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Preprints
https://doi.org/10.5194/os-2019-134
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/os-2019-134
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 17 Jan 2020

Submitted as: research article | 17 Jan 2020

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A revised version of this preprint was accepted for the journal OS and is expected to appear here in due course.

Pending recovery in the strength of the meridional overturning circulation at 26° N

Ben I. Moat1, David A. Smeed1, Eleanor Frajka-Williams1, Damien G. Desbruyères2, Claudie Beaulieu3, William E. Johns4, Darren Rayner1, Alejandra Sanchez-Franks1, Molly O. Baringer5, Denis Volkov5, and Harry L. Bryden6 Ben I. Moat et al.
  • 1National Oceanography Centre, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, UK
  • 2Ifremer, University of Brest, CNRS, IRD, Laboratoire d’Océanographie Physique et Spatiale, IUEM, Ifremer centre de Bretagne, Plouzané, 29280, France
  • 3Ocean Sciences Department, University of California Santa Cruz, CA, USA
  • 4Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
  • 5Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, FL, USA
  • 6School of Ocean and Earth Science, University of Southampton Waterfront Campus, European Way, Southampton, SO14 3ZH, UK

Abstract. The strength of the Atlantic meridional overturning circulation (AMOC) at 26° N has now been continuously measured by the RAPID array over the period Apr 2004–Sept 2018. This record provides unique insight into the variability of the large-scale ocean circulation, previously only measured by sporadic snapshots of basin-wide transports from hydrographic sections. The continuous measurements have unveiled striking variability on timescales of days to a decade, driven largely by wind-forcing, contrasting with previous expectations about a slowly-varying, buoyancy forced large-scale ocean circulation. However, these measurements were primarily observed during a warm state of the Atlantic Multidecadal Variability (AMV) which has been steadily declining since a peak in 2008–2010. In 2013–2015, a period of strong buoyancy-forcing by the atmosphere drove intense watermass transformation in the subpolar North Atlantic and provides a unique opportunity to investigate the response of the large-scale ocean circulation to buoyancy forcing. Modelling studies suggest that the AMOC in the subtropics responds to such events with an increase in overturning transport, after a lag of 3–9 years. At 45° N, observations suggest that the AMOC my already be increasing. We have therefore examined the record of transports at 26° N to see whether the AMOC in the subtropical North Atlantic is now recovering from a previously reported low period commencing in 2009. Comparing the two latitudes, the AMOC at 26° N is higher than its previous low. Extending the record at 26° N with ocean reanalysis from GloSea5, the transport fluctuations follow those at 45° N by 0–2 years, albeit with lower magnitude. Given the short span of time and anticipated delays in the signal from the subpolar to subtropical gyres, it is not yet possible to determine whether the subtropical AMOC strength is recovering.

Ben I. Moat et al.

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Ben I. Moat et al.

Data sets

Atlantic meridional overturning circulation observed by the RAPID-MOCHA-WBTS (RAPID-Meridional Overturning Circulation and Heatflux Array-Western Boundary Time Series) array at 26N from 2004 to 2018 D. Smeed, B. Moat, D. Rayner, W. E. Johns, M. O. Baringer, D. Volkov, and E. Frajka-Williams https://doi.org/10.5285/8cd7e7bb-9a20-05d8-e053-6c86abc012c2

Ben I. Moat et al.

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Short summary
The RAPID 26° N array has been measuring the Atlantic Meridional Overturning Circulation (AMOC) since 2004. Since 2009 the AMOC has, compared with previous years, been in a low state. In 2013–2015 in the northern North Atlantic, strong cooling was observed in the ocean and anticipated to intensify the strength of the AMOC some years later. Here, we analyse the latest results from 26° N and conclude that while the AMOC is increasing since 2009, this increase is not statistically significant.
The RAPID 26° N array has been measuring the Atlantic Meridional Overturning Circulation (AMOC)...
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