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Historic observations of dissolved oxygen (O2) in the ocean are analyzed to quantify multidecadal trends and variability from 1958 to 2015. Additional quality control is applied, and the resultant oxygen anomaly field is used to quantify upper ocean O2trends at global and hemispheric scales. A widespread negative O2trend is beginning to emerge from the envelope of interannual variability. Ocean reanalysis data are used to evaluate relationships with changes in ocean heat content (OHC) and oxygen solubility (O2,sat). Global O2decline is evident after the 1980s, accompanied by an increase in global OHC. The global upper ocean O2inventory (0–1000 m) changed at the rate of -243 ± 124 T mol O2per decade. Further, the O2inventory is negatively correlated with the OHC (r= -0.86; 0–1000 m) and the regression coefficient of O2to OHC is approximately -8.2 ± 0.66 nmol O2J-1, on the same order of magnitude as the simulated O2-heat relationship typically found in ocean climate models. Variability and trends in the observed upper ocean O2concentration are dominated by the apparent oxygen utilization component with relatively small contributions from O2,sat. This indicates that changing ocean circulation, mixing, and/or biochemical processes, rather than the direct thermally induced solubility effects, are the primary drivers for the observed O2changes. The spatial patterns of the multidecadal trend include regions of enhanced ocean deoxygenation including the subpolar North Pacific, eastern boundary upwelling systems, and tropical oxygen minimum zones. Further studies are warranted to understand and attribute the global O2trends and their regional expressions. This new paper describes the analysis of the dissolved oxygen in the global ocean using the most recent version of the World Ocean Database for the period of 1955 to 2015. After careful examination of the data, we found that a statistically significant, widespread O2decline is emerging beyond the envelope of natural fluctuations. Our study also reveals a tight relationship between O2inventories and the ocean heat content. The spatial pattern and magnitude of this relationship are consistent with expectations derived from mechanistic ocean climate models forced under climate warming scenarios. Taken together, the trends we document here are suggestive of the effects of the ocean warming beginning to supersede natural variability and emerge as a recognizable signal. This merits additional scrutiny over the coming years. A widespread negative O2trend is beginning to emerge from the envelope of interannual variabilityThe global ocean O2inventory is negatively correlated with the global ocean heat contentVariability and trends in the observed upper ocean O2concentration are dominated by the apparent oxygen utilization

Continental margins play an essential role in global ocean biogeochemistry and the carbon cycle; however, global assessments of this role remain highly uncertain. This uncertainty arises from the large variability over a broad range of temporal and spatial scales of the processes that characterize these environments. High‐resolution simulations with ocean biogeochemical models have emerged as essential tools to advance biogeochemical assessments at regional scales. Here, we examine the processes and balances for carbon, oxygen, and nitrogen cycles along the U.S. West Coast in an 11‐year hindcast simulation with a submesoscale‐permitting oceanic circulation coupled to a biogeochemical model. We describe and quantify the biogeochemical cycles on the continental shelf, and their connection to the broader regional context encompassing the California Current System. On the shelf, coastal and wind stress curl upwelling drive a vigorous overturning circulation that supports biogeochemical rates and fluxes that are approximately twice as large as offshore. Exchanges with the proximate sediments, submesoscale shelf currents, bottom boundary layer transport, and intensified cross‐shelf export of shelf‐produced materials further impact coastal and open‐ocean balances. While regional variability prevents extrapolation of our results to global margins, our approach provides a powerful tool to identify the dominant dynamics in different shelf setting and quantify their large‐scale consequences. The biogeochemical balances along the U.S. West Coast shelf are characterized using a submesoscale‐permitting oceanic biogeochemical modelAlongshore wind stress, intensified curl, eddies, and boundary layer dynamics generate biogeochemical rates twice as large as offshoreIntense mean and eddy cross‐shore exchanges, mainly in the boundary layers, transport nutrients offshore, where they enhance productivity The biogeochemical balances along the U.S. West Coast shelf are characterized using a submesoscale‐permitting oceanic biogeochemical model Alongshore wind stress, intensified curl, eddies, and boundary layer dynamics generate biogeochemical rates twice as large as offshore Intense mean and eddy cross‐shore exchanges, mainly in the boundary layers, transport nutrients offshore, where they enhance productivity

Earth System Models project a decline of dissolved oxygen in the oceans due to climate warming. Observational studies suggest that the ratio of O2inventory to ocean heat content is several fold larger than what can be explained by solubility alone, but the ratio remains poorly understood. In this work, models of different complexity are used to understand the factors controlling the air‐sea O2flux to heat flux ratio (O2/heat flux ratio) during deep convection. Our theoretical analysis based on a one‐dimensional convective adjustment model indicates that the vertical stratification and distribution of oxygen before the convective mixing determines the upper bound for the O2/heat flux ratio. Two competing rates, the mean entrainment rate of deeper waters into the mixed layer and the rate of air‐sea gas exchange, determine how much the actual ratio departs from the upper bound. The theoretical predictions are tested against the outputs of a regional ocean model. The model sensitivity experiments broadly agree with the theoretical predictions. Our results suggest that the relative vertical gradients of temperature and oxygen at sites of deep water formation are an important local metric to quantify the marginal changes between years with high and lower heat loss. Numerical simulations suggest that the dissolved oxygen (O2) in the ocean decreases in a warming climate. An important metric to consider to quantify such decrease is the ratio between the rates of oxygen loss and heat gain, in particular for the high latitude oceans, that ventilate the mid‐depth and deep oceans globally. As the O2in the deep ocean can only be supplied from the surface during deep mixing events at high latitudes in the cold season, it is important to know how much oxygen can enter the ocean for a given amount of cooling. It will help us estimate how the oceanic oxygen uptake may change if global warming reduces cooling at the ocean surface. This study investigates the ratio between oxygen uptake and cooling during deep winter mixing events using models of different complexities. Our results suggest that this ratio differs under different cooling scenarios. At the convection site, the differences in density and O2concentration between the surface water and deep water right before the convective season control this ratio. A hierarchy of models is used to examine what controls the ratio between air‐sea oxygen flux and heat loss in high latitude oceansThe O2/heat flux ratio of convective events depends on both the surface forcing and background vertical gradients of temperature and O2The vertical gradients of O2and temperature are essential for the sensitivity of the oxygen fluxes to changes in heat fluxes A hierarchy of models is used to examine what controls the ratio between air‐sea oxygen flux and heat loss in high latitude oceans The O2/heat flux ratio of convective events depends on both the surface forcing and background vertical gradients of temperature and O2 The vertical gradients of O2and temperature are essential for the sensitivity of the oxygen fluxes to changes in heat fluxes