Title: The South Atlantic Ocean: Circulation from an Inverse Box Model and a Multi-data Set Analysis of the Freshwater Transport by the AMOC
Abstract: The South Atlantic plays a key role in heat transport, as it hosts the returning flow of the Atlantic Meridional Overturning Circulation (AMOC). To gain insights on this role, using data from three hydrographic cruises conducted in the South Atlantic at 34.5ºS, 24ºS, and 10ºW, we identify water masses and compute absolute geostrophic circulation using inverse modeling. Our results describe the South Atlantic anticyclonic gyre in the upper layers, and the northward mass transports in the western basin and a loop in the eastern basin corresponding to the abyssal layers. The NADW flows southward in the western and eastern boundaries with an eastward flowing branch located north of 24ºS. Furthermore, we compute the upper AMOC strength (14.8±1.0 and 17.5±0.9 Sv), the northward heat (0.30±0.05 and 0.80±0.05 PW), and freshwater transports (0.18±0.02 and -0.07±0.02 Sv) at 34.5ºS and 24ºS, respectively.
Next, the freshwater transport by the AMOC (Mov) at 34.5ºS is computed using 49 XBT transects between 2002 and 2019. The Mov at 34.5ºS serves as an indicator of the AMOC stability, with Mov<0 indicating a possible bistable AMOC regime and Mov>0 indicating a monostable regime. A negative Mov mean is estimated from the XBT transects, consistent with a bistable AMOC regime. The results are complemented with Argo floats, numerical ocean models, and CMIP6 coupled models. Surprisingly, 20 out of 32 coupled models showed positive Mov values. To investigate the causes of the differing signs of Mov across the models, we examine the salinity vertical structure, indicating fresher upper and saltier deep waters in models with positive Mov. Additionally, the South Atlantic meridional fluxes show a linear relationship: Mov/MOC and Mov/MHT present a negative slope, while MHT/MOC present a positive slope. Seasonally, from April to August across 34.5ºS, Mov shows higher negative values and MOC and MHT present higher positive values.