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Discussion papers
https://doi.org/10.5194/os-2018-134
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2018-134
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 07 Dec 2018

Research article | 07 Dec 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).

Cold vs. warm water route – sources for the upper limb of the AMOC revisited in a high-resolution ocean model

Siren Rühs1, Franziska U. Schwarzkopf1, Sabrina Speich2, and Arne Biastoch1 Siren Rühs et al.
  • 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2LMD-IPSL, UMR 8539, Département de Géosciences, ENS, PSL Research University, Paris, France

Abstract. The northward flow of the upper limb of the Atlantic Meridional Overturning Circulation (AMOC) is fed by waters entering the South Atlantic from the Indian Ocean mainly via the Agulhas Current (AC) system and by waters entering from the Pacific through Drake Passage (DP), commonly referred to as the warm and cold water routes, respectively. However, there is no final consensus on the relative importance of these two routes for the upper limb’s volume transport and thermohaline properties. In this study we revisited the AC and DP contributions by performing Lagrangian analyzes between the two source regions and the North Brazil Current (NBC) at 6°S in a realistically forced high-resolution (1/20°) ocean model. Our results agree with the prevailing conception that the AC contribution is the major source for the upper limb transport of the AMOC. However, they also suggest a non-negligible DP contribution of at least 40%, which is substantially higher than estimates from previous Lagrangian studies with coarser resolution models, but now better matches estimates from Lagrangian observations. Moreover, idealized analyzes of decadal changes in the DP and AC contributions indicate that the ongoing increase in Agulhas leakage indeed may have evoked an increase in the AC contribution to the upper limb of the AMOC while the DP contribution decreased. In terms of thermohaline properties, our study highlights that the AC and DP contributions cannot be unambiguously distinguished by their temperature, as the commonly adopted terminology may imply, but rather by their salinity when entering the South Atlantic. During their transit towards the NBC the bulk of DP waters experiences a net density loss through a net warming, whereas the bulk of AC waters experiences a slight net density gain through a net increase in salinity. Notably, these density changes are nearly completely captured by those Lagrangian particle trajectories that reach the surface mixed layer at least once during their transit, which amount to 66% and 49% for DP and AC waters, respectively. This implies that more than half of the water masses supplying the upper limb of the AMOC are actually formed within the South Atlantic, and do not get their characteristic properties in the Pacific and Indian Oceans.

Siren Rühs et al.
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Short summary
We revisit the sources for the upper limb of the overturning circulation in the South Atlantic by tracking fluid particles in a high-resolution ocean model. Our results suggest that the upper limb’s transport is dominantly supplied by waters with Indian Ocean origin, but the contribution of waters with Pacific origin is substantially larger than previously estimated with coarse-resolution models. Yet, a large part of upper limb waters get their thermohaline properties within the South Atlantic.
We revisit the sources for the upper limb of the overturning circulation in the South Atlantic...
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