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 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: Highlights: • First eddy–mean flow interaction analysis in the Deep Western Boundary Current between 5°S and 16°S. • Eddy kinetic energy is mainly generated via barotropic instability. • Enhanced upstream mean flow induces intensification in the downstream eddy field. Abstract: Thirty-six years output of a 1/10° eddy-resolving Ocean General Circulation Model are used to analyze the energetics of eddy–mean flow interactions in the Deep Western Boundary Current (DWBC) region of the tropical South Atlantic between 5°S and 16°S. The DWBC flow has a coherent structure between 5°S and 8°S but breaks up into a train of eddies downstream of a region of strong bathymetric curvature at 8°S. In the train of eddies area, the seasonal cycle of eddy kinetic energy (EKE) exhibits poleward phase propagation from May to September. The connection between the seasonal cycle of mean kinetic energy and EKE indicates an intensification of the downstream eddy field associated with enhanced upstream mean flow. The magnitudes of the baroclinic conversion and vertical eddy density flux terms are small in the DWBC core layer depth but somewhat elevated 500 m above and below the core. Eddy processes, including eddy generation and propagation, are accompanied by high EKE and large barotropic conversion. While in the global ocean baroclinic conversion is thought to dominate the energy transfer to EKE, our results suggest that barotropic energy conversion is the primary source of EKE and modulates its variability in the DWBC region of the deeper ocean.