The mechanisms involved in defining the northern boundary of the shallow oxygen minimum zone in the eastern tropical Pacific Ocean off Mexico

We identify the dynamic northern boundary of the shallow Oxygen Minimum Zone (OMZ) in the eastern tropical Pacific off Mexico (ETPM; 16–23°N). Spatial and temporal variability of the upper limit of the OMZ (DO=9 μmol L−1) is studied analyzing the World Ocean Database 2009 (WOD09) and regional data collected during nine oceanographic surveys (2002–2010). The mean depth of the upper limit of the OMZ ranges from 300 to 400 m from 20° to 23°N, where the California Current induces the advection of oxygen-rich water. Further south (from 16° to 20°N), the shallow upper limit of the OMZ (∼60 m) results from the poleward transport of hypoxic Subtropical Subsurface Water (StSsW) near the surface layer (∼25.6 kg m−3). A transitional zone is located in the north (20–23°N), where the influence of StSsW disappears, and the top of the OMZ is forced deeper by eddies generating southward intrusion of California Current Water (CCW). This oxygen-rich water (50–150 μmol L−1), transported overlying the 18 °C isotherm, defines the northern boundary of the shallow OMZ in the ETPM. The mechanisms involved in the definition of the northward distribution of the shallow OMZ are associated with seasonal advection of CCW and StSsW as well as the intensification of regional mesoscale circulation. Regional and temporal dynamics of these two subsurface water masses largely determine the position of the northern boundary of the shallow OMZ in the ETPM.