9-Year Challenge of the Oxygen Minimum Zone in the Mexican Tropical Eastern Pacific: 2018 vs 2009 and interannual variability

The oxygen minimum zone (OMZ) is an intrinsic feature of the water masses in the Tropical Eastern Pacific Ocean (TEPO) off the Mexican coast. Although its direct effects on the organisms living in the water column are not fully understood, the OMZ has indeed an important influence on the distribution of the zooplankton and higher-level consumers in the TEPO. In recent years, several authors have reported changes in the extension (i.e., horizontal and vertical expansion) and magnitude (i.e., oxygen depletion) of the OMZ’s in the ocean world, including the Eastern Pacific, a result attributable to the global climate change. Herein we explore the possible changes of the OMZ in the Mexican TEPO, centered in a region in the coordinate range of 18-21°N and 105-112°W, where suitable oceanographic-campaign observations are available. A comparison between the hydrographic conditions in April 2019 and those in November 2009 was carried out, after removing the mean seasonal cycle and possible sensor-dependent bias. Robust differences between the two campaigns are clearly identifiable in the upper layer (above 200 m depth) but they are subtle in the lower layer (200-1000 m depth) where the lowest oxygen values are observed. In order to complement the information provided by the hydrographic campaigns, we implemented a physical-biogeochemical coupled numerical ocean model which, among other biochemical variables, solves the oxygen dynamics. This model, together with the available observations, is a very useful tool to identify the physical and biogeochemical processes involved in the interannual variability of the OMZ in the study area.