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Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations

Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations
Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations
Optimal perturbations of sea surface salinity are obtained for an idealized North Atlantic basin using a 3D planetary geostrophic model—optimality is defined with respect to the intensity of the meridional over- turning circulation. Both optimal initial and stochastic perturbations are computed in two experiments corresponding to two different formulations of the surface boundary conditions: the first experiment uses mixed boundary conditions (i.e., restoring surface temperature and prescribed freshwater flux), whereas the second experiment uses flux boundary conditions for both temperature and salinity. The latter reveals greater responses to both initial and stochastic perturbations that are related to the existence of a weakly damped oscillatory eigenmode of the Jacobian matrix, the optimal perturbations being closely related to its bi- orthogonal. The optimal initial perturbation induces a transient modification of the circulation after 24 yr. The spectral response to the optimal stochastic perturbation reveals a strong peak at 35 yr, corresponding to the period of this oscillatory eigenmode. This study provides an upper bound of the meridional overturning response at multidecadal time scales to freshwater flux perturbation: for typical amplitudes of Great Salinity Anomalies, initial perturbations can alter the circulation by 12.25 Sv (1 Sv [ 10^6 m3 s-1; i.e., 12.5% of the mean circulation) at most; stochastic perturbations with amplitudes typical of the interannual variability of the freshwater flux in midlatitudes induce a circulation variability with a standard deviation of 1 Sv (i.e., 5.5% of the mean circulation) at most.
thermohaline circulation, salinity, climate variability
0022-3670
852-872
Sévellec, Florian
01569d6c-65b0-4270-af2a-35b0a77c9140
Huck, Thierry
3de8511b-db7b-4ed1-bc27-92d2bbc7c6b9
Ben Jelloul, Mahdi
bb143e60-6a9a-4949-8a97-a0e52b79ec38
Vialard, Jérôme
165dd2c7-d5ec-4bd6-9f20-02e182e38f36
Sévellec, Florian
01569d6c-65b0-4270-af2a-35b0a77c9140
Huck, Thierry
3de8511b-db7b-4ed1-bc27-92d2bbc7c6b9
Ben Jelloul, Mahdi
bb143e60-6a9a-4949-8a97-a0e52b79ec38
Vialard, Jérôme
165dd2c7-d5ec-4bd6-9f20-02e182e38f36

Sévellec, Florian, Huck, Thierry, Ben Jelloul, Mahdi and Vialard, Jérôme (2009) Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations. Journal of Physical Oceanography, 39 (4), 852-872. (doi:10.1175/2008JPO3998.1).

Record type: Article

Abstract

Optimal perturbations of sea surface salinity are obtained for an idealized North Atlantic basin using a 3D planetary geostrophic model—optimality is defined with respect to the intensity of the meridional over- turning circulation. Both optimal initial and stochastic perturbations are computed in two experiments corresponding to two different formulations of the surface boundary conditions: the first experiment uses mixed boundary conditions (i.e., restoring surface temperature and prescribed freshwater flux), whereas the second experiment uses flux boundary conditions for both temperature and salinity. The latter reveals greater responses to both initial and stochastic perturbations that are related to the existence of a weakly damped oscillatory eigenmode of the Jacobian matrix, the optimal perturbations being closely related to its bi- orthogonal. The optimal initial perturbation induces a transient modification of the circulation after 24 yr. The spectral response to the optimal stochastic perturbation reveals a strong peak at 35 yr, corresponding to the period of this oscillatory eigenmode. This study provides an upper bound of the meridional overturning response at multidecadal time scales to freshwater flux perturbation: for typical amplitudes of Great Salinity Anomalies, initial perturbations can alter the circulation by 12.25 Sv (1 Sv [ 10^6 m3 s-1; i.e., 12.5% of the mean circulation) at most; stochastic perturbations with amplitudes typical of the interannual variability of the freshwater flux in midlatitudes induce a circulation variability with a standard deviation of 1 Sv (i.e., 5.5% of the mean circulation) at most.

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More information

Published date: April 2009
Keywords: thermohaline circulation, salinity, climate variability
Organisations: Physical Oceanography

Identifiers

Local EPrints ID: 339737
URI: https://eprints.soton.ac.uk/id/eprint/339737
ISSN: 0022-3670
PURE UUID: ade5e779-e698-47f1-aaf8-bdc4c0b32a24

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Date deposited: 29 May 2012 16:14
Last modified: 18 Jul 2017 05:52

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