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We analyse simulations of the Pacific Ocean oxygen minimum zones (OMZs) from11 Earth system model contributions to the Coupled Model IntercomparisonProject Phase 5, focusing on the mean state and climate change projections.The simulations tend to overestimate the volume of the OMZs, especially inthe tropics and Southern Hemisphere. Compared to observations, five modelsintroduce incorrect meridional asymmetries in the distribution of oxygenincluding larger southern OMZ and weaker northern OMZ, due tointerhemispheric biases in intermediate water mass ventilation. Seven modelsshow too deep an extent of the tropical hypoxia compared to observations,stemming from a deficient equatorial ventilation in the upper ocean, combinedwith too large a biologically driven downward flux of particulate organiccarbon at depth, caused by particle export from the euphotic layer that is too highand remineralization in the upper ocean that is too weak.  At interannual timescales, the dynamics of oxygen in the eastern tropicalPacific OMZ is dominated by biological consumption and linked to naturalvariability in the Walker circulation. However, under the climate changescenario RCP8.5, all simulations yield small and discrepant changes inoxygen concentration at mid depths in the tropical Pacific by the end of the21st century due to an almost perfect compensation betweenwarming-related decrease in oxygen saturation and decrease in biologicaloxygen utilization. Climate change projections are at odds with recentobservations that show decreasing oxygen levels at mid depths in thetropical Pacific.  Out of the OMZs, all the CMIP5 models predict a decrease of oxygen over mostof the surface and deep ocean at low latitudes and over all depths at high latitudes due to an overall slow-down ofventilation and increased temperature

Oxygen minimum zones in the tropical Pacific across CMIP5 models: mean state differences and climate change trends