“Young Get Older, But Old Get Younger”: Warming‐Induced Circulation Change Explains Contrasting Pattern of Projected Pacific Deoxygenation

Abstract

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. Warming in the mid‐latitude Southern Ocean reduces downwelling locally, but remotely reduces upwelling of old, low‐oxygen water in the Tropical and North Pacific via a Kelvin wave‐mediated adjustment of the circulation. The combination of these two effects drives the “young get older, but old get younger” response of the Pacific thermocline. Continued monitoring of upper ocean oxygen through the Biogeochemical‐Argo float program and improved model representation of mid‐ and high‐latitude ventilation processes are critical to inform how marine ecosystems will adapt to a warmer and more stratified ocean.