Antarctic Circumpolar Current Transport Variability in GRACE Gravity Solutions and Numerical Ocean Model Simulations

The Antarctic Circumpolar Current (ACC) is the main contributor to the inter-basin mass exchange in the ocean carrying a mean total transport of about 134 Sv (1 Sv = 10⁶ m³/s) of water around the globe. A large part of its transport is found to be related to the prevailing westerly winds over the Southern Ocean. The Southern Annular Mode (SAM) explains 20–30% of the zonal wind component’s variability and enforces (at least to a certain extent) the variability of the oceanic transport. In this study, the representation of the ACC transport variability in the GRACE time-variable gravity solutions is investigated. Monthly GRACE gravity solutions over the ocean are commonly connected to ocean bottom pressure (OBP) fluctuations. Due to the connection between OBP gradients and geostrophic transport, a part of the ACC transport variability can be explained by gradients of OBP anomalies. Model simulations with the Finite Element Sea Ice-Ocean Model (FESOM) feature variations of about 2–3 Sv on annual and semi-annual periods; more than 50% of the total ACC transport variability is explained by OBP variability. GRACE-derived transport anomalies show variations at semi-annual and annual periods of about 6–9 Sv. Variations detected by GRACE coincide with model results with correlations of about 0.75. Atmospheric variability as described by SAM is found to be related to FESOM- and GRACE-derived OBP on short time scales up to annual. The spatial pattern of SAM is reflected in GRACE OBP by showing an annular band of highly negative correlation around Antarctica south of the southern boundary of the ACC. Model simulations with FESOM confirm that these spatial patterns are related to f/H contours in the Southern Ocean.