Meridional Overturning Circulation Transport Variability at 34.5°S During 2009–2017: Baroclinic and Barotropic Flows and the Dueling Influence of the Boundaries

Abstract

Six years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin-wide meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning~20 months during 2009-2010 and~4 years during 2013-2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak-to-peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year-to-year variations.Plain Language Summary Changes in the meridional overturning circulation, characterized by north-south flows throughout the Atlantic Ocean basin and vertical exchange between the surface and the deep ocean, are related to changes in important ocean-atmosphere-climate signals like precipitation patterns, sea level, and extreme weather (e.g., drought, heat waves, and hurricane intensification). This study presents, for the first time, a multiyear daily record of the meridional overturning circulation flow based on direct measurements in the South Atlantic Ocean at 34.5°S. The roughly six years of observations presented in this study provided the ability to study seasonal and interannual changes in these important flows with continuous daily data, and they demonstrated a complexity of the ocean circulation as compared to other latitudes where this flow has been studied in the past.

Recent Contributions of Theory to Our Understanding of the Atlantic Meridional Overturning Circulation

“…However, several of the developments over the last decade have been catalyzed by recent observations such as those made by the RAPID‐MOCHA array at 26°N (e.g., our understanding of the mechanisms via which wind forcing dominates the AMOC seasonal cycle, and of the role of eddies and boundary propagation). The unprecedented amount of data currently being collected by existing and new arrays throughout the Atlantic (e.g., RAPID‐MOCHA (McCarthy et al, ), Overturning in the Subpolar North Atlantic Programme (OSNAP, Lozier et al, ), SAMOC at 34°S (Meinen et al, ), MOVE at 16°N (Send et al, ), etc. See Cunningham & coauthors, , for a summary) together with long‐term global ocean observing systems such as Argo (http://www.argo.ucsd.edu) and satellite altimetry (https://www.aviso.altimetry.fr/en/) mean that there has never been such an exciting time to be thinking about the Atlantic Ocean.…”

Shallow and Deep Eastern Boundary Currents in the South Atlantic at 34.5°S: Mean Structure and Variability

“…Unfortunately, the bottom current meters on the CPIES moorings are too far apart (>3.5° spacing) relative to the spatial decorrelation scales of the deep circulation (~1°; Donohue et al, ; Purkey & Johnson, ; Meinen et al, ) to use those to provide the time‐mean velocity for the reference flows. As in previous studies in the South Atlantic (Meinen et al, , , ), the time‐mean reference geostrophic velocity between pairs of CPIES was derived at 1,500 dbar from the output of a long (36‐year) run of the Ocean general circulation model For the Earth Simulator (OFES; Masumoto et al, ; Sasaki et al, , ) at eddy‐resolving resolution. Combining the CPIES‐GEM derived relative velocity profile time series with the time series of the reference velocity created by merging the model time‐mean and bottom pressure‐derived time‐varying reference velocity yields estimates of the full‐water‐column time series of absolute geostrophic velocity between each pair of CPIES.…”

“…Moored arrays have been deployed at several locations as part of an international initiative to study the South Atlantic MOC (Ansorge et al, ; Garzoli & Matano, ; Hummels et al, ; Kopte et al, ; Meinen et al, , , ). One of the main components of this initiative is the South Atlantic MOC Basin‐wide Array (SAMBA; Figure a) that has been under development along 34.5°S since 2008–2009.…”

“…One of the main components of this initiative is the South Atlantic MOC Basin‐wide Array (SAMBA; Figure a) that has been under development along 34.5°S since 2008–2009. The SAMBA array was designed to capture the transport associated with the western boundary currents and eastern boundary currents (EBC) as well as the interior flows via estimation of geostrophic transports (Ansorge et al, ; Meinen et al, , ; Perez et al, ). The choice of 34.5°S for the array was based on analyses indicating that latitude was crucial for evaluating MOC variability and the impact of interocean exchanges (e.g., Schiermeier, ).…”

Sustainable Observations of the AMOC: Methodology and Technology

“…Inverted echo sounders equipped with a bottom pressure sensor (PIES) in combination with hydrographic profiles from Argo and shipboard CTD profiles provide a powerful tool to calculate continuous transport time series using the geostrophic method (Meinen & Watts, , ; Mertens et al, ; Rhein et al, ). PIES moorings form the backbone of the SAMBA array measuring the AMOC at 34.5°S (Meinen et al, ; Meinen, Speich, et al, ). Furthermore, through comparisons when the PIES are deployed, altimetry can be used to extend the transport time series back to 1993, the start of the satellite altimetry (Roessler et al, ).…”