A multi‐data set analysis of the freshwater transport by the Atlantic Meridional overturning circulation at nominally 34.5°S

Abstract

The freshwater transport (Mov) by the Atlantic Meridional Overturning Circulation (AMOC) across 34.5°S is computed using observations from 49 eXpendable BathyThermograph (XBT) transects between 2002 and 2019. The Mov at 34.5°S serves as a possible indicator of the AMOC stability, with a negative (southward) freshwater transport indicating a possible bistable AMOC regime and positive (northward) transport indicating a monostable regime. A negative Mov mean of −0.15 ± 0.09 Sv is estimated from the repeated XBT transects, suggesting a bistable AMOC regime. Results are complemented with two data sets derived from Argo float observations, numerical ocean models, and coupled climate models. More than half of the coupled models examined, 20 out of 32, present positive Mov mean values. To investigate the causes of the differing signs of the Mov across the models, we examine the salinity vertical structure in models with positive and negative Mov, indicating fresher upper and saltier deep waters in models with positive Mov. The South Atlantic meridional fluxes show linear relationships, with a negative slope (positively correlated in magnitude) between Mov/MOC and Mov/MHT, and a positive slope (positively correlated) between MHT/MOC. Seasonally, the South Atlantic meridional fluxes from most of the data sets considered here, show a more negative Mov and a more positive MOC and MHT in austral fall and winter from April to August across 34.5°S.