Projections of hypoxia: Abiotic tracer-based insights into model parameter uncertainty

There is growing concern that global warming will lead to declining oxygen levels and the expansion of so-called "dead zones”. This endangers local ecosystems. Model-based projections are essential for assessing the impact of respective political management strategies and for implementing early warning systems. However, simulating dissolved oxygen dynamics in the oceans remains challenging. While the underlying processes are well understood, their representation in contemporary coupled biogeochemical-ocean models crucially depends on poorly constrained model parameters. This parameter uncertainty can map onto to diverging projections. In a step forward to more robust projections we advocate the use of abiotic tracers to assess the effects of different parameter choices among models. In addition to common tracers, such as artificial “clocks” that measure residence times and the timescales of (surface) ventilation, we propose to introduce of argon saturation as an additional tracer to the ocean models to diagnose ocean mixing, which is key to setting oxygen concentrations in the interior. We provide illustrative examples from the Baltic Sea and the North Atlantic Ocean off Mauretania.