The Southern Ocean's complex hydrographic changes over recent decades, including surface cooling in the subpolar sector and warming north of the Antarctic Circumpolar Current (ACC), are accompanied by near-surface freshening in the subpolar region. These changes significantly impact regional and global climate, influencing large-scale circulation, sea ice, and surface cooling. While several studies suggest that subpolar freshening is due to an amplified global hydrological cycle and increased high-latitude precipitation, other mechanisms like ice shelf and iceberg melting, and sea ice freshwater transport, have also been proposed. Climate models, however, struggle to accurately represent these processes, particularly Antarctic meltwater and sea ice. This paper addresses these uncertainties by analyzing long-term observations of seawater oxygen isotopes and salinity in the Indian sector of the Southern Ocean (40°E to 90°E) to identify the causes of surface freshening. The study focuses on austral summer observations (spanning 1993–2021) from the French Océan Indien Service d'Observations (OISO) program and additional hydrographic sections. This combined dataset provides concurrent δ¹⁸O (oxygen isotope ratio) observations, crucial for distinguishing between freshwater sources with varying isotopic signatures. In open ocean surface waters, δ¹⁸O is primarily controlled by evaporation and precipitation, while in polar regions, it is also affected by continental ice melt and sea ice processes. This makes δ¹⁸O a valuable tracer to differentiate between meteoric (precipitation and continental ice) and oceanic (sea ice) freshwater sources. The researchers aim to disentangle the contributions of these factors to the observed salinity changes.

Prior research on Southern Ocean hydrographic changes has highlighted contrasting temperature trends – cooling in the subpolar region and warming north of the ACC. Near-surface freshening in the subpolar Southern Ocean has been linked to an amplified global hydrological cycle and increased precipitation at high latitudes. However, alternative explanations have been suggested, including increased meltwater from ice shelves and icebergs, and intensified sea ice freshwater transport. While climate models indicate a human-induced component to salinity changes, their representation of Antarctic meltwater, sea ice, and precipitation remains imperfect, leading to uncertainties in interpreting observed trends. Existing studies demonstrate inconsistencies among atmospheric reanalysis products regarding precipitation minus evaporation (P-E) trends, further complicating the analysis of Southern Ocean surface salinity changes. The existing literature has conflicting interpretations regarding the main drivers of subpolar freshening, highlighting the need for a robust study capable of disentangling the diverse freshwater inputs.

The study utilizes a long-term dataset (1993–2021) of surface seawater oxygen isotope (δ¹⁸O) and salinity observations from the Indian sector of the Southern Ocean, incorporating data from the OISO program and earlier hydrographic sections. The analysis focuses on austral summer data. A streamwise coordinate system, based on mean dynamic topography, is employed to account for the non-zonal mean circulation structure of the region, allowing for more accurate analysis of meridional gradients. Zonally averaged meridional profiles of salinity and δ¹⁸O are created, revealing distinct regimes north and south of the Subantarctic Front (SAF-N) at approximately 46°S. The relationship between salinity and δ¹⁸O anomalies is investigated to identify the dominant processes controlling surface properties in each region. Long-term trends in salinity and δ¹⁸O are computed using weighted linear regression models, accounting for variability in the data. To explain the observed changes, a coupled salinity and δ¹⁸O budget is used, incorporating freshwater fluxes from precipitation-evaporation (P-E), sea ice, and glacial meltwater. The uncertainty in these calculations is addressed by incorporating estimates from the literature, and conducting Monte Carlo simulations to account for the uncertainties of the input parameters. The study relies on existing estimates for sea ice freshwater fluxes (1993–2008) and glacial meltwater fluxes (1993–2021) from literature review, employing the latest IPCC reports as references. Given the uncertainties associated with atmospheric reanalysis data for P-E, the study takes an alternative approach of computing plausible P-E changes from the observed salinity and δ¹⁸O trends and the best estimates of glacial meltwater and sea ice changes. This method allows reconciliation of the observed oceanographic changes with the literature values of different freshwater fluxes, resolving inconsistencies among reanalysis products. The use of oxygen isotope data alongside salinity allows for a better determination of the relative importance of various freshwater inputs.

