AMOC Transport from the TRACOS Array at 11°S and Associated Methodological Uncertainties

The Atlantic meridional overturning circulation (AMOC) is a key component of the global ocean circulation. On longer time scales, it is considered one of the potential tipping points of the climate system. On shorter time scales, specifically at 11°S, it is linked to tropical Atlantic variability, primarily through the seasonally to interannually varying equatorward heat transport. Here, we present a 10-year time series of the upper branch of the AMOC transport at 11°S, utilizing data from the Tropical Atlantic Circulation and Overturning at 11°S (TRACOS) array. The TRACOS array consists of Pressure equipped Inverted Echo Sounders (PIES), tall moorings, and ship sections at the eastern and western boundaries. Supplementary data are taken from Argo floats and satellites. The sensitivity of the array setup and assumptions used to compute the geostrophic AMOC transport are evaluated by performing an observing system simulation experiment (OSSE) using a NEMO based high-resolution ocean model (VIKING20X). We find that, using a combination of PIES and tall moorings, the geostrophic AMOC transport can be reconstructed, explaining more than 70% (60%) of the variance in the high-pass (low-pass) filtered transport time series. A linear AMOC trend would be underestimated by less than 40%. We propose that part of the limitations caused by the drift of the bottom pressure sensors can be mitigated by incorporating PIES travel time data. The resulting geostrophic AMOC transport at 11°S is dominated by seasonal variability with peak-to-through amplitudes of about 20 Sv. Overall, we find that, despite the relatively sparse design of the TRACOS array (particularly when compared to other AMOC observing systems), it is capable of capturing AMOC signals. However, we also identify potential improvements in the array design that could reduce uncertainties, especially with respect to longer-term variability.