Modulation of the Equatorial Pacific Oxygen Minima By Tropical Instability Vortices

The distribution of dissolved oxygen in the tropical Pacific acts as a major control on marine ecosystems habitats and the foraging range of tuna fisheries in this region. A basic understanding of processes driving the mean structure and variability of the oxygen minimum zones (OMZs) in this region, however, remains challenged by sparse observations and coarse model resolution. In this study, we examine the influence of mesoscale processes on equatorial Pacific oxygen distribution and variability, with a particular focus on tropical instability vortices (TIVs). We employ an eddy-resolving configuration of the Community Earth System Model (CESM) and Lagrangian analysis to evaluate the impacts and governing mechanisms by which TIVs influence oxygen distribution and budgets in this region. The westward seasonal propagation of TIVs from summer through winter is found to drive a deepening of the oxygen minima along the equatorial Pacific band (10ºN-10ºS), and thus a seasonal expansion of the equatorial oxygenated tongue separating the north and south tropical Pacific OMZs. Strong hemispheric asymmetry is evident in TIV impacts on oxygen due to relatively weaker TIV activity and less pronounced oxygen gradients south of the equator. Mechanisms governing TIV oxygenation of the upper equatorial Pacific include a complex interplay of physical and biogeochemical processes. Isopycnal displacements act in concert with vortex trapping and lateral stirring to mix oxygenated waters from the eastward flowing equatorial zonal jets into the equatorial boundaries of the north and south tropical Pacific OMZs. TIV-induced upwelling, on the other hand, intensifies nutrient supply to the surface, increasing productivity, organic carbon export, and oxygen respiration demand at depth, thus acting to slightly counteract the physical effects. The influence of these processes varies with TIV phase, from vortex generation in the eastern Pacific through vortex dissipation in the west. TIVs are found to have a profound influence on upper equatorial Pacific oxygen distribution and budget, with major implications for understanding the coupling between oxygen and ocean circulation, modeling and predicting marine ecosystem dynamics at the mesoscale, and designing observation networks in this region.