Western boundary current - driven shelf sea deoxygenation on the Agulhas bank

Abstract

Dissolved oxygen is a fundamental component of healthy marine ecosystems and the livelihoods and economies they support. Despite its importance, dissolved oxygen is declining globally, and the processes driving deoxygenation in western boundary current systems remain poorly constrained. Here we present new autonomous ocean glider data from the eastern Agulhas Bank, a temperate shelf system off South Africa strongly influenced by the Agulhas Current. We identify a two-stage self-enhancing deoxygenation mechanism: shelf-edge exchange injects cold, nutrient-rich but oxygen-deficient South Indian Central Water onto the shelf, establishing and maintaining strong water column stratification; while wind-driven coastal upwelling fuels intense primary production and organic matter sinking that further enhances oxygen decline within the shelf sea interior. The first turbulence measurements in this region show that vertical mixing is too weak to sufficiently ventilate subsurface and near bed layers on the shelf, allowing low-oxygen conditions to persist. These results demonstrate that western boundary currents can precondition shelf seas for episodic oxygen depletion, with important implications for ecosystem resilience under projected climate-driven intensification of boundary current dynamics.