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Coherent circulation changes in the deep North Atlantic from 16N and 26N transport arrays

Coherent circulation changes in the deep North Atlantic from 16N and 26N transport arrays
Coherent circulation changes in the deep North Atlantic from 16N and 26N transport arrays
The meridional overturning circulation (MOC) has been measured by boundary arrays in the Atlantic since 2000. Over the past decade of measurements, however, the reported tendencies in overturning circulation strength have differed between 16N and 26N. Here, we investigate these differences by diagnosing their origin in the observed hydrography, finding that both arrays show deep waters (below 1100 dbar) at the western boundary becoming fresher and less dense. The associated change in geopotential thickness is about 0.15 m2/s2 between 2004-2009 and 2010-2014, with the shift occurring between 2009- 2010 and earlier at 26N than 16N. In the absence of a similar density change on the east of the Atlantic, this mid-depth reduction in water density at the west would drive an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at 26N and 3.9 Sv at 16N. These transport anomalies result in an intensifying tendency in the MOC estimate at 16N, but at 26N, the method of correcting the geostrophic reference level results in an opposing (reducing) tendency of the MOC. The results indicate that both arrays are observing coherent, low frequency changes, but that there remain discrepancies in the methods of addressing the geostrophic reference level for boundary arrays measuring ocean circulation.
2169-9275
3427-3443
Frajka-Williams, E.
da86044e-0f68-4cc9-8f60-7fdbc4dc19cb
Lankhorst, M.
4e7b4983-e77f-4108-9275-229639d4355d
Koelling, J.
ea52057d-7efa-43d0-9b66-5cedb435fa2a
Send, U.
6e900fbe-b504-4e74-bda7-9e80e178d33a
Frajka-Williams, E.
da86044e-0f68-4cc9-8f60-7fdbc4dc19cb
Lankhorst, M.
4e7b4983-e77f-4108-9275-229639d4355d
Koelling, J.
ea52057d-7efa-43d0-9b66-5cedb435fa2a
Send, U.
6e900fbe-b504-4e74-bda7-9e80e178d33a

Frajka-Williams, E., Lankhorst, M., Koelling, J. and Send, U. (2018) Coherent circulation changes in the deep North Atlantic from 16N and 26N transport arrays. Journal of Geophysical Research: Oceans, 123 (5), 3427-3443. (doi:10.1029/2018JC013949).

Record type: Article

Abstract

The meridional overturning circulation (MOC) has been measured by boundary arrays in the Atlantic since 2000. Over the past decade of measurements, however, the reported tendencies in overturning circulation strength have differed between 16N and 26N. Here, we investigate these differences by diagnosing their origin in the observed hydrography, finding that both arrays show deep waters (below 1100 dbar) at the western boundary becoming fresher and less dense. The associated change in geopotential thickness is about 0.15 m2/s2 between 2004-2009 and 2010-2014, with the shift occurring between 2009- 2010 and earlier at 26N than 16N. In the absence of a similar density change on the east of the Atlantic, this mid-depth reduction in water density at the west would drive an increase in the shear between the upper and lower layers of North Atlantic Deep Water of about 2.6 Sv at 26N and 3.9 Sv at 16N. These transport anomalies result in an intensifying tendency in the MOC estimate at 16N, but at 26N, the method of correcting the geostrophic reference level results in an opposing (reducing) tendency of the MOC. The results indicate that both arrays are observing coherent, low frequency changes, but that there remain discrepancies in the methods of addressing the geostrophic reference level for boundary arrays measuring ocean circulation.

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Accepted/In Press date: 11 April 2018
e-pub ahead of print date: 19 April 2018
Published date: May 2018

Identifiers

Local EPrints ID: 419994
URI: http://eprints.soton.ac.uk/id/eprint/419994
ISSN: 2169-9275
PURE UUID: 47ac6883-46cf-4ec7-8c7c-6519b2356ece
ORCID for E. Frajka-Williams: ORCID iD orcid.org/0000-0001-8773-7838

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Date deposited: 25 Apr 2018 16:30
Last modified: 18 Feb 2021 17:14

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