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Diel vertical migrations in the ocean play a key role in predator-prey dynamics and the functioning of the biological carbon pump. However, changes in ocean conditions including warming and deoxygenation threaten to significantly perturb vertical migration patterns over the twenty-first century. Specifically, vertical migrations over regions of critically low oxygen, known as oxygen minimum zones (OMZs), are likely to be most sensitive to changes in temperature and oxygen. In this study, we apply a simplified prognostic ecosystem model (APECOSM-1D) to changing conditions in the Pacific Ocean OMZ as simulated by 13 Earth System Models from the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that modeled fish migration depths at a given location in the region may deepen or shoal by over 100 m by the end of the century; however, there are large uncertainties across the CMIP6 ensemble for the geographic pattern of migration depth changes. To reconcile this, we adopt a water mass based approach which aggregates changes into regions defined by their vertical oxygen minimum value. In this framework, we find that fish migration depths over the lowest oxygen core of the OMZ remain stable due to compensating changes in temperature and oxygen. Meanwhile, away from the OMZ core, ocean warming and deoxygenation together drive shallower migration depths in projected conditions. 

Abstract Climate change reduces ocean oxygen levels, posing a serious threat to marine ecosystems and their benefits to society. State‐of‐the‐art Earth System Models (ESMs) project an intensification of global oxygen loss in the future, but poorly constrain its patterns and magnitude, with contradictory oxygen gain or loss projected in tropical oceans. We introduce an oxygen water mass framework—grouping waters with similar oxygen concentrations from lowest to highest levels—and separate oxygen changes into two components: thetransformationof oxygen in water masses by biological, chemical, or physical processes along their pathways in “ventilation‐space,” and theredistributionof these water masses in “geographic‐space.” The redistribution of water masses explains the large projection uncertainties in the tropics. ESMs with more realistic representations of water masses provide tighter constraints on future redistribution than less skilled ESMs, leading to over a third more of tropical area exhibiting consistent oxygen projections (58% vs. 22%), and a 30% reduction in model spread for tropical oxygen projections. These higher‐skilled ESMs also project weaker global deoxygenation than less skilled models (median of −2.9 vs. −4.2 Pmol per °C of surface warming) controlled by an increase in global water residence times, and they project a stronger increase in oxygen minimum zone ventilation by ocean mixing. These tighter constraints on future oxygen changes are critical to anticipate and mitigate impacts for ecosystems and inform management and conservation strategies of marine resources. 

Since 1980, atmospheric pollutants in South Asia and India have dramatically increased in response to industrialization and agricultural development, enhancing the atmospheric deposition of anthropogenic nitrogen in the northern Indian Ocean and potentially promoting primary productivity. Concurrently, ocean warming has increased stratification and limited the supply of nutrients supporting primary productivity. Here, we examine the biogeochemical consequences of increasing anthropogenic atmospheric nitrogen deposition and contrast them with the counteracting effect of warming, using a regional ocean biogeochemical model of the northern Indian Ocean forced with atmospheric nitrogen deposition derived from an Earth System Model. Our results suggest that the 60% recent increase in anthropogenic nitrogen deposition over the northern Indian Ocean provided external reactive nitrogen that only weakly enhanced primary production (+10 mg C.m^–2.d^–1.yr^–1in regions of intense deposition) and secondary production (+4 mg C.m^–2.d^–1.yr^–1). However, we find that locally this enhancement can significantly offset the declining trend in primary production over the last four decades in the central Arabian Sea and western Bay of Bengal, whose magnitude are up to -20 and -10 mg C.m^–2.d^–1.yr^–1respectively. 

Abstract. We introduce and evaluate the regional ocean model MOM6-COBALT-IND12 version 1 coupling the MOM6 ocean dynamics model to the Carbon, Ocean Biogeochemistry and Lower Trophics (COBALT) biogeochemical model at a horizontal resolution of 1/12°. The model covers the northern Indian Ocean (from 8.6° S to the northern continental boundaries), central to the livelihoods and economies of countries that comprise about one-third of the world’s population. We demonstrate that the model effectively captures the key physical and biogeochemical basin-scale features related to seasonal monsoon reversal, interannual Indian Ocean Dipole and multi-decadal variability, as well as intraseasonal and fine-scale variability (e.g., eddies and planetary waves), which are all essential for accurately simulating patterns of coastal upwelling, primary productivity, temperature, salinity, and oxygen levels. Well represented features include the timing and amplitude of the monsoonal blooms triggered by summer coastal upwelling and winter mixing, the strong contrast between the high evaporation/high salinity Arabian Sea and high precipitation/high runoff/low salinity Bay of Bengal, the seasonality of the Great Whirl gyre and coastal Kelvin upwelling/downwelling waves, as well as the physical and biogeochemical patterns associated with intraseasonal and interannual variability. Quantitatively, the model exhibits relatively small biases, as reflected by root mean square error (RMSE) values in key variables: sea surface temperature (0.25–0.3 °C), mixed layer depth (7–8.09 m), sea level anomaly (0.02 m), sea surface salinity (0.53–0.71 psu), vertical chlorophyll (0.03–0.3 mg m−3), subsurface temperature (0.33 °C), and subsurface salinity (0.07 psu). A major model bias (16 µmol kg−1 of oxygen) is the larger oxygen minimum zone simulated in the Bay of Bengal, a common challenge of ocean and Earth system models in this region. This bias was partly mitigated by improving the representation of the export and burial of organic detritus to the deep ocean (e.g., sinking speed, riverine lithogenic material inputs that protect organic material and burial fraction), and water-column denitrification (e.g., nitrate-based respiration at higher oxygen levels) using observational constraints. These results indicate that the regional MOM6-COBALT-IND12 v1.0 model is well suited for physical and biogeochemical studies on timescales ranging from weeks to decades, in addition to supporting marine resource applications and management in the northern Indian Ocean. 

ABSTRACT Coastal hypoxia in the densely populated Indian Ocean regions has dramatic consequences for ecosystems and ecosystem services such as fisheries and coastal protection. However, inadequate understanding of the spatio‐temporal variability of coastal hypoxia in the region, and its physical and biological drivers, poses a major challenge for anticipating these risks. Here we usein‐situobservations and a high‐resolution (1/12 degree) biophysical model of the Indian Ocean to map areas with heightened vulnerability to coastal hypoxia and identify the drivers of coastal oxygen dynamics. We find strong regional disparities in the mechanisms and temporal scales that govern coastal hypoxia: seasonal upwelling in the eastern Arabian Sea, interannual upwelling/downwelling linked to Indian Ocean Dipole events in the eastern Bay of Bengal, intraseasonal fluctuations introduced by coastal currents, Kelvin waves, and eddies along the western Bay of Bengal, and biologically driven oxygen variations at the mouth of major rivers in the northern Bay of Bengal (e.g., Ganges–Brahmaputra and Irrawaddy–Sittang Deltas). This basin‐scale mapping identifies regions where intraseasonal hypoxia (e.g., western Bay of Bengal, river deltas) makes prediction of these events challenging and calls for intense monitoring, and regions where seasonal and interannual hypoxia (e.g., eastern Arabian Sea, eastern Bay of Bengal) facilitates the prevention of the adverse impacts on coastal ecosystems and their services. 

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.