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Ocean Science An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/os-2017-87
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
09 Nov 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Ocean Science (OS).
Atlantic meridional overturning circulation at 14.5° N and 24.5° N during 1989/1992 and 2013/2015: volume, heat and freshwater fluxes
Yao Fu1, Johannes Karstensen1, and Peter Brandt1,2 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
2Christian-Albrechts-Universität zu Kiel, Kiel, Germany
Abstract. The Atlantic meridional overturning circulation (AMOC) is analyzed using hydrographic data from trans-Atlantic sections along 14.5° N, occupied in 1989 and 2013, and along 24.5° N, occupied in 1992 and 2015. Comparison between the periods shows that the Antarctic Intermediate Water (AAIW) became warmer and saltier at 14.5° N, and the density of the densest Antarctic Bottom Water decreased at both sections. By applying a box inverse model, the absolute meridional velocity across the sections and dianeutral velocity across neutral surfaces were determined. Corresponding to the warming and salinification of the AAIW at 14.5° N, the intermediate layer transport was also considerably weaker in 2013 than in 1989. The AMOC was generally stronger during 1989/1992 than during 2013/2015 (18.6 ± 2.7 vs. 14.7 ± 3.9 Sv at 14.5° N, and 19.2 ± 1.7 vs. 16.9 ± 1.6 Sv at 24.5° N, respectively). The inverse solution suggests that the transport changes are caused by reduction in the northward thermocline and intermediate water transport, which is balanced by decrease in the southward upper North Atlantic Deep Water transport at both sections. The AMOC strength of the inverse solution agrees well with that of dynamically consistent and data-constrained ocean state estimate GECCO2 (15.8 ± 3.4 Sv at 14.5° N, and 17.7 ± 3.6 Sv at 24.5° N) and derived by the RAPID array data (16.9 ± 4.4 Sv), but is generally smaller than that derived by the MOVE array data (24.1 ± 4.1 Sv). Instead of any long-term trend, the GECCO2 shows strong seasonal to interannual variability of the AMOC at both latitudes, which may explain the observed changes of the AMOC in the box inverse model. Sensitivity tests of the inverse solution suggest that the overturning structure and heat flux across the 14.5° N section are sensitive to the Ekman transport, while freshwater flux is sensitive to the transport-weighted salinity at the western boundary.

Citation: Fu, Y., Karstensen, J., and Brandt, P.: Atlantic meridional overturning circulation at 14.5° N and 24.5° N during 1989/1992 and 2013/2015: volume, heat and freshwater fluxes, Ocean Sci. Discuss., https://doi.org/10.5194/os-2017-87, in review, 2017.
Yao Fu et al.
Yao Fu et al.
Yao Fu et al.

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Short summary
Analysis of hydrographic data in the Atlantic along 14.5° N and along 24.5° N shows that between the periods of 1989/92 and 2013/15, the Antarctic Intermediate Water became warmer and saltier at 14.5° N, and that the Antarctic Bottom Water became lighter at both latitudes. By applying a box inverse model, the Atlantic Meridional Overturning Circulation (AMOC) was determined. The AMOC was weaker during 2013/15 than during 1989/92 in the inverse solution, but likely due to the AMOC variability.
Analysis of hydrographic data in the Atlantic along 14.5° N and along 24.5° N shows that between...
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