The vertical variability in the oxygen minimum zone (OMZ) in the eastern South Pacific (ESP; 0‐40°S) is characterized by the influence of anticyclonic intrathermocline eddies (ITEs), which are subsurface‐intensified mesoscale features that are frequently generated in the coastal upwelling zone off Peru and Chile. The unique lens‐shaped signatures that ITEs leave on temperature, salinity and dissolved oxygen vertical distributions are used as proxies to assess their influences on vertical OMZ boundaries and thickness. Data from in situ profiles in the region (World Ocean Database and Argo databases) are used in correlation analyses between anomalous depths (and vertical displacements) of oxyclines and isopycnals/isotherms, together with an objective eddy detection method based on satellite altimetry to identify the location of such profiles (i.e., outside or inside cyclonic and anticyclonic eddies). The results indicate that most of the vertical fluctuations in the climatological OMZ have a lens‐shaped signature and that those at the mesoscale are largely due to ITEs. ITEs are a main driver of the upper oxycline variability in the coastal band, the coastal transition zone (CTZ, 3‐10° from the coast) and beyond at mid‐latitudes (12‐26°S). The influence of ITEs on the upper and lower oxycline variability is mostly observed within the horizontal (offshore) climatological boundary of the OMZ, suggesting that ITEs play a role in the OMZ offshore extension in the ESP. ITEs may produce mesoscale variations in the thickness of the surface‐oxygenated layer, resulting in potential changes in prey‐predator interactions and in food web functioning.

Plain language summary

The eastern South Pacific hosts one of the most intense zones of the world ocean where oxygen concentrations are extremely low, known as the oxygen minimum zones (OMZ), located at intermediate depths (from 100 m from the surface to a maximum of approximately 800 m depth). The vertical boundaries of this OMZ are influenced by a variety of physical processes; however, their specific contributions to total variability remain poorly known. This study focuses on the role of mesoscale intrathermocline eddies (ITEs), which produce a strong lens‐shaped signal in the water column as they move from the coast to offshore, containing high‐salinity and low‐oxygen waters. The results show that ITEs drive most of the variability in the upper boundary of the OMZ, and thus, they may influence the overall horizontal and vertical extension of the OMZ. The mesoscale variability of the thickness of the surface‐oxygenated layer may structure marine food web functioning and carbon export, and global warming might amplify this impact if eddy activity increases and the OMZ shoals.