Chaotic variability of the meridional overturning circulation on subannual to interannual timescales
Chaotic variability of the meridional overturning circulation on subannual to interannual timescales
Observations and numerical simulations have shown that the meridional overturning circulation (MOC) exhibits substantial variability on sub- to interannual timescales. This variability is not fully understood. In particular it is not known what fraction of the MOC variability is caused by processes such as mesoscale ocean eddies and waves which are ubiquitous in the ocean. Here we analyse twin experiments performed with a global ocean model at eddying (1/4°) and non-eddying (1°) resolutions. The twin experiments are forced with the same surface fluxes for the 1958 to 2001 period but start from different initial conditions. Our results show that on subannual to interannual timescales a large fraction of MOC variability directly reflects variability in the surface forcing. Nevertheless, in the eddy-permitting case there is an initial-condition-dependent MOC variability (hereinafter referred to as "chaotic" variability) of several Sv (1Sv = 106 m3 s?1) in the Atlantic and the Indo-Pacific. In the Atlantic the chaotic MOC variability represents up to 30% of the total variability at the depths where the maximum MOC occurs. In comparison the chaotic MOC variability is only 5–10% in the non-eddying case. The surface forcing being almost identical in the twin experiments suggests that mesoscale ocean eddies are the most likely cause for the increased chaotic MOC variability in the eddying case. The exact formation time of eddies is determined by the initial conditions which are different in the two model passes, and as a consequence the mesoscale eddy field is decorrelated in the twin experiments. In regions where eddy activity is high in the eddy-permitting model, the correlation of sea surface height variability in the twin runs is close to zero. In the non-eddying case in contrast, we find high correlations (0.9 or higher) over most regions. Looking at the sub- and interannual MOC components separately reveals that most of the chaotic MOC variability is found on subannual timescales for the eddy-permitting model. On interannual timescales the amplitude of the chaotic MOC variability is much smaller and the amplitudes are comparable for both the eddy-permitting and non-eddy-permitting model resolutions. Whereas the chaotic MOC variability on interannual timescales only accounts for a small fraction of the total chaotic MOC variability in the eddy-permitting case, it is the main contributor to the chaotic variability in the non-eddying case away from the Equator.
805-823
Hirschi, J.J.-M.
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Blaker, A.T.
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Sinha, B.
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Coward, A.
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de Cuevas, B.
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Alderson, S.
00ee9859-a11f-4040-a963-d88d4ae2740d
Madec, G.
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9 September 2013
Hirschi, J.J.-M.
c8a45006-a6e3-4319-b5f5-648e8ef98906
Blaker, A.T.
94efe8b2-c744-4e90-87d7-db19ffa41200
Sinha, B.
544b5a07-3d74-464b-9470-a68c69bd722e
Coward, A.
53b78140-2e65-476a-b287-e8384a65224b
de Cuevas, B.
01cc697c-2832-4de6-87bf-bf9f16c1f906
Alderson, S.
00ee9859-a11f-4040-a963-d88d4ae2740d
Madec, G.
7e2ec04b-896a-4861-b2d0-b74f39d748c2
Hirschi, J.J.-M., Blaker, A.T., Sinha, B., Coward, A., de Cuevas, B., Alderson, S. and Madec, G.
(2013)
Chaotic variability of the meridional overturning circulation on subannual to interannual timescales.
Ocean Science, 9 (5), .
(doi:10.5194/os-9-805-2013).
Abstract
Observations and numerical simulations have shown that the meridional overturning circulation (MOC) exhibits substantial variability on sub- to interannual timescales. This variability is not fully understood. In particular it is not known what fraction of the MOC variability is caused by processes such as mesoscale ocean eddies and waves which are ubiquitous in the ocean. Here we analyse twin experiments performed with a global ocean model at eddying (1/4°) and non-eddying (1°) resolutions. The twin experiments are forced with the same surface fluxes for the 1958 to 2001 period but start from different initial conditions. Our results show that on subannual to interannual timescales a large fraction of MOC variability directly reflects variability in the surface forcing. Nevertheless, in the eddy-permitting case there is an initial-condition-dependent MOC variability (hereinafter referred to as "chaotic" variability) of several Sv (1Sv = 106 m3 s?1) in the Atlantic and the Indo-Pacific. In the Atlantic the chaotic MOC variability represents up to 30% of the total variability at the depths where the maximum MOC occurs. In comparison the chaotic MOC variability is only 5–10% in the non-eddying case. The surface forcing being almost identical in the twin experiments suggests that mesoscale ocean eddies are the most likely cause for the increased chaotic MOC variability in the eddying case. The exact formation time of eddies is determined by the initial conditions which are different in the two model passes, and as a consequence the mesoscale eddy field is decorrelated in the twin experiments. In regions where eddy activity is high in the eddy-permitting model, the correlation of sea surface height variability in the twin runs is close to zero. In the non-eddying case in contrast, we find high correlations (0.9 or higher) over most regions. Looking at the sub- and interannual MOC components separately reveals that most of the chaotic MOC variability is found on subannual timescales for the eddy-permitting model. On interannual timescales the amplitude of the chaotic MOC variability is much smaller and the amplitudes are comparable for both the eddy-permitting and non-eddy-permitting model resolutions. Whereas the chaotic MOC variability on interannual timescales only accounts for a small fraction of the total chaotic MOC variability in the eddy-permitting case, it is the main contributor to the chaotic variability in the non-eddying case away from the Equator.
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Published date: 9 September 2013
Organisations:
Marine Systems Modelling
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Local EPrints ID: 346906
URI: http://eprints.soton.ac.uk/id/eprint/346906
ISSN: 1812-0792
PURE UUID: a541b8ee-3909-43b5-a9ec-017c8f9cdf64
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Date deposited: 11 Jan 2013 14:08
Last modified: 14 Mar 2024 12:43
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Author:
J.J.-M. Hirschi
Author:
A.T. Blaker
Author:
B. Sinha
Author:
A. Coward
Author:
B. de Cuevas
Author:
S. Alderson
Author:
G. Madec
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