What question did this study set out to answer?

This study aims to understand long-term changes in salinity in the South Atlantic Ocean and the factors driving these changes.

June 19, 2026

Attribution of Subsurface Salinity Change in the South Atlantic

Key Points

This study aims to understand long-term changes in salinity in the South Atlantic Ocean and the factors driving these changes.
Analyzed observational data sets and CMIP6 simulations from 1955 to 2014
Examined trends in salinity on isopycnal surfaces to distinguish between thermodynamic and dynamical contributions
Assessed the influence of processes like pure warming and pure freshening on salinity trends
In the upper layer, pure freshening accounts for approximately 75% of the salinity increase, with heaving contributing about 25%.
In the Agulhas Leakage region, upper-layer salinification trend is about 0.03 g kg−1 decade−1, with approximately 60-65% driven by pure freshening.
A southward shift of the Subtropical Front at about 0.5° decade−1 further influences circulation and salinity dynamics.

Abstract

Abstract Using observational data sets and CMIP6 simulations, this study investigated the long‐term changes of salinity in the upper 2,000 m in the South Atlantic Ocean (SAO) during 1955–2014. Salinity trends are analyzed on isopycnal surfaces to separate thermodynamic and dynamical contributions, including spiciness, heave, and three pure processes: pure warming (PW), pure freshening (PF), and pure heaving ( PH). In the subtropical SAO upper layer, salinity increases are primarily driven by PF, which explains ∼75% of the total trend, while PH contributes ∼25% and PW has negligible net influence. In contrast, intermediate waters in the Subantarctic Mode Water density range exhibit freshening dominated by PF, partially offset by PH. In the Agulhas Leakage (AL) region, upper‐layer salinification is significantly amplified relative to the basin mean (≈0.03 g kg −1 decade −1 ) and shows a stronger dynamical imprint: PF explains ∼60–65% of the trend, while PH contributes ∼35%, indicating enhanced circulation‐driven influence compared with the broader SAO. AL salt flux into the SAO has increased since the 1950s, with velocity changes accounting for ∼60% of the trend. A southward shift of the Subtropical Front (∼0.5° decade −1 ) provides a secondary contribution by widening the inter‐basin gateway. These results demonstrate that while surface freshwater forcing governs basin‐scale upper‐ocean trends, AL strengthens the dynamical contribution locally and redistributes salt across the SAO, thereby modulating basin‐wide salinity structure with potential implications for stratification and Atlantic Meridional Overturning Circulation (AMOC) variability.

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