The Agulhas Current facilitates (sub)mesoscale nutrient supply that increases subtropical fertility

There is considerable uncertainty surrounding the influence of (sub)mesoscale processes on ocean fertility. The Agulhas Current, like all western boundary currents (WBCs), is characterised by high (sub)mesoscale energy as well as oligotrophic surface waters and nutrient-rich thermocline waters, making it a potentially significant region for upward nutrient supply to the surface ocean. Recently published in AGU Advances, Marshall et al. use the difference between the δ15N and δ18O of nitrate (termed Δ(15-18)) as a novel tracer of (sub)mesoscale nutrient supply to characterise deep nitrate injection into the euphotic zone, made possible because Δ(15-18) is conserved during both physical mixing and phytoplankton assimilation of nitrate. An increase in the upward nitrate supply in WBC systems driven by an ongoing increase in their eddy kinetic energy could partially offset the decline in subtropical ocean productivity predicted under global warming.

Reference: Marshall, T.A., Beal, L., Sigman, D.M. & Fawcett, S.E. (2023). Instabilities across the Agulhas Current enhance upward nitrate supply in the southwest subtropical Indian Ocean. AGU Adv., 4, e2023AV000973. https://doi.org/10.1029/2023AV000973