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1. “Young Get Older, But Old Get Younger”: Warming‐Induced Circulation Change Explains Contrasting Pattern of Projected Pacific Deoxygenation

   https://doi.org/10.1029/2025AV002278  

   Taylor, Benjamin A; Eddebbar, Yassir A; Takano, Yohei; Talley, Lynne D; Xie, Shang‐Ping (September 2026, AGU Advances)

   Abstract Future projections of oxygen and water mass age change in the upper Pacific Ocean show a consistent pattern: strong deoxygenation and increased age in the well‐ventilated subtropical gyres, contrasted by weak deoxygenation and younger age in the poorly ventilated tropical oxygen minimum zones. We investigate the underlying dynamics of this pattern by forcing an intermediate‐complexity ocean circulation and biogeochemistry model with globally uniform and regionally‐distinct idealized warming experiments. We find that this meridional contrast in oxygen and age change arises from a slowdown of vertical velocities driven by a combination of Pacific basin and remote Southern Ocean heat forcing. Warming and stratifying the upper Pacific basin compresses the subtropical gyre circulation into a shallower, flatter layer, thereby reducing ventilation along the base of the North Pacific ventilated thermocline. Warming in the mid‐latitude Southern Ocean reduces downwelling locally, but remotely reduces upwelling of old, low‐oxygen water in the Tropical and North Pacific via a Kelvin wave‐mediated adjustment of the circulation. The combination of these two effects drives the “young get older, but old get younger” response of the Pacific thermocline. Continued monitoring of upper ocean oxygen through the Biogeochemical‐Argo float program and improved model representation of mid‐ and high‐latitude ventilation processes are critical to inform how marine ecosystems will adapt to a warmer and more stratified ocean. 

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2. Unique ocean circulation pathways reshape the Indian Ocean oxygen minimum zone with warming

   https://doi.org/10.5194/bg-20-4711-2023  

   Ditkovsky, Sam; Resplandy, Laure; Busecke, Julius (January 2023, Biogeosciences)

   Abstract. The global ocean is losing oxygen with warming. Observations and Earth system model projections, however, suggest that this global ocean deoxygenation does not equate to a simple and systematic expansion of tropical oxygen minimum zones (OMZs). Previous studies have focused on the Pacific Ocean; they showed that the outer OMZ deoxygenates and expands as oxygen supply by advective transport weakens, the OMZ core oxygenates and contracts due to a shift in the composition of the source waters supplied by slow mixing, and in between these two regimes oxygen is redistributed with little effect on OMZ volume. Here, we examine the OMZ response to warming in the Indian Ocean using an ensemble of Earth system model high-emissions scenario experiments from the Coupled Model Intercomparison Project Phase 6. We find a similar expansion–redistribution–contraction response but show that the unique ocean circulation pathways of the Indian Ocean lead to far more prominent OMZ contraction and redistribution regimes than in the Pacific Ocean. As a result, only the outermost volumes (oxygen>180 µmol kg−1) expand. The Indian Ocean experiences a broad oxygenation in the southwest driven by a reduction in waters supplied by the Indonesian Throughflow in favor of high-oxygen waters supplied from the southern Indian Ocean gyre. Models also project a strong localized deoxygenation in the northern Arabian Sea due to the rapid warming and shoaling of marginal sea outflows (Red Sea and Persian Gulf) and increases in local stratification with warming. We extend the existing conceptual framework used to explain the Pacific OMZ response to interpret the response in the Indian Ocean. 

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3. A machine learning emulator for dissolved oxygen in earth system models

   https://doi.org/10.1088/1748-9326/ae6fd3  

   Zhang, Qi; Ito, Takamitsu; Bracco, Annalisa (June 2026, Environmental Research Letters)

   Abstract The current generation of earth system models (ESMs) exhibit substantial biases and inter-model spread in representing ocean deoxygenation and the expansion of tropical oxygen minimum zones (OMZs). Improving dissolved oxygen simulation is challenging as it depends on uncertain physical and biogeochemical processes and their interactions. A machine learning-based emulator, O₂EMU, aims to reduce model biases and inter-model spread by replacing ESMs’ biogeochemical parameterizations with the learned relationships between dissolved oxygen and physical variables. The emulator first learns from the historical shipboard and autonomous observations of dissolved oxygen and temperature and salinity from ocean reanalyses. The learned relationships are then applied to the temperature and salinity outputs from an ensemble of ESM historical simulations. The results show significantly improved climatological OMZ structure and long-term oxygen trends including the shoaling and expansion of the tropical Atlantic OMZs. Application of O₂EMU to ESM simulations can reduce the inter-model spread by 70%–75% and the root mean square error by 50%–60%, offering a computationally efficient and scalable alternative to standard biogeochemical models, and providing a stepping stone towardshybridbiogeochemical projections that blend mechanistic ESM dynamics with observationally constrained tracer distributions. 

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4. Slower nutrient stream suppresses Subarctic Atlantic Ocean biological productivity in global warming

   https://doi.org/10.1073/pnas.2000851117  

   Whitt, Daniel B.; Jansen, Malte F. (July 2020, Proceedings of the National Academy of Sciences)

   null (Ed.)

   Earth system models (ESMs) project that global warming suppresses biological productivity in the Subarctic Atlantic Ocean as increasing ocean surface buoyancy suppresses two physical drivers of nutrient supply: vertical mixing and meridional circulation. However, the quantitative sensitivity of productivity to surface buoyancy is uncertain and the relative importance of the physical drivers is unknown. Here, we present a simple predictive theory of how mixing, circulation, and productivity respond to increasing surface buoyancy in 21st-century global warming scenarios. With parameters constrained by observations, the theory suggests that the reduced northward nutrient transport, owing to a slower ocean circulation, explains the majority of the reduced productivity in a warmer climate. The theory also informs present-day biases in a set of ESM simulations as well as the physical underpinnings of their 21st-century projections. Hence, this theoretical understanding can facilitate the development of improved 21st-century projections of marine biogeochemistry and ecosystems. 

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5. Vertical distribution of planktic foraminifera through an oxygen minimum zone: how assemblages and test morphology reflect oxygen concentrations

   https://doi.org/10.5194/bg-2020-280  

   Davis, C.V.; Wishner, K.; Renema, W.; Hull, P.M. (January 2021, Biogeosciences)

   null (Ed.)

   Abstract. Oxygen-depleted regions of the global ocean are rapidly expanding, with important implications for global biogeochemical cycles. However, our ability to make projections about the future of oxygen in the ocean is limited by a lack of empirical data with which to test and constrain the behavior of global climatic and oceanographic models. We use depth-stratified plankton tows to demonstrate that some species of planktic foraminifera are adapted to life in the heart of the pelagic oxygen minimum zone (OMZ). In particular, we identify two species, Globorotaloides hexagonus and Hastigerina parapelagica, living within the eastern tropical North Pacific OMZ. The tests of the former are preserved in marine sediments and could be used to trace the extent and intensity of low-oxygen pelagic habitats in the fossil record. Additional morphometric analyses of G. hexagonus show that tests found in the lowest oxygen environments are larger, more porous, less dense, and have more chambers in the final whorl. The association of this species with the OMZ and the apparent plasticity of its test in response to ambient oxygenation invites the use of G. hexagonus tests in sediment cores as potential proxies for both the presence and intensity of overlying OMZs. 

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