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Warming-driven expansion of the oxygen minimum zone (OMZ) in the equatorial Pacific would bring very low oxygen waters closer to the ocean surface and possibly impact global carbon/nutrient cycles and local ecosystems. Global coarse Earth System Models (ESMs) show, however, disparate trends that poorly constrain these future changes in the upper OMZ. Using an ESM with a high-resolution ocean (1/10 degrees), we show that a realistic representation of the Equatorial Undercurrent (EUC) dynamics is crucial to represent the upper OMZ structure and its temporal variability. We demonstrate that coarser ESMs commonly misrepresent the EUC, leading to an unrealistic "tilt" of the OMZ (e.g., shallowing toward the east) and an exaggerated sensitivity to EUC changes overwhelming other important processes like diffusion and biology. This shortcoming compromises the ability to reproduce the OMZ variability and could explain the disparate trends in ESMs projections.

Plain Language Summary We expect an expansion of the ocean low-oxygen areas as the climate warms. This expansion would bring low-oxygen waters closer to the ocean surface in the Pacific Ocean and affect the global uptake of carbon and interactions between marine animals. We show that commonly used low-resolution climate models misrepresent characteristics of one of the major ocean currents-the Equatorial Undercurrent-with implications for the depth of the low-oxygen waters. Using a higher-resolution climate model we refine the representation of the Equatorial Undercurrent and the low-oxygen areas in the equatorial Pacific. We postulate that shortcomings of low-resolution models compromise our ability to reproduce areas of low oxygen and their response to climate change.