Control of spatio-temporal variability of ocean nutrients in the East Australian Current

Abstract. The East Australian Current (EAC), the South Pacific's southward-flowing western boundary current, dominates the marine environment of the eastern coast of Australia. Upwelling of deep EAC nutrient-rich water into the oligotrophic surface waters is very important for maintaining upper-ocean productivity. However, the role of EAC dynamics in upper-ocean nutrient variability and resulting productivity is poorly understood. In this study, we use physical and biogeochemical data collected from 2012–2022 to improve understanding of the variability of the nutrients in the upper water column at ∼ 27° S, a subtropical region strongly influenced by the EAC. The 10-year data set shows that there is a seasonal increase in nutrient concentrations in the upper water column (0–200 m) in the austral spring (September–November) and autumn (March–May) and a minimum in winter (June–August). We also find that the nutrient concentrations in the upper water column are influenced by the position of the EAC jet. Two main modes of variability in the EAC's position are identified: an inshore mode with the jet flowing along the continental slope and an offshore mode with the current core detached from the continental slope and flowing over the adjacent abyssal plain. The position of the EAC jet influences the location of upwelling of nutrient-rich water at depth (> 200 m). For the EAC inshore mode, cooler nutrient-rich waters are restricted to the area of the continental shelf and slope that is inshore of the EAC. The offshore mode exhibits a wider distribution of nutrient-rich waters over both the inshore shelf and slope and the offshore abyssal Tasman Sea. Our analysis highlights the important interactions between the highly variable EAC and the distribution of high-nutrient waters, which has implications for primary production, fisheries, and the biological carbon pump.