Extratropical forcing of low-latitude subsurface oxygenation under future warming

The global ocean is losing oxygen under climate warming, yet most climate models project rising oxygen levels in low-latitude subsurface waters (~100–500 m), partly due to their enhanced ventilation. However, underlying drivers for the enhanced ventilation remain unclear. Here we demonstrate that the enhanced tropical subsurface ventilation is driven by extratropical forcing. While extratropical warming/freshening are typically associated with weakened subsurface ventilation in the subtropics due to reduced subduction of mode and intermediate waters, they also induce cross-hemispheric overturning responses that deepen isopycnals and thereby effectively push well-ventilated, oxygen-rich shallower waters to greater depths, which improves ventilation and increases oxygen in tropical subsurface waters. These processes can reduce the deoxygenation stress in tropical subsurface depths on centennial timescales, but their effects diminish on longer time scales. Our findings highlight critical links between extratropical dynamics and tropical subsurface oxygen fluctuations, with critical implications for projecting climatic impacts on marine ecosystems.