Investigating the Southern Ocean Biogeochemical Divide

Abstract

The Southern Ocean plays a crucial role in Earth's climate and biogeochemical (BGC) cycles due to its unique features, such as strong westerly winds, intense ocean circulation, and its high-latitude location. This study focuses on the Southern Ocean Biogeochemical Divide (SOBD), a region where distinct ocean circulation patterns significantly influence nutrient distribution, marine primary production, and carbon sequestration. Utilizing data from the Norwegian Earth System Model version 2 (NorESM2-MM) and observational data from the World Ocean Atlas 2018 (WOA18), we examine the spatial and temporal variability of key BGC tracers-phosphate (PO4), dissolved oxygen (DO), and silicate (Si). Our analysis reveals that PO4 concentrations show a clear latitudinal gradient with higher levels near the Antarctic continent. DO concentrations are higher near the Antarctic continent at the surface, decreasing with depth. Si concentrations display similar patterns to PO4 but with generally higher values. The SOBD is identified as a consistent feature of upwelling water at approximately 250m depth, where significant changes in nutrient concentrations align with the 1036.5 kg/m^3 isopycnal line, marking the transition between different water masses. This alignment helps identify the SOBD's location, which falls between the Polar Front (PF) and the Southern Boundary (SB) of the Antarctic Circumpolar Current (ACC). Time series analysis highlights significant temporal fluctuations in BGC tracer concentrations, with notable shifts around the mid-20th century. These shifts are associated with the Early Twentieth Century Warming (ETCW) and changes in the Southern Annular Mode (SAM) index, influencing the strength of westerly winds and upwelling processes. Despite these fluctuations, the SOBD location remains stable over the study period (1850 to 2014).