Sevellec F., Huck T., Ben Jelloul M., Vialard Jérôme. (2009). Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations. Journal of Physical Oceanography, 39 (4), p. 852-872. ISSN 0022-3670.
Titre du document
Nonnormal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations
Sevellec F., Huck T., Ben Jelloul M., Vialard Jérôme
Source
Journal of Physical Oceanography, 2009,
39 (4), p. 852-872 ISSN 0022-3670
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 overturning 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 +2.25 Sv (1 Sv equivalent to 10(6) m(3) 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.
