”Young Get Older, but Old Get Younger”: Warming-Induced Circulation Change Explains Contrasting Pattern of Projected Pacific Deoxygenation

Future projections of oxygen and water mass age change in the upper Pacific Ocean show a consistent pattern: strong deoxygenation and increased age in the well-ventilated subtropical gyres, contrasted by weak deoxygenation and younger age in the poorly ventilated tropical oxygen minimum zones. We investigate the underlying dynamics of this pattern by forcing an intermediate-complexity ocean circulation and biogeochemistry model with globally uniform and regionally-distinct idealized warming experiments. We find that this meridional contrast in oxygen and age change arises from a slowdown of vertical velocities driven by a combination of Pacific basin and remote Southern Ocean heat forcing. Warming and stratifying the upper Pacific basin compresses the subtropical gyre circulation into a shallower, flatter layer, thereby reducing ventilation along the base of the North Pacific ventilated thermocline. On the other hand, warming in Southern Ocean water mass formation regions decreases age and increases oxygen in the tropical and North Pacific by reducing upwelling of old waters from depth via a wave-mediated adjustment. The combination of these two effects drives the “young get older, but old get younger” response of the Pacific thermocline. Continued observing and improved modeling of the spatial patterns of ocean deoxygenation is key to informing how marine ecosystems will adapt to a warmer and more stratified ocean.