Highly variable upper and abyssal overturning cells in the South Atlantic

Abstract

The Meridional Overturning Circulation (MOC) is a primary mechanism driving oceanic heat redistribution on Earth, thereby affecting Earth’s climate and weather. However, the full-depth structure and variability of the MOC are still poorly understood, particularly in the South Atlantic. This study presents unique multiyear records of the oceanic volume transport of both the upper (<~3100 meters) and abyssal (>~3100 meters) overturning cells based on daily moored measurements in the South Atlantic at 34.5°S. The vertical structure of the time-mean flows is consistent with the limited historical observations. Both the upper and abyssal cells exhibit a high degree of variability relative to the temporal means at time scales, ranging from a few days to a few weeks. Observed variations in the abyssal flow appear to be largely independent of the flow in the overlying upper cell. No meaningful trends are detected in either cell.

The South Atlantic Meridional Overturning Circulation and Mesoscale Eddies in the First GO‐SHIP Section at 34.5°S

“…At interannual time scales, large (∼3 Sv) changes in the AMOC max have been observed both at 26.5°N and 34.5°S (e.g., C. S. Meinen et al., 2018; Smeed et al., 2018). A recent study which used the entire SAMBA moored array has shown that the amplitude of the AMOC max variability at 34.5°S is even larger than previously estimated with a standard deviation of 15.5 Sv for the upper‐cell and 6.2 Sv for the abyssal‐cell (Kersalé et al., 2020). The strongest variability occurs on timescales between 30 and 90 days tightly linked to the passage of eddies across the section (Kersalé et al., 2019).…”

“…Previous estimates of the AMOC max variability at this latitude from blended altimetry and XBT sections data (e.g., Dong et al., 2015) or Argo float data (e.g., Majumder et al., 2016) have revealed substantial seasonal to interannual variability, where the Ekman as well as the geostrophic transport contribution play a role both showing annual cycles, which are out of phase. The SAMBA‐SAMOC observations have also demonstrated a significant high‐frequency variability (Kersalé et al., 2020; C. S. Meinen et al., 2018). Both baroclinic (density) and barotropic (bottom pressure) variations at the eastern boundary at 34.5°S show significant or even dominant contributions to the overall AMOC variability at 34.5°S.…”

“…The SAMBA line of moorings began in a pilot mode in 2009, with a much better resolved array put in place beginning in 2013. SAMBA consists primarily of pressure‐equipped inverted echo sounders (PIES) and current‐and‐pressure‐equipped inverted echo sounders (CPIES), with four additional tall dynamic height/current meter moorings added on the South African continental margin in 2014 (e.g., Kersalé et al., 2018, 2020, 2019; C. S. Meinen et al., 2013; 2018). Hydrographic observations collected during the mooring maintenance cruises provide regional snapshots of the flow patterns and water mass regimes in the boundary regions.…”

“…In addition, as no measurements exist inshore of 1,350 dbar on both boundaries, estimates in those shelf areas are also based on model velocities. As such, only the variability of AMOC max from the SAMBA PIES/CPIES array is a complete robust observational assessment (Kersalé et al., 2020).…”

Seasonal variability of the Atlantic Meridional Overturning Circulation at 11° S inferred from bottom pressure measurements

“…The western tropical South Atlantic constitutes a key region for the exchange of water masses, heat and salt between the Southern Hemisphere and Northern Hemisphere (Biastoch et al, 2008b;Schmidtko and Johnson, 2012;Kolodziejczyk et al,2014;Hummels et al, 2015;Lübbecke et al, 2015;Herrford et al, 2017). Several observational and modelling studies (e.g.…”