Eddy-Mediated Turbulent Mixing of Oxygen in the Equatorial Pacific

In the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O2) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and vertical mixing as potential pathways of O2 supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O2 budget balance and find that vertical mixing of O2, which is modulated by the surface wind speed and the vertical shear of the eddying currents, contributes substantially to the replenishment of O2 in the upper equatorial Pacific thermocline, complementing the advective supply of O2 by the ECS and meridional circulation at depth. These transport processes vary seasonally in conjunction with the wind: mixing of O2 into the upper thermocline is strongest during boreal summer and fall when the vertical shear and eddy kinetic energy are intensified. The relationship between eddy activity and the downward mixing of O2 arises from the modulation of equatorial turbulence by Tropical Instability Waves via their impacts on the vertical shear. This interaction of processes across scales sustains a local pathway of O2 delivery into the equatorial Pacific interior and highlights the need for adequate observations and models of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O2 content in a warmer and more stratified ocean. The eastern tropical Pacific interior is an O2 deficient environment, due to intense O2 consumption by microbial communities that is not vigorously replenished by ocean circulation at depth. In this study, we use a high resolution simulation of ocean circulation and biogeochemistry to understand the role of finer scale processes such as turbulence and eddies in injecting O2 locally. We find that mixing due to turbulence along the equator supplies a key portion of O2 into the ocean by exchanging waters between the well-aerated mixed layer near the surface and the ocean's interior where O2 falls precipitously with depth. We also find that this mixing varies considerably with the seasons. This annual cycle in mixing arises from the seasonal variability in wind stress and the passage of eddies, which amplify turbulence through their influence on the subsurface currents along the equator, and represents a previously unexplored but potentially important route of O2 delivery into the ocean's interior. As the upper ocean warms and becomes less dense, the ocean's O2 content is expected to decrease, and thus observing and accurately modeling these O2 pathways will be crucial to monitoring how marine ecosystem habitats will shift in a warmer climate. Mesoscale-resolving ocean biogeochemistry simulations show vertical mixing is a key source of oxygen to the equatorial Pacific thermocline The supply of oxygen by advection and vertical mixing is strongly seasonal and is driven by seasonal variability in the wind The vertical mixing of oxygen is modulated by the wind stress and mesoscale eddy impacts on equatorial shear-driven turbulence