Subtropical Contribution to Sub‐Antarctic Mode Waters

Abstract

Sub‐Antarctic Mode Waters (SAMW) form to the north of the Antarctic Circumpolar Current (ACC) through deep winter mixing. SAMW connect the atmosphere with the oceanic pycnocline, transferring heat and carbon into the ocean interior and supplying nutrients to the northern ocean basins. The processes controlling SAMW ventilation and properties remain poorly understood. Here, we investigate the significance and origin of a ubiquitous feature of SAMW formation regions: The seasonal build‐up of a subsurface salinity maximum. With biogeochemical Argo floats, we show that this feature influences SAMW mixed‐layer dynamics, and that its formation is associated with a decline in preformed nutrients comparable to biological drawdown in surface waters (∼0.15 mol m−2 y−1). Our analysis reveals that these features are driven by advection of warm, salty, nutrient‐poor waters of subtropical origin along the ACC. This influx represents a leading‐order term in the SAMW physical and biogeochemical budgets, and can impact large‐scale nutrient distributions.

Mixing and Overturning Across the Brazil‐Malvinas Confluence

“…When these transports are integrated vertically and scaled by a characteristic along‐front length of the BMC (1,000 km; see e.g., Figure 1b), they give rise to a poleward injection of O(1–10 Sv) (1 Sv = 10 6 m 3 s −1 ) of mode waters lighter than γ n = 27.2 kg m −3 in the top 500 m, and an equatorward subduction into the subtropical gyre of O(1 Sv) of AAIW. The substantial poleward transfer of subtropical mode waters is consistent with these water's recently proposed role in shaping SAMW properties (Fernández Castro et al., 2022). Further, the subduction of intermediate waters into the subtropical zone is similar to estimates of the rate of cross‐BMC subduction of AAIW reported from numerical models (Tanaka & Hasumi, 2008), although lower than that diagnosed from inverse models (Jullion et al., 2010).…”

Subantarctic Mode Water Biogeochemical Formation Properties and Interannual Variability

“…Using data from biogeochemical Argo floats, Fernández Castro et al. (2022) similarly documented the influence of salty, nutrient‐poor subtropical waters on SAMW formation properties which decreases from the Indian to Pacific regions and is an important factor influencing the pre‐formed nutrient content of SAMW.…”

“…An early analysis of the Southern Ocean State Estimate found SAMW forming in the Indian Ocean to have a greater fraction of volume transformed from the lighter (TW) waters than the SAMW forming in the Pacific (Cerovečki & Mazloff, 2016). Using data from biogeochemical Argo floats, Fernández Castro et al (2022) similarly documented the influence of salty, nutrient-poor subtropical waters on SAMW formation properties which decreases from the Indian to Pacific regions and is an important factor influencing the pre-formed nutrient content of SAMW.…”

The Southern Ocean Hub for Nutrients, Micronutrients, and Their Isotopes in the Global Ocean

“…The subducting waters are a mixture of both northward-flowing Southern Ocean surface waters and subtropical waters flowing southward in western boundary currents (Figure 2b). While many nutrients exhibit negligible concentrations in the subtropical source waters, it is still necessary to consider contributions from both northern and southern source waters to understand nutrient distributions in the mode waters (Fernández Castro et al, 2022). Many conceptual frameworks and schematics of mode water formation focus exclusively on the role of the two-dimensional longitudinally averaged overturning circulation (e.g., Sarmiento et al, 2004;Marinov et al, 2006).…”

“…This poses an interesting question that remains unresolved: is the marine Ni distribution driven by low-latitude Ni demand by prokaryotes, or instead by the limited Ni requirement of highlatitude eukaryote-dominated ecosystems? In the latter case, the global Ni distribution is controlled from the south much like those of the macronutrients as well as Zn and Cd (John et al, 2022); in the former, the reduction of Ni concentrations in the Subantarctic, and with it the global Ni distribution, is at least partially driven by subtropical Ni drawdown, and the contribution of subtropical waters to Southern Ocean mode waters (Figure 2b; Fernández Castro et al, 2022). This would make the biogeochemical controls on the marine Ni cycle unique among the elements considered here.…”