The role of physical transport processes for the evolution of anoxia in a semi-enclosed marine system

Semi-enclosed marginal seas like the Baltic Sea are often characterized by permanently anoxic deep layers, and may therefore serve as important model systems to study the causes and consequences of the predicted global expansion of oxygen minimum zones. Here, we focus on the role and potential of physical transport processes in ventilating the deep anoxic bottom water of the central Baltic Sea. Based on long-term deployments of an autonomous profiling system in the central Baltic Sea, we show that small scale oxic mid-water intrusions, that are advectively transported, are ubiquitous features, providing an important oxygen source. The turbulent transport of oxygen was measured by a novel combination of fast optodes with shear microstructure data and show strongly increased transport rates during the autumn and winter months as well as a confinement to the basin boundaries, caused by boundary mixing. On the long-term average, mid-water intrusions were shown to inject about 15 Gmol of oxygen per year into the deep-water region below the permanent halocline, turbulent mixing contributes with about 23 Gmol. This is about one order of magnitude larger than the average amount of oxygen imported during the massive deep-water inflow events (Major Baltic Inflows) that occur on an approximately decadal time scale. These results emphasize that a precise understanding of the oxygen transport pathways is fundamental for a projection of the state of the Baltic Sea in a changing climate.
The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)