Impact of Thermohaline Variability on Sea Level Changes in the Southern Ocean

Abstract

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°S to 70°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.

Global ocean colour trends in biogeochemical provinces

“…We found a negative relationship between Chl-a and salinity in the open water province of the Southern Ocean (ANTA: r 0.3). In contrast, we found that the surface salinity is increasing near the Antarctic coast, which was also reported by Chaigneau and Morrow (2002) and Kolbe et al (2021). In the coastal Southern Ocean province the salinity is positively related to Chl-a (APLR: r 0.4), which may be caused by the stronger vertical mixing in these shallower coastal waters and larger brine rejection in the local autumn and winter because of ice cover thinning (Chaigneau and Morrow, 2002;Holland et al, 2006).…”