which are on average quasi‐Gaussian in shape. Regionally, the Ertel potential vorticity anomaly is marked at the surface by a temporal variability with winter intensification, directly reflected in the seasonal cycle of the eddies number. Noting the growth of the baroclinic Rossby radius in winter, this suggests baroclinic processes as essential for these eddies generation.

Abstract. Harmonic analysis of time series from 20 years of MODIS-Aqua ocean color observations (2002–2022) is conducted to identify periodic variability of near-surface chlorophyll (Chl-a) inferred from ocean color. As they are based on satellite imagery, the Chl-a observations are characterized by significant gaps in both spatial and temporal coverage due to the masking of clouds in the images. Results yield a coherent picture of surface Chl-a associated with the time mean, annual and semiannual cycles, and spring-neap tidal variability. Spring-neap variability is heterogeneous and associated with regions of significant baroclinic tides as well as coastal regions with strong tidal currents. The observations provide another line of evidence for the significant contribution of ocean tides to mixing in the Indonesian Seas.

The rate of turbulent kinetic energy dissipation and diapycnal diffusivity are estimated along 10 hydrographic sections across the Indian Ocean from a depth of 500 m to the seabed. Six sections were occupied twice. On the meridional section, which is nominally along 95°E, spatial patterns were observed to persist throughout the three occupations. Since the variability in diffusivity exceeds the variability in the vertical gradients of temperature and salinity, we conclude that the diffusive diapycnal fluxes vary mostly with diffusivity. In high latitudes, diapycnal diffusions of both temperature and salinity contribute almost equally to density diffusion, particularly across isopycnals just above the salinity maximum, while mainly temperature contributes in other latitudes. The known zonal difference in turbulence is reproduced. Diffusivity from the seabed to 4,000 m above the seabed has an exponential profile with a mode value of 4 × 10−4 m2s−1 at 1,000 m above the seabed and is positively correlated with topographic roughness as reported previously. It is found that the diffusivity also correlates with wind power injected through the surface at near‐inertial frequencies 10–80 days before the observations. These correlations were used to interpolate the observation‐based turbulence quantities to the entire Indian Ocean. Although the dissipation averaged along selected neutral‐density surfaces is less than the dissipation needed to explain the meridional overturning circulation evaluated across 32°S latitude, this may be explained by effects not captured by the ship‐based observations and parameterization. These effects likely include unobserved high‐mixing events, near bottom processes (e.g., hydraulic jumps), and deep equatorial jets.

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