Variability of the Western Boundary Current System and AMOC at 11°S

The circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flows. The zonally integrated meridional flow is associated with the streamfunction of the Atlantic Meridional Overturning Circulation (AMOC) — a major component of the global climate system. In the tropics, the northward, upper branch of the AMOC flow is superimposed by the shallower overturning associated with the wind-driven Subtropical cells (STC), where both overturning systems substantially contribute to tropical Atlantic variability. At 11°S, the AMOC has been monitored for more than 10 years by the Tropical Atlantic Circulation and Overturning at 11°S (TRACOS) observing system, which combines dedicated moorings and Pressure equipped inverted echo sounders (PIES) at the western and eastern boundaries with available observations across the basin.

Here, we investigate specifically the variability of the deep western boundary current (DWBC) and different estimates for the AMOC anomaly transport at 11°S based on different parts of the observing system. DWBC transport estimates are derived from full-depth alongshore velocity fields using regression-based techniques combining the moored observations with the shipboard velocity sections. The sensitivity and robustness of the AMOC estimate is evaluated through an observing system simulation experiment (OSSE) employing the high-resolution ocean model VIKING20X.

The results reveal that the Deep Western Boundary Current (DWBC) transport is dominated by strong intraseasonal variability associated with the passage of deep eddies, while longer-term variability becomes apparent as the observational record lengthens. The geostrophic upper AMOC transport at 11°S is primarily characterized by pronounced seasonal variability, with peak-to-trough amplitudes of approximately 20 Sv. Despite its comparatively sparse design, the TRACOS array is capable of capturing key aspects of AMOC variability, while also identifying opportunities for further improvements, particularly with respect to reducing uncertainties on longer time scales.