Impact of Thermohaline Variability on Sea Level Changes in the South ocean

The Southern Ocean is responsible for the majority of the global oceanic heat uptake that contributes to global sea level rise. At the same time, ocean temperatures do not change at the same rate in all regions and sea level variability is also affected by changes in salinity. This study investigates 10 years of steric height variability (2008-2017) in the Southern Ocean (30 degrees S to 70 degrees S) by analyzing temperature and salinity variations obtained from the GLORYS-031 model provided by the European Copernicus Marine Environment Monitoring Service. The thermohaline variability is decomposed into thermohaline modes using a functional Principal Component Analysis. Thermohaline modes provide a natural basis to decompose the joint temperature-salinity vertical profiles into a sum of vertical modes weighted by their respective principal components that can be related to steric height. Interannual steric height trends are found to differ significantly between subtropical and subpolar regions, simultaneously with a shift from a thermohaline stratification dominated by the first thermal mode in the north to the second 'saline' mode in the South. The Polar Front appears as a natural boundary between the two regions, where steric height variations are minimized Despite higher melt rates and atmospheric temperatures, steric height in Antarctic waters (0-2,000 m) has dropped since 2008 due to higher salt content in the surface and upper intermediate layer and partially colder waters, while subtropical waters farther north have mostly risen due to increased heat storage. Plain Language Summary Sea level variations on longer timescales mainly arise from mass changes or the thermo- and halosteric effects of temperature and salinity on water density. Recent variability in steric height in the Southern Ocean was investigated from 2008 to 2017 by analyzing potential temperature and salinity variations obtained from a global ocean reanalysis. The work was performed using a functional approach to a standard Principal Component Analysis that was applied on vertical temperature and salinity profiles (2,000 m). The resulting thermohaline modes contain information about the general temperature and salinity structure and their variations can be attributed to steric height changes. The results have shown that Antarctic waters above 2,000 m have dropped since 2008 due to higher salt content and colder waters, while subtropical waters farther north have mostly risen due to increased heat storage. Those spatial differences in recent steric height trends also display on the total sea level rise (SLR) observed from satellite data, which shows a significantly higher rate of SLR in subtropical waters compared to higher latitudes of the Southern Ocean.

Automated summary

The Southern Ocean plays a key role in global oceanic heat uptake and sea level rise, with significant interregional differences in steric height trends. Changes in thermohaline stratification have been observed over the past decade, with Antarctic waters experiencing a drop in steric height while subtropical waters have risen.