Abstract
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.
Highlights
Photosynthesis and respiration are important processes in all ecosystems on earth, with carbon and oxygen being the two main elements
We focus on model–data comparisons over 200– 400, 400–700, and 700–1000 m that broadly represent the upper oxygen minimum zones (OMZs), core OMZ, and lower OMZ, respectively
There is evidence that larger-scale mass transport due to circulation and ventilation is more efficient in the South Pacific than in the North Pacific (Kuntz and Schrag, 2018), and the transit time from the surface to the OMZ is much longer in the eastern tropical North Pacific (ETNP) than in the ETSP (Fu et al, 2018). All these analyses indicate that vertical mixing is largely responsible for asymmetric distribution of mid-depth Dissolved oxygen (DO), and physical processes play a major role in shaping the asymmetry of the OMZs in the tropical Pacific
Summary
Photosynthesis and respiration are important processes in all ecosystems on earth, with carbon and oxygen being the two main elements. The carbon cycle has garnered much attention, with significant advances in both observations (Feely et al, 1999; Takahashi et al, 2009) and modeling (DeVries et al, 2019; Le Quéré et al, 2010; McKinley et al, 2016) of biological processes (e.g., uptake of CO2 and respiration) and physical/chemical processes (e.g., carbon fluxes between the atmosphere, land and ocean). The oxygen cycle has received much less attention despite its large role in the earth system (Breitburg et al, 2018; Oschlies et al, 2018). Dissolved oxygen (DO) is a sensitive indicator of physical and biogeochemical processes in the ocean and a key parameter for understanding the ocean’s role in the climate system (Stramma et al, 2010). Unlike most dissolved nutrients that display an increase in concentration with depth, DO concentration
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