Observing the Local Emergence of the Southern Ocean Residual-Mean Circulation

• Published in: Geophysical Research Letters
• Main Authors: Sévellec, Florian, Naveira Garabato, A, Vic, Clement, Ducousso, N.
• Format: Article in Journal/Newspaper
• Language: English
• Published: American Geophysical Union (AGU) 2019
• Subjects: Southern Ocean; meridional overturning; mesoscale turbulence; residual-mean circulation; Eulerian circulation; mooring measurements; Antarctic; Drake Passage; Antarc*
• Online Access: https://archimer.ifremer.fr/doc/00485/59655/62687.pdf; https://archimer.ifremer.fr/doc/00485/59655/62688.pdf; https://archimer.ifremer.fr/doc/00485/59655/62689.tex; https://doi.org/10.1029/2018GL081382; https://archimer.ifremer.fr/doc/00485/59655/

The role of mesoscale turbulence in maintaining the mean buoyancy structure and overturning circulation of the Southern Ocean is investigated through a 2‐year‐long, single‐mooring record of measurements in Drake Passage. The buoyancy budget of the area is successively assessed within the Eulerian and the Temporal‐Residual‐Mean frameworks. We find that a regime change occurs on timescales of 1 day to 100 days, characteristic of mesoscale dynamics, whereby the eddy‐induced turbulent horizontal advection balances the vertical buoyancy advection by the mean flow. We use these diagnostics to reconstruct the region's overturning circulation, which is found to entail an equatorward downwelling of Antarctic Intermediate and Bottom Waters and a poleward upwelling of Circumpolar Deep Water. The estimated eddy‐induced flow can be accurately parameterized via the Gent‐McWilliams closure by adopting a diffusivity of ~2,000 m2 s‐1 with a mid‐depth increase to 2,500 m2 s‐1 at 2,100 m, immediately underneath the maximum interior stratification. Plain Language Summary In this study we use a 2‐yr long, high‐resolution (both temporally and vertically) mooring together with a state‐of‐the‐art method (Temporal‐Residual‐Mean framework) to disentangle the role of mesoscale eddy turbulence for sustaining the Meridional Overturning Circulation leeward of the Drake Passage.Because of its role in carbon sequestration and ocean heat uptake the Meridional Overturning Circulation is a key component of the climate system. However, it is driven by a wide range of physical processes from persistent, large‐scale wind to meso‐ and small‐scale turbulence. Hence disentangling how these processes contribute to the maintenance of this climatically relevant ocean circulation is one of the current challenges in physical oceanography. Here we apply a state‐of‐the‐art method (Temporal‐Residual‐Mean framework) and shows the emergence of a balance between mesoscale eddies and Eulerian mean circulation for buoyancy advection. In particular, we demonstrate that the effect mesoscale eddies can be accurately parameterized with a Gent‐McWilliams type diffusivity of ~2,000 m2 s‐1. Beyond this result, this study also provides an observational evidence of the typical large‐scale picture of the Meridional Overturning Circulation and shows its relevance at local scales.