Mesoscale and Submesoscale Processes in the Southeast Atlantic and Their Impact on the Regional Thermohaline Structure

Abstract

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.

Submesoscale Impacts on Mesoscale Agulhas Dynamics

“…The signal of present-day satellite altimetry exceeds the associated noise at scales larger than about 50 km in the greater Agulhas region (Dufau et al, 2016). Dufau et al (2016) further point out that in winter time, when the submesoscale dynamics are most active (Callies et al, 2015), the rough sea state shifts this signal-equals-noise scale to even larger scales. Present-day altimetry is thus not able to resolve submesoscale dynamics.…”

“…Ro > 1 are found to occur only barely in INALT20r.L46.HighDiff (Figure 2a); on average in only 0.17% of the domain area ( Figure 2b). A snapshot of the surface Rossby number on the model day 4 September 2012 shows that even in the Southern Hemisphere winter, when the submesoscales should be most active (Callies et al, 2015), the surface flow is mainly associated with low Rossby numbers and small-scale features are almost absent from the simulation (Figure 3a). There are no submesoscale flows simulated in the Agulhas…”

“…Recent high-resolution model studies found that submesoscale dynamics play an essential role in the mixing of Agulhas ring waters into the Cape Basin and thus in setting the properties of the water masses that are exported into the Atlantic (Capuano, Speich, Carton, & Blanke, 2018;Sinha et al, 2019). On the basis of a high-resolution regional model ((1/108) • ) for a very limited domain, Capuano, Speich, Carton, & Blanke (2018) found that submesoscale dynamics in the Cape Basin drive a strong exchange of both upper and intermediate waters of Atlantic and Indian Ocean origin. They conclude that the simulation of a realistic Indo-Atlantic exchange requires the resolution of a large portion of the submesoscale spectrum.…”

Answer to: Anonymous Referee #3

“…This 1 mm threshold step is a bit smaller than the typical value used in the literature; however, recent work has shown that it leads to more accurate eddy sizes and amplitudes (Faghmous et al, 2015). This criterion prevents the misidentification of large eddies as two smaller ones (Chaigneau et al, 2008(Chaigneau et al, , 2009. The spatially largest closed ADT contour line encompassing one extremum corresponds to the eddy edge.…”

“…The sea surface height (SSH) and geostrophic velocity fields are derived from the Delayed Time Maps of Absolute Dynamic Topography (MADT) mapped daily on a 1/4 • Mercator grid (Pujol et al, 2016). An eddy detection method based on the algorithm developed by Chaigneau et al (2008Chaigneau et al ( , 2009 was applied to these absolute dynamic topography (ADT) fields. This new method detects local ADT extrema to identify potential eddy centers.…”