The study reveals distinct trends in surface salinity and δ¹⁸O north and south of the SAF-N. North of 46°S (subtropical region), both salinity and δ¹⁸O increased significantly (0.06 ± 0.07 g kg⁻¹ per decade and 0.03 ± 0.04‰ per decade, respectively). This strong correlation between salinity and δ¹⁸O anomalies suggests that P-E changes are the main driver of this trend. South of 46°S (subpolar region), salinity decreased (-0.02 ± 0.01 g kg⁻¹ per decade) and δ¹⁸O showed a slight decrease (-0.01 ± 0.02‰ per decade). A weaker correlation between salinity and δ¹⁸O anomalies in this region indicates that additional processes beyond P-E are at play, primarily sea ice and glacial meltwater contributions. By analyzing a coupled salinity and δ¹⁸O budget, the study quantifies the contribution of each freshwater source to the observed changes. The results reveal that the subpolar freshening is predominantly driven by increased net precipitation (an increase by a factor of two), while the effects of decreased sea ice melt are largely compensated by the contribution of glacial meltwater. Glacial meltwater from the Antarctic ice shelves has a limited impact on open-ocean salinity changes in this region, only contributing to a small negative salinity trend (-0.008 ± 0.003 g kg⁻¹ per decade). Sea ice decline contributes to a positive salinity trend (0.008 ± 0.002 g kg⁻¹ per decade), but this is outweighed by the negative trend due to increased P-E (-0.02 ± 0.008 g kg⁻¹ per decade), resulting in net freshening. The authors' analysis demonstrates that the coupled salinity and δ¹⁸O budget, applied to a long-term observation dataset, is a robust way to quantify different freshwater forcing contributions. These findings align with independent analyses from a much larger dataset.

The findings of this study provide crucial insights into the drivers of Southern Ocean salinity changes. The significant increase in salinity north of the SAF-N is strongly linked to changes in P-E, highlighting the role of atmospheric processes in shaping the subtropical ocean. In contrast, the subpolar freshening is attributed to a complex interplay of increased net precipitation, decreased sea ice melt, and glacial meltwater inputs. The limited influence of Antarctic ice discharge on open-ocean salinity, despite increased ice sheet mass loss, suggests that the impact of meltwater is mainly confined to coastal regions. The study's robust methodology, combining salinity and δ¹⁸O data, allows for a more precise quantification of freshwater forcing than previous studies. This approach highlights the importance of long-term, multi-parameter observations in resolving the complex interplay of factors influencing Southern Ocean surface water properties. This is crucial for a better understanding of the Southern Ocean's role in global climate and for the improvement of climate models.

This study demonstrates the use of long-term oxygen isotope and salinity observations to disentangle complex processes influencing Southern Ocean surface water properties. The results highlight the intensified hydrological cycle in the Indian sector, with a dominant role of increased precipitation in subpolar freshening, and a limited effect of Antarctic glacial meltwater on open ocean salinity. Continued monitoring of these surface ocean processes, using similar methodologies, is crucial for establishing an early warning system for the impact of ice sheet mass loss on ocean characteristics. The results offer a framework for improved climate model development, ensuring better representation of critical polar processes and aiding in understanding the human impact on ocean properties.

The study focuses solely on austral summer observations, potentially limiting the scope of interpretations for annual or seasonal cycles. The use of existing estimates for sea ice and glacial meltwater fluxes introduces uncertainties. The spatial coverage is limited to the Indian sector of the Southern Ocean; hence the study's findings may not be universally applicable to the entire Southern Ocean. Despite these limitations, the study's robust methodology and comprehensive analysis provide valuable insights into the drivers of Southern Ocean salinity changes.