ALBERT

Description: 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: Here we provide CO2-system properties that were continuously measured in a southeast-northwest transect in the South Atlantic Ocean in which six Agulhas eddies were sampled. The Following Ocean Rings in the South Atlantic (FORSA) cruise occurred between 27th June and 15th July 2015, from Cape Town – South Africa to Arraial do Cabo – Brazil, on board the first research cruise of the Brazilian Navy RV Vital de Oliveira, as part of an effort of the Brazilian High Latitude Oceanography Group (GOAL). Finally, it contributed to the activities developed by the following Brazilian networks: GOAL, Brazilian Ocean Acidification Network (BrOA), Brazilian Research Network on Global Climate Change (Rede CLIMA). The focus of the first study using this dataset (Orselli et al. 2019a) was on investigate the role played by the Agulhas eddies on the sea-air CO2 net flux along their trajectories through the South Atlantic Ocean and model the seawater CO2–related properties as function of environmental parameters. This data has been used to contribute to the scientific discussion about the Agulhas eddies impact on the changes of the marine carbonate system, which is an expanding oceanographic subject (Carvalho et al. 2019; Orselli et al. 2019b; Ford et al. 2023). Seawater and atmospheric CO2 molar fraction (xCO2sw and xCO2atm, respectively) were continuously measured during the cruise track, as well as the sea surface temperature (T) and salinity (S). The following sampling methodology is fully described in Orselli et al. (2019a). The underway xCO2 sampling was taken using an autonomous system GO–8050, General Oceanic®, equipped with a non-dispersive infrared gas analyzer (LI–7000, LI–COR®). The underway T and S were sampled using a Sea-Bird® Thermosalinograph SBE21. Seawater intake to feed the continuous systems of the GO-8050 and the SBE21 was set at ~5 m below the sea surface. The xCO2 system was calibrated with four standard gases (CO2 concentrations of 0, 202.10, 403.20, and 595.50 uatm) within a 12 h interval along the entire cruise. Every 3 h the system underwent a standard reading, to check the derivation and allow the xCO2 corrections. The xCO2 measurements were taken within 90 seconds interval. After a hundred of xCO2sw readings, the system was changed to atmosphere and five xCO2atm readings were taken (Pierrot et al., 2009). xCO2 (umol mol–1) inputs were corrected by the CO2 standards (Pierrot et al., 2009). Thermosalinograph data were corrected using the CTD surface data. Then, together with the pressure data, these data were used to calculate the pCO2 of the equilibrator and atmosphere (pCO2eq and pCO2atm, respectively, uatm), following Weiss & Price (1980). Using the pCO2eq, which is calculated at the equilibrator temperature, it is possible to calculate the pCO2 at the in situ temperature (pCO2sw, uatm), according to Takahashi et al. (2009). Another common calculation regarding pCO2sw data, is the temperature-normalized pCO2sw (NpCO2sw, uatm). This means that the temperature effect is removed when one calculates the NpCO2sw for the mean cruise temperature. The procedure followed the Takahashi et al. (2009) and considered the mean cruise temperature of 20.39°C. The results obtained allow one to investigate the exchanges of CO2 at the ocean-atmosphere interface by calculating the pCO2 difference between these two reservoirs (DeltapCO2, DpCO2=pCO2sw–pCO2atm, uatm). Negative (positive) DpCO2 results indicate that the ocean acts as a CO2 sink (source) for the atmosphere. To determine the FCO2, the monthly mean wind speed data of July 2015 (at 10 m height) were extracted from the ERA-Interim atmospheric reanalysis product of the European Centre for Medium Range Weather Forecast (http://apps.ecmwf.int/datasets/data/interim-full-moda/levtype=sfc/) since the use of long-term mean is usual (e.g., Takahashi et al., 2009). The average wind speed for the period and whole area was 6.8 ± 0.6 m s−1, ranging from 5.6 to 8.3 m s−1. The CO2 transfer coefficients proposed by Takahashi et al. (2009) and Wanninkhof (2014) were used. With all these data together, the FCO2 was determined according to Broecker & Peng (1982), where FCO2 is the sea-air CO2 net flux (mmol m–2 d–1; FT09 and FW14 are the Sea-air CO2 flux calculated using the coefficients described in Takahashi et al. (2009) and Wanninkhof (2014), respectively).

Description: We present a high-resolution spatial study of ocean surface carbon dioxide partial pressure (pCO2), temperature and salinity coupled with a seismic survey performed in subpolar waters with a variable presence of glaciers along the coastal margins of Admiralty Bay and the Bransfield Strait, northern Antarctic Peninsula, during the late spring season. Three zones were identified in this bay. The shallow and relatively fresh SHALLOW GLACIER THAW zone in the inner portion of the bay had high freshwater inputs from active glacial meltwater channels, representing higher pCO2 levels (median ~438 μatm) than the shallow and relatively salty SHALLOW zone without glaciers along the margins and dominated by macroalgae communities at the bottom, which showed relatively low pCO2 levels (median ~371 μatm). The deep and relatively salty CENTRE zone was highly influenced by seawater intrusions from the Bransfield Strait, representing intermediate pCO2 levels (median ~397 μatm). The net sea-air CO2 fluxes in late spring obtained from the high-resolution surface survey in Admiralty Bay indicate a condition of near neutral air-sea CO2 flux, with a median (25–75% interquartile range) value of -0.07 mmol m-2 day-1 (ranging from -12.21 to +4.33 mmol m-2 day-1), contrasting with the slight source to the atmosphere estimated from measurements only in the CENTRE zone. This finding suggests that temperature-sensitive metabolic and physical-chemical processes may cause significant variability in the ocean surface distribution of CO2 over short shoreline distances in the northern Antarctic Peninsula.