Controls of recent patterns and trends in global oceanic oxygen content

The global oceanic oxygen (O2) inventory has declined by more than 2% over the last half century, threatening marine ecosystems and altering biogeochemical cycles. This study uses a high-resolution ocean model in hindcast mode, forced by atmospheric reanalysis data, to investigate how changing atmospheric forcing may have affected global and regional O2 variability and long-term trends through changes in solubility, ventilation (diagnosed using CFC-12), and biological consumption. Alongside the standard hindcast run, two sensitivity experiments were performed to isolate the effects of interannual variability in wind stress and buoyancy forcing on the modulation of these dynamics. The time series of simulated global oceanic O2 content can be clustered into four periods: (1) From 1958 to 1967, the O2 inventory increased (218.7 ± 33.9 teramoles per decade) largely due to a buoyancy-induced increase in O2 solubility. (2) From 1967 to 1994, O2 gradually decreased by -46.6 ± 4.5 teramoles per decade due to buoyancy-induced decreases in both solubility and ocean ventilation. (3) Between 1994 and 2002, there was a transient low in the global oceanic O2 inventory, likely linked to strong El Niño conditions in 1997-1998. (4) Thereafter, the decline continued, but at an accelerated rate of -108.6 ± 7.6 teramoles per decade; threefold less than the observed decline over the same period. For the past five decades, changes in wind stress have acted continuously to mitigate the dominant buoyancy-driven decline in global oceanic O2, mainly in the intermediate waters of the Southern Hemisphere. This mitigation is primarily attributed to the intensification and poleward shift of westerly winds, and raises concerns about a potential acceleration of oxygen loss following the projected weakening of wind stress intensification in the Southern Hemisphere. On a regional scale, oxygen changes show substantial temporal and spatial variability, as do their underlying drivers. By identifying regional structures of dominant influences, this analysis contributes to a much-needed improved mechanistic understanding of O2 changes, allows to better understand differences between simulated and observed O2 changes, and in turn facilitates the anticipation of future global and regional O2 inventory changes forced by ongoing climate change.