Cold vs. warm water route - sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model

Siren Rühs, Franziska U. Schwarzkopf, Sabrina Speich, Arne Biastoch

Research output: Contribution to journalArticleAcademicpeer-review

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 La-grangian analyses between the two source regions and the North Brazil Current (NBC) at 6 degrees S in a realistically forced high-resolution (1/20 degrees) 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 in the tropical South Atlantic. However, they also suggest a non-negligible DP contribution of around 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 analyses of decadal changes in the DP and AC contributions indicate that the ongoing increase in Agulhas leakage indeed may have induced an increase in the AC contribution to the upper limb of the AMOC in the tropics, while the DP contribution decreased. In terms of thermohaline properties, our study highlights the fact that the AC and DP contributions cannot be unambiguously distin-guished 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 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.
Original languageEnglish
Pages (from-to)489-512
Number of pages24
JournalOcean Science
Volume15
Issue number3
DOIs
Publication statusPublished - 9 May 2019
Externally publishedYes

Funding

The OGCM and trajectory simulations were performed at the High Performance Computing Center in Hanover (HLRN). The project received funding from the following: Cluster of Excellence 80 "The Future Ocean" within the framework of the Excellence Initiative by the Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governments (grant CP1412, SR); the German Federal Ministry of Education and Research (BMBF) for the SPACES-AGULHAS project (grant 03F0750A, AB and FUS); the European Union Horizon 2020 research and innovation program as part of the AtlantOS project (grant 633211, SS); and the SAMOC project (grant 11-ANR-56004, SS). The authors further wish to thank Susan Lozier and Erik van Sebille for inspiring discussions that added value to this paper, Bruno Blanke and Nicolas Grima for realizing and helping to tackle the Lagrangian ARIANE software, Willi Rath for technical support, Jonathan Durgadoo for discussing Lagrangian analyses techniques, and two anonymous reviewers for their constructive criticism, in particular regarding the framing and discussion of our work.

FundersFunder number
Cluster of Excellence 80 "The Future Ocean" within the framework of the Excellence Initiative by the Deutsche Forschungsgemeinschaft (DFG) on behalf of the German federal and state governmentsCP1412
German Federal Ministry of Education and Research (BMBF)
SPACES-AGULHAS project03F0750A
European Union Horizon 2020 research and innovation program as part of the AtlantOS project633211, 11-ANR-56004

    Keywords

    • Western boundary circulation
    • Agulhas leakage
    • South-atlantic
    • Tropical atlantic
    • Transport variability
    • Interocean exchange
    • Heat-transport
    • North-atlantic
    • Impact
    • Shallow

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