Physical and Biological Controls of the Drake Passage pCO2 Variability

The Southern Ocean is an important region of ocean carbon uptake, and recent observations indicate that its air-sea carbon flux can fluctuate from seasonal to decadal timescales. Carbon fluxes at regional scales remain an area of high uncertainty due to the sparse observation and intrinsic complexity of the biogeochemical processes. Drake Passage is the narrowest and southernmost constriction of the Antarctic Circumpolar Current and a relatively well sampled sector of the Southern Ocean. The objective of this study is to better understand the mechanisms influencing the seasonal to interannual variability of the carbon uptake in the Drake Passage. A new regional circulation and biogeochemistry model is developed based on the MITgcm configured for the regional Drake Passage domain at the lateral resolution of 10km. The model includes six phytoplankton species and four macro and micro nutrient elements (P, N, Si and Fe). We use this model as a tool to examine the interplay between horizontal and vertical advection, ocean stratification and biological carbon export that determines the surface pCO₂variability. The results are validated against the in-situ and satellite observations, demonstrating that the model can capture the observed seasonal to interannual variability. The complex seasonal cycles of the surface pCO₂ is well represented, reflecting the competing influences of temperature-solubility relationship and biological carbon drawdown. This compensation often produces two annual peaks of pCO₂ in the Drake Passage. It reveals the central role played by the mesoscale dynamics and stratification in the vertical transfer of carbon and micro-nutrient, iron, which then regulates the variability of dissolved inorganic carbon (DIC) and pCO₂. The budget analysis highlights the importance of the vertical entrainment of carbon during the cool season and the biological carbon export during the warm seasons.