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The oxygen response in the subpolar North Atlantic (SPNA) to future climate change is poorly understood. We investigate the multidecadal variability in interior oxygen and its association with the subpolar gyre index (a gyre strength proxy) for models and data. During positive phases, persistent strong Labrador Sea (LS) lateral and vertical mixing entrains oxygen-rich water into the interior southern SPNA and vice versa during negative phases. This is indicated by the observed anomalously fresh, cold, and low apparent oxygen utilization, resembling LS water mass during positive phases. We use this relationship to benchmark Earth system models. Under a high CO2 future, the best performing models project a steady decline in SPNA oxygen, driven partly by lower solubility and increases in apparent oxygen utilization. The deoxygenation depends on the sensitivity of the LS mixing to warming. The time of emergence of interior oxygen is projected to be decades earlier than that of temperature and salinity.

Plain Language Summary An important indicator for marine ecosystem health and biodiversity is oceanic oxygen, which is projected to decline considerably with the ongoing climate change. Understanding when, where, and how fast the oxygen may change is of importance for society and the broader scientific community. In this study, we show that the interior oxygen in the North Atlantic subpolar gyre is sensitive to the gyre strength and winter mixing in the Labrador Sea. We demonstrate that in a future warmer climate where the ocean becomes more stratified, the interior water mass will experience rapid deoxygenation due to both lower oxygen solubility and increased biological consumption. The latter is caused by a reduced ventilation and a more sluggish circulation. The deoxygenation signals due to climate change are projected to be evident in the early 21st century. The study provides a compelling argument to establish and maintain a long-term monitoring system for the interior oxygen, especially in the study region.