Preprints
https://doi.org/10.5194/os-2020-124
https://doi.org/10.5194/os-2020-124

  25 Jan 2021

25 Jan 2021

Review status: this preprint is currently under review for the journal OS.

The depth scales of the AMOC on a decadal timescale

Tim Rohrschneider1,2, Johanna Baehr3, Veit Lüschow1, Dian Putrasahan1, and Jochem Marotzke1,4 Tim Rohrschneider et al.
  • 1Max Planck Institute for Meteorology, Hamburg, Germany
  • 2International Max Planck Research School on Earth System Modelling, Hamburg, Germany
  • 3Institute of Oceanography, Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, Germany
  • 4Center for Earth System Research and Sustainability, Universität Hamburg, Hamburg, Germany

Abstract. We use wind sensitivity experiments to understand the wind forcing dependencies of the level of no motion and the e-folding pycnocline scale as well as their relationship to northward transport of the mid-depth Atlantic meridional overturning circulation (AMOC) south and north of the equator. In contrast to previous studies, we investigate the interplay of nonlocal and local wind effects on a decadal timescale. We use 30-year simulations with a high-resolution ocean general circulation model (OGCM) which is an eddy-resolving version of the Max Planck Institute Ocean Model (MPIOM). Our findings deviate from the common perspective that the AMOC is a nonlocal phenomenon only, because northward transport in the inter-hemispheric cell can only be understood by analyzing nonlocal Southern Ocean wind effects and local wind effects in the northern hemisphere downwelling region where Ekman pumping takes place. Southern Ocean wind forcing predominantly determines the magnitude of the pycnocline scale throughout the basin, whereas northern hemisphere winds additionally influence the level of no motion locally. In that respect, the level of no motion is a better proxy for northward transport and mid-depth velocity profiles despite the Ekman return flow which is found to be baroclinic. We compare our results inferred from the wind experiments and a 100-year global warming experiment in which the atmospheric CO2 concentration is quadrupled, using MPIOM coupled to an atmospheric model. We find that the evolution of the level of no motion in response to global warming represents changes in vertical velocity profiles or northward transport, whereas the changes of the pycnocline scale are opposite to the changes of the level of no motion over time. Using the level of no motion as depth scale, the analysis of the wind experiments and the warming experiment suggests a hemisphere-dependent scaling of the strength of AMOC. Furthermore, we put forward the idea that the ability of numerical models to capture the spatial and temporal variations of the level of no motion is crucial to reproduce the mid-depth cell in an appropriate way.

Tim Rohrschneider et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on os-2020-124', Anonymous Referee #1, 26 Feb 2021
  • RC2: 'Comment on os-2020-124', Anonymous Referee #2, 23 Mar 2021

Tim Rohrschneider et al.

Data sets

The depth scales of the AMOC on a decadal timescale Tim Rohrschneider, Johanna Baehr, Veit Lüschow, Dian Putrasahan, and Jochem Marotzke http://hdl.handle.net/21.11116/0000-0007-D24C-7

Tim Rohrschneider et al.

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Short summary
This paper presents an analysis of wind and warming sensitivity experiments in order to provide insight into the Atlantic inter-hemispheric circulation by understanding the behavior of the depth scale(s) of the AMOC. We put into question the common view that the AMOC is a nonlocal phenomenon only. The analysis of the wind experiments and the warming experiment suggests a hemisphere-dependent scaling of the strength of AMOC and temporal adjustment to global warming.