Description: Highlights • Shifts in WSBW properties to less dense varieties likely equate to less formation of WSBW. • The decline of WSBW volume ceased around 2005 and likely recovering after that. • Dense Shelf Waters drive and modulate the recent WSBW variability. • WSBW is composed by 71% of mWDW and 29% of Dense Shelf Waters. Abstract The role of Antarctic Bottom Water (AABW) in changing the ocean circulation and controlling climate variability is widely known. However, a comprehensive understanding of the relative contribution and variability of Antarctic regional deep water mass varieties that form AABW is still lacking. Using a high-quality dataset comprising three decades of observational shipboard surveys in the Weddell Sea (1984–2014), we updated the structure, composition and hydrographic properties variability of the Weddell Sea deep-layer, and quantified the contribution of the source waters composing Weddell Sea Bottom Water (WSBW) in its main formation zone. Shifts in WSBW hydrographic properties towards less dense varieties likely equate to less WSBW being produced over time. WSBW is primarily composed of 71 ± 4% of modified-Warm Deep Water (mWDW) and 29 ± 4% of Dense Shelf Waters, with the latter composed by ~ two-thirds (19 ± 2%) of High Salinity Shelf Water and ~ one-third (10 ± 6%) of Ice Shelf Water. Further, we show evidence that WSBW variability in the eastern Weddell Sea is driven by changes in the inflow of Dense Shelf Waters and bottom water from the Indian Sector of the Southern Ocean. This was observed through the rise of the WSBW contribution to the total mixture after 2005, following a twenty-year period (1984–2004) of decreasing contribution.

Description: We investigate the spatiotemporal variability of the source water masses (i.e., varieties of Subtropical Mode Water – STMW) that contribute to the South Atlantic Central Water (SACW) in the South Atlantic Ocean. Thus, the composition of the SACW layer is updated. For this investigation, we applied an optimum multiparameter (OMP) analysis and used the conservative and semi-conservative parameters available from the World Ocean Database and Argo floats for the South Atlantic Ocean. The STMW18 (at upper levels) sourced in the central and eastern regions of the South Atlantic and the STMW12 (at lower levels) sourced at the boundaries of the South Atlantic Subtropical Front are the main contributors to the SACW. Although also important, the contribution of STMW14 (sourced in the Brazil-Malvinas Confluence zone) is regionally confined by the Brazil Current recirculation gyre. The contributions from Subtropical Indian Mode Water (SIMW) increased westward along the Agulhas Corridor, while the contribution from STMW12 decreased. The relatively low contribution from SIMW matches the results of previous studies regarding the influence of these waters in the climatology of the South Atlantic Ocean. However, it cannot be ignored, since the results bring new light to further investigations of the mixing processes in the ocean interior of the South Atlantic Ocean.

Description: The Southern Ocean is a key region for analyzing environmental drivers that regulate sea-air CO2 exchanges. These CO2 fluxes are influenced by several mesoscale structures, such as meanders, eddies and other mechanisms responsible for energy dissipation. Aiming to better understand sea-air CO2 dynamics in the northern Antarctica Peninsula, we investigated an anticyclonic stationary eddy located south of Clarence Island, in the eastern basin of Bransfield Strait – named the Antarctica Slope Front bifurcation (ASFb) eddy. Physical, chemical and biological data were sampled, and remote sensing measurements taken, in the region during late summer conditions in February 2020. The eddy’s core consisted of cold (0.31 °C), salty (34.38) and carbon-rich (2247 μmol kg−1) waters with dissolved oxygen depletion (337 μmol kg−1). The core retains a mixture of local surface waters with waters derived from Circumpolar Deep Water (i.e., Warm Deep Water from the Weddell Sea and modified Circumpolar Deep Water from the Bransfield Strait) and Dense Shelf Water. The ASFb eddy acts as a CO2 outgassing structure that reaches a CO2 emission to the atmosphere of ∼1.5 mmol m−2 d–1 in the eddy’s core, mostly due to enhanced dissolved inorganic carbon (DIC). The results suggest that surface variation in DIC in the eddy’s core is modulated by (i) the entrainment of CO2-rich intermediate waters at ∼500 m, (ii) low primary productivity, associated with small phytoplankton cells such as cryptophytes and green flagellates, and (iii) respiration processes caused by heterotrophic organisms (i.e., zooplankton community). By providing a comprehensive view of these physical and biogeochemical properties of this stationary eddy, our results are key to adding new insights to a better understanding of the behavior of mesoscale features influencing sea-air CO2 exchanges in polar environments.