Simulations of a Line W-based observing system for the Atlantic meridional overturning circulation
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In a series of observing system simulations, we test whether the Atlantic meridional overturning circulation (AMOC) can be observed based on the existing Line W deep western boundary array. We simulate a Line W array, which is extended to the surface and to the east to cover the basin to the Bermuda Rise. In the analyzed ocean circulation model ORCA025, such an extended Line W array captures the main characteristics of the western boundary current. Potential trans-basin observing systems for the AMOC are tested by combining the extended Line W array with a mid-ocean transport estimate obtained from thermal wind “measurements” and Ekman transport to the total AMOC (similarly to Hirschi et al., Geophys Res Lett 30(7):1413, 2003). First, we close Line W zonally supplementing the western boundary array with several “moorings” in the basin (Line W-32°N). Second, we supplement the western boundary array with a combination of observations at Bermuda and the eastern part of the RAPID array at 26°N (Line W-B-RAPID). Both, a small number of density profiles across the basin and also only sampling the eastern and western boundary, capture the variability of the AMOC at Line W-32°N and Line W-B-RAPID. In the analyzed model, the AMOC variability at both Line W-32°N and Line W-B-RAPID is dominated by the western boundary current variability. Away from the western boundary, the mid-ocean transport (east of Bermuda) shows no significant relation between the two Line W-based sections and 26°N. Hence, a Line W-based AMOC estimate could yield an estimate of the meridional transport that is independent of the 26°N RAPID estimate. The model-based observing system simulations presented here provide support for the use of Line W as a cornerstone for a trans-basin AMOC observing system.
KeywordsAtlantic meridional overturning circulation Observing system simulations Line W RAPID
We thank John Toole for providing information about Line W and valuable scientific discussions. We thank both reviewers for their constructive and valuable comments which helped to significantly improve the manuscript. This work was supported by the Cooperative Project “RACE - Regional Atlantic Circulation and Global Change” funded by the German Federal Ministry for Education and Research (BMBF), 03F0651A, and by the CliSAP Cluster of Excellence at the University of Hamburg funded through the Deutsche Forschungsgemeinschaft (M.F. and J.B.). The research leading to the presented results has received funding from the European Union’s Seventh Framework Programme (FP7/2007-2013) under grant agreement no 212643 (EU-THOR) (A.B., E.B). The model integration was performed at the North-German Supercomputing Alliance (HLRN). Data from the RAPID-WATCH MOC monitoring project are funded by the Natural Environment Research Council and are freely available from http://www.noc.soton.ac.uk/rapidmoc.
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