Subantarctic Mode Water Biogeochemical Formation Properties and Interannual Variability

Subantarctic mode water (SAMW) is a key water mass for the transport of nutrients, oxygen, and anthropogenic carbon into the ocean interior. However, a lack of biogeochemical observations of SAMW properties during wintertime formation precluded their detailed characterization. Here we characterize for the first time SAMW properties across their entire wintertime formation regions based primarily on biogeochemical profiling floats. Observations show that the SAMW properties differ between the two main formation regions in the Pacific and Indian sectors of the Southern Ocean. SAMW formed in the Pacific is colder, fresher, and higher in oxygen, nitrate, and dissolved inorganic carbon (DIC) than its Indian Ocean counterpart. The relationship between potential density and biogeochemical water properties is nearly identical between the two formation regions; property differences thus predominantly reflect the difference in mean densities of SAMW formed in each region. SAMW is undersaturated in oxygen during formation, which will impact calculations of derived quantities that assume preformed oxygen saturation. SAMW is at or above atmospheric pCO(2) during wintertime and therefore not a direct sink of contemporary carbon dioxide during the formation period. Results from the Biogeochemical Southern Ocean State Estimate suggest anti-correlated interannual variability of DIC, nitrate, and oxygen between the central and southeastern Pacific formation regions similar to previously established patterns in mixed layer physical properties. This indicates that the mean properties of SAMW will vary depending on which sub-region has a stronger formation rate, which is in turn linked to the Southern Annual Mode and the El-Nino Southern Oscillation.

Automated summary

Subantarctic mode water (SAMW) is a crucial water mass for transporting nutrients, oxygen, and anthropogenic carbon into the deep ocean. This study characterizes the properties of SAMW during its wintertime formation, utilizing biogeochemical profiling floats. Results show that SAMW properties differ between the Pacific and Indian sectors of the Southern Ocean, with Pacific-formed SAMW being colder, fresher, and higher in oxygen, nitrate, and dissolved inorganic carbon compared to its Indian Ocean counterpart. The study also reveals the interannual variability of SAMW properties, which is linked to the Southern Annual Mode and the El-Nino Southern Oscillation.