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A decomposition of the Atlantic Meridional Overturning

A decomposition of the Atlantic Meridional Overturning
A decomposition of the Atlantic Meridional Overturning
A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure-dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25 degrees S and 50 degrees N. A stronger dependency on the external mode and Ekman components between 8 degrees and 35 degrees N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.
0022-3670
2253-2270
Sime, Louise C.
27e4dd9a-2b54-4f4c-b345-31fff4a2f2b6
Stevens, David P.
80cd1121-2231-443b-a5e2-32235739fca0
Heywood, Karen J.
83d91436-76bc-4d55-ae41-9af6a6fc8869
Oliver, Kevin I.C.
588b11c6-4d0c-4c59-94e2-255688474987
Sime, Louise C.
27e4dd9a-2b54-4f4c-b345-31fff4a2f2b6
Stevens, David P.
80cd1121-2231-443b-a5e2-32235739fca0
Heywood, Karen J.
83d91436-76bc-4d55-ae41-9af6a6fc8869
Oliver, Kevin I.C.
588b11c6-4d0c-4c59-94e2-255688474987

Sime, Louise C., Stevens, David P., Heywood, Karen J. and Oliver, Kevin I.C. (2006) A decomposition of the Atlantic Meridional Overturning. Journal of Physical Oceanography, 36 (12), 2253-2270. (doi:10.1175/JPO2974.1).

Record type: Article

Abstract

A decomposition of meridional overturning circulation (MOC) cells into geostrophic vertical shears, Ekman, and bottom pressure-dependent (or external mode) circulation components is presented. The decomposition requires the following information: 1) a density profile wherever bathymetry changes to construct the vertical shears component, 2) the zonal-mean zonal wind stress for the Ekman component, and 3) the mean depth-independent velocity information over each isobath to construct the external mode. The decomposition is applied to the third-generation Hadley Centre Coupled Ocean-Atmosphere General Circulation Model (HadCM3) to determine the meridional variability of these individual components within the Atlantic Ocean. The external mode component is shown to be extremely important where western boundary currents impinge on topography, and also in the area of the overflows. The Sverdrup balance explains the shape of the external mode MOC component to first order, but the time variability of the external mode exhibits only a very weak dependence on the wind stress curl. Thus, the Sverdrup balance cannot be used to determine the external mode changes when examining temporal change in the MOC. The vertical shears component allows the time-mean and the time-variable upper North Atlantic MOC cell to be deduced at 25 degrees S and 50 degrees N. A stronger dependency on the external mode and Ekman components between 8 degrees and 35 degrees N and in the regions of the overflows means that hydrographic sections need to be supplemented by bottom pressure and wind stress information at these latitudes. At the decadal time scale, variability in Ekman transport is less important than that in geostrophic shears. In the Southern Hemisphere the vertical shears component is dominant at all time scales, suggesting that hydrographic sections alone may be suitable for deducing change in the MOC at these latitudes.

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Published date: December 2006

Identifiers

Local EPrints ID: 68800
URI: http://eprints.soton.ac.uk/id/eprint/68800
ISSN: 0022-3670
PURE UUID: a5c49352-c57c-4a90-a1b5-80c32788c39b

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Date deposited: 05 Oct 2009
Last modified: 13 Mar 2024 19:06

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Contributors

Author: Louise C. Sime
Author: David P. Stevens
Author: Karen J. Heywood

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