A Multi‐Data Set Analysis of the Freshwater Transport by the Atlantic Meridional Overturning Circulation at Nominally 34.5°S

• Published in: Journal of geophysical research. Oceans 2024-06, Vol.129 (6), p.n/a
• Main Authors: Arumí‐Planas, Cristina, Dong, Shenfu, Perez, Renellys, Harrison, Matthew J., Farneti, Riccardo, Hernández‐Guerra, Alonso
• Format: Article
• Language: English
• Subjects: AMOC; Atlantic Meridional Overturning Circulation (AMOC); Bathythermographs; bistable AMOC regime; Circulation; Climate; Climate models; Climate system; Data analysis; Datasets; Deep water; Eddies; Fluxes; Fresh water; Freshwater; freshwater transport; heat transport; Inland water environment; Mass transport; Mathematical models; Numerical models; Ocean currents; Ocean models; Oceans; Salinity; Seasonal variability; Seasonal variation; Seasonal variations; South Atlantic Ocean; Vertical profiles; Winds; XBTs
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2023JC020558

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. Plain Language Summary It is well known that the meridional (north‐south) overturning circulation, a large system of ocean currents driven by winds, buoyancy (density) differences, mixing, and eddies, has a significant impact on the world's climate system. Based on observations and numerical model data, this study presents a multi‐data set analysis of the freshwater transported by this circulation system across the nominal latitude of 34.5°S in the South Atlantic. The observed southward meridional freshwater transport (out of the South Atlantic) derived from all the observational data sets considered indicates a bistable regime of the meridional overturning circulation. Some coupled models that suggest a mono‐stable (one stable state) regime have fresher upper and saltier deep waters than the models that indicate a bistable regime (two stable states). We confirm that there is a linear relationship between mass transport by the meridional overturning circulation with the meridional freshwater and heat transports. Finally, we de