Submesoscale processes can become dominant contributors to mixed-layer dynamics in regions with strong lateral density gradients. Such conditions are ubiquitous in Eastern Boundary Upwelling Systems, where cold, nutrient-rich waters upwell along the coast. Here, we present observations of a pronounced and active density front in the northeastern tropical Atlantic that strongly influences mixed layer depth and associated heat and nutrient fluxes. Our analysis combines data from autonomous ocean gliders equipped with MicroRider turbulence sensors and an Pathfinder DVL, extensive ship-based measurements, and satellite products, recorded during the main upwelling season in March 2025. We find that active mesoscale frontogenesis and mixed layer instabilities lead to a subduction, or an effective vertical decoupling, of the lower half the mixed layer within a spatially confined region of about 5km, collocated with the strongest density gradients. The associated (re)stratification is about an order of magnitude larger than the average cumulative nighttime heat flux. Re-entrainment of the subducted layer into the mixed layer would require dissipation rates far exceeding those observed, suggesting that the subduction is effectively permanent. There is evidence that this process contributes to the overall shoaling of the mixed layer in boreal spring in this area when the main upwelling season starts. Additionally, this process might facilitate the decoupling of strongly sheared high-mode near-inertial oscillations from atmospheric forcing. These contribute significantly to nutrient and heat fluxes in the deeper ocean. Overall, frontal instabilities enhance diapycnal exchange between the nutrient-rich cold upwelled water of coastal origin and the pelagic ocean, while diverting a fraction of the mixed-layer water into permanent subduction.