Abstract
Abstract. Open-ocean oxygen minimum zones (OMZs) occur in regions with high biological productivity and weak ventilation. They restrict marine habitats and alter biogeochemical cycles. Global models generally show a large model–data misfit with regard to oxygen. Reliable statements about the future development of OMZs and the quantification of their interaction with climate change are currently not possible. One of the most intense OMZs worldwide is located in the Arabian Sea (AS). We give an overview of the main model deficiencies with a detailed comparison of the historical state of 10 climate models from the 5th Coupled Model Intercomparison Project (CMIP5) that present our present-day understanding of physical and biogeochemical processes. Most of the models show a general underestimation of the OMZ volume in the AS compared to observations that is caused by an overly shallow layer of oxygen-poor water in the models. The deviation of oxygen values in the deep AS is the result of oxygen levels that are too high simulated in the Southern Ocean formation regions of Indian Ocean Deep Water in the models compared to observations and uncertainties in the deepwater mass transport from the Southern Ocean northward into the AS. Differences in simulated water mass properties and ventilation rates of Red Sea Water and Persian Gulf Water cause different mixing in the AS and thus influence the intensity of the OMZ. These differences in ventilation rates also point towards variations in the parameterizations of the overflow from the marginal seas among the models. The results of this study are intended to foster future model improvements regarding the OMZ in the AS.
Highlights
Just like on land, marine animals need oxygen to breathe, and they suffer if the oxygen concentration in the ocean falls below certain thresholds
The models that overestimate the Arabian Sea oxygen minimum zone (ASOMZ) area of less than 50 μmol L−1 show oxygen values that are too low compared to observations in the whole Arabian Sea (AS) and a southward expansion of the ASOMZ with one exception: in the NorESM1-ME model the ASOMZ is shifted to the southeastern boundary of the AS and is located between 15◦ N and the Equator (Fig. 1c)
All in all this wider horizontal expansion of the oxygen-poor areas in the models compared to the observations (Fig. 1c) cannot compensate for the reduced thickness of the low-oxygen layers, which is responsible for the general underestimation of the ASOMZ volume in the CMIP5 models (Fig. 1a)
Summary
Marine animals need oxygen to breathe, and they suffer if the oxygen concentration in the ocean falls below certain thresholds. Oxygen concentrations below the oceanic permanent thermocline depend on two mechanisms: (i) atmospheric oxygen that enters the ocean at the surface mixed layer and is transported into the ocean interior by subduction and mixing and (ii) biological consumption by microbial respiration of sinking organic matter and respiration by higher trophic organisms. Main ventilation regions of the ocean are found at higher latitudes, where mode water and deepwater masses are formed (McCartney and Woodgate-Jones, 1991; Sverdrup, 1938). There is a close connection between the age and oxygen concentration of a water mass (Jenkins, 1977). The water mass age is defined by the time passed since the last surface contact, where its properties can be changed by gas exchange with the atmosphere. Older water masses typically feature lower oxygen concentrations because oxygen consumption has accumulated over longer time periods
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