Sensitivity of asymmetric oxygen minimum zones to mixing intensity and stoichiometry in the tropical Pacific using a basin-scale model (OGCM-DMEC V1.4)

Abstract

The tropical Pacific Ocean holds the two largest oxygen minimum zones (OMZs) in the world's oceans, showing a prominent hemispheric asymmetry, with a much stronger and broader OMZ north of the Equator. However, many models have difficulties in reproducing the observed asymmetric OMZs in the tropical Pacific. Here, we apply a fully coupled basin-scale model to evaluate the impacts of stoichiometry and the intensity of vertical mixing on the dynamics of OMZs in the tropical Pacific. We first utilize observational data of dissolved oxygen (DO) to calibrate and validate the basin-scale model. Our model experiments demonstrate that enhanced vertical mixing combined with a reduced O:C utilization ratio can significantly improve our model capability of reproducing the asymmetric OMZs. Our study shows that DO concentration is more sensitive to biological processes over 200–400 m but to physical processes below 400 m. Applying an enhanced vertical mixing causes a modest increase in physical supply (1–2 mmol m−3 yr−1) and a small increase (< 0.5 mmol m−3 yr−1) in biological consumption over 200–1000 m, whereas applying a reduced O:C utilization ratio leads to a large decrease (2–8 mmol m−3 yr−1) in both biological consumption and physical supply in the OMZs. Our analyses suggest that biological consumption (greater rate to the south than to the north) cannot explain the asymmetric distribution of mid-depth DO in the tropical Pacific, but physical supply (stronger vertical mixing to the south) plays a major role in regulating the asymmetry of the tropical Pacific's OMZs. This study also highlights the important roles of physical and biological interactions and feedbacks in contributing to the asymmetry of OMZs in the tropical Pacific.