Satellite-derived steric height in the Southern Ocean: trends, variability, and climate drivers

The Southern Ocean circulation plays a central role in regulating the global ocean overturning, ventilating the deep ocean, and driving sea level rise by delivering heat to Antarctic ice shelves. Understanding heat and freshwater content in this region is key to monitoring these global processes and identifying multi-year changes; however, in situ observations are limited and often do not offer the spatial or temporal consistency needed to study long-term variability. Perturbations in steric height can reveal changes in oceanic heat and freshwater content inasmuch as they impact the density of the water column. Here, we show for the first time that the monthly steric height anomaly of the Southern Ocean south of 50° S can be assessed using satellite altimetry and GRACE gravimetry data from 2002 to 2018. Steric height anomalies are validated against in situ Argo float and conductivity–temperature–depth (CTD) data from tagged elephant seals. We find good agreement north of 65° S, but there is increasing uncertainty towards the Antarctic continental shelf due to insufficient validation data, the leakage error, and anti-aliasing in GRACE. The Southern Ocean steric height anomalies capture the expected seasonal cycle of low (high) steric height in winter (summer) and show regionally variable trends during 2002–2018. We find that the variability in steric height is driven predominantly by anomalies in surface heat and freshwater content associated with positive and negative phases of the two major modes of Southern Hemisphere climate variability (the El Niño–Southern Oscillation and Southern Annular Mode). This steric height dataset provides a uniquely comprehensive insight into density anomalies and presents opportunities for further analysis of heat and freshwater fluxes, changes in stratification, or convective regimes across the Southern Ocean.