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 ten years of steric height variability (2008 to 2017) in
the Southern Ocean (30°S to 70°S) by analysing temperature and salinity
variations obtained from the GLORYS-031 model provided by the European
Copernicus Marine Environment Monitoring Service (CMEMS). The
thermohaline variability is decomposed into thermohaline modes using a
functional Principal Component Analysis (fPCA). 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 (PCs) that can be related to steric
height variability. 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 (0-2000
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.