Can the Surface Quasi‐Geostrophic (SQG) Theory Explain Upper Ocean Dynamics in the South Atlantic?

Satellite altimeters provide quasi‐global measurements of sea surface height, and from those the vertically integrated geostrophic velocity can be directly estimated, but not its vertical structure. This study discusses whether the mesoscale (30–400 km) dynamics of three regions in the South Atlantic can be described by the surface quasi‐geostrophic (SQG) theory, both at the surface and in depth, using outputs from an ocean general circulation model. At these scales, the model surface eddy kinetic energy (EKE) spectra show slopes close to k−5/3 (k−3) in winter (summer), characterizing the SQG and quasi‐geostrophic (QG) turbulence regimes. We use surface density and temperature to (a) reconstruct the stream function under the SQG theory, (b) assess its capability of reproducing mesoscale motions, and (c) identify the main parameters that improve such reconstruction. For mixed layers shallower than 100 m, the changes in the mixed‐layer depth contributes nine times more to the surface SQG reconstruction than the EKE, indicating the strong connection between the quality of the reconstruction and the seasonality of the mixed layer. To further explore the reconstruction vertical extension, we add the barotropic and first baroclinic QG modes to the surface solution. The SQG solutions reproduce the model density and geostrophic velocities in winter, whereas in summer, the interior QG modes prevail. Together, these solutions can improve surface correlations (>0.98) and can depict spatial patterns of mesoscale structures in both the horizontal and vertical domains. Improved spatial resolution from upcoming altimeter missions poses a motivating scenario to extend our findings into future observational studies. Plain Language Summary Altimeters provide sea surface height measurements from which geostrophic velocities can be calculated. However, the measurements are strict to the ocean surface and obtaining its vertical structure is an ongoing challenge. Using outputs from an ocean general circulation model, we focus on describing the dynamics of mesoscale motions (30–400 km) in three regions of the South Atlantic under the surface‐quasi‐geostrophic (SQG) theory. We reconstruct the stream function taking a snapshot of density (and temperature) and assess the capability of the SQG method to correctly reproduce surface and vertical fields. Our results indicate that density may drive mesoscale dynamics under specific environmental conditions, and the role played by the se