Processes that impact the O2-distribution in the oceans SA06
O2 is supplied to the surface ocean through air-sea exchange, which is dependent on ocean temperature as O2 is more soluble in colder water. Ventilation leads to a physical exchange of O2 between surface and interior, which mainly occurs along isopycnal surfaces. The main oceanic O2 sink in the interior ocean is respiratory O2 consumption, driven by the break-down of organic matter. As the organic matter is created through photosynthesis at the surface and subsequently sinks through the water column, the decrease in O2 due to respiration is occurring below the surface. Though respiration is a biological process and ventilation is a physical driver, we note that ventilation is not only driving the exchange of O2 between the surface and interior ocean, but also driving the nutrient supply needed for photosynthesis and hence respiratory O2 consumption.
The climate change induced O2-decline in the oceans
From 1960-2010, climate change has led globally to a 2% decline of O2 in the oceans SC17. About half of the O2-decline in the upper 1000m is induced by an increase in ocean surface temperature, leading to a decreased O2-solubility and hence less O2 supply SC17, HE11. Yet, the decreased solubility explains only about 15% of the total changes as 75% of the O2-loss is located below 1200m SC17. Related studies indicate that the remaining O2-decline results from climate change effects on stratification and circulation (ventilation), possibly respiration, changes in marine biology and biogeochemical feedbacks. However, a quantitative and mechanistic understanding of each of the individual processes remains difficult OS19.
Despite these challenges, a tight relationship between changes in OHC and O2 inventory have been found in multiple studies e.g., IT17. The O2-heat ratio is not spatially uniform but increases with depth, which is potentially related to the fact that O2 trends above the main thermocline are driven by changes in O2 solubility and that respiration and ventilation become dominant in deeper waters IT17. However, the linkages between O2- and OHC-trends are not fully understood.
We note that the advent of biogeochemical (BGC) Argo floats has the potential to substantially boost our knowledge of O2-dynamics. More than 226202 O2-profiles have been collected already BI21, with a substantial density in the North Atlantic Subpolar gyre and Nordic Seas e.g., LE20.
Climate Models and their representation of O2
ESMs struggle to accurately reproduce the observed mean state of O2-distributions and tend to have too low O2-values BO13, SE20. They also significantly underestimate the ongoing O2-decline in our oceans SC17, and do not reproduce observed patterns for O2-changes in the ocean’s thermocline OS18. For future projections of subsurface O2, simulated trends differ substantially between ESMs and this disagreement between ESMs increases with depth KW20. So far, it has not been possible to quantify the contributions of specific processes to the model uncertainty, though possible reasons for the inadequacy of models have been listed OS18.
Despite these qualitatively very uncertain ESM projections, studies have shown that ESM also show a tight relationship between O2 and OHC trends e.g., IT17, though their spatial accuracy has not been evaluated yet. Moreover, regional studies in the high latitude North Atlantic have shown that it is possible to trace model error in the upper ocean O2 distribution back to circulation features like the North Atlantic Current T17. Most promising, it has been shown that, by constraining ESM via their ability to accurately simulate the Labrador Sea (LS) water mass, the O2-trend in the LS could be estimated to be nearly twice as high than that for of the unconstrained ESM ensembleTJ18.
O2Ocean: Towards better understanding and representation of O2-mechanisms
We use the advent of BGC-Argo, the tight-relationship O2 and OHC trends with the promising constraint of water mass properties, as well as the previously found circulation dependent O2-distributions in the North Atlantic to validate the importance of modelled physics and biology for O2-distribution and trends. We make use of (i) the possibility to estimate new and improved biology parameterizations through the usage of BGC-Argo, and (ii) the usage of a model’s water mass representation and/or circulation as a possible constraint for O2-trends. We focus not only on global ESMs, but also (iii) utilize regional climate models, which are likely to feature a better water mass representation and are additionally used to test our new biology parameterizations.
References
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