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Article
Article Type:
Research Article

Mechanisms of Multidecadal Atlantic Meridional Overturning Circulation Variability Diagnosed in Depth versus Density Space

Young-Oh Kwon Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Claude Frankignoul Sorbonne (UPMC, University of Paris 06)-CNRS-IRD-MNHN, LOCEAN/IPSL, Paris, France

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Print Publication:
15 Dec 2014
DOI:
https://doi.org/10.1175/JCLI-D-14-00228.1
Page(s):
9359–9376
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Abstract

Multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is examined based on a comparison of the AMOC streamfunctions in depth and in density space, in a 700-yr present-day control integration of the fully coupled Community Climate System Model, version 3. The commonly used depth-coordinate AMOC primarily exhibits the variability associated with the deep equatorward transport that follows the changes in the Labrador Sea deep water formation. On the other hand, the density-based AMOC emphasizes the variability associated with the subpolar gyre circulation in the upper ocean leading to the changes in the Labrador Sea convection. Combining the two representations indicates that the ~20-yr periodicity of the AMOC variability in the first half of the simulation is primarily due to an ocean-only mode resulting from the coupling of the deep equatorward flow and the upper ocean gyre circulation near the Gulf Stream and North Atlantic Current. In addition, the density-based AMOC reveals a gradual change in the deep ocean associated with cooling and increased density, which is likely responsible for the transition of AMOC variability from strong ~20-yr oscillations to a weaker red noise–like multidecadal variability.

Corresponding author address: Dr. Young-Oh Kwon, Physical Oceanography Department, Woods Hole Oceanographic Institution, MS #21, Woods Hole, MA 02543. E-mail: yokwon@whoi.edu

Abstract

Multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is examined based on a comparison of the AMOC streamfunctions in depth and in density space, in a 700-yr present-day control integration of the fully coupled Community Climate System Model, version 3. The commonly used depth-coordinate AMOC primarily exhibits the variability associated with the deep equatorward transport that follows the changes in the Labrador Sea deep water formation. On the other hand, the density-based AMOC emphasizes the variability associated with the subpolar gyre circulation in the upper ocean leading to the changes in the Labrador Sea convection. Combining the two representations indicates that the ~20-yr periodicity of the AMOC variability in the first half of the simulation is primarily due to an ocean-only mode resulting from the coupling of the deep equatorward flow and the upper ocean gyre circulation near the Gulf Stream and North Atlantic Current. In addition, the density-based AMOC reveals a gradual change in the deep ocean associated with cooling and increased density, which is likely responsible for the transition of AMOC variability from strong ~20-yr oscillations to a weaker red noise–like multidecadal variability.

Corresponding author address: Dr. Young-Oh Kwon, Physical Oceanography Department, Woods Hole Oceanographic Institution, MS #21, Woods Hole, MA 02543. E-mail: yokwon@whoi.edu
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