Non-normal multidecadal response of the thermohaline circulation induced by optimal surface salinity perturbations

Florian Sévellec, Thierry Huck, Mahdi Ben Jelloul and Jérôme Vialard

Optimal perturbations of the 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 condition: the first one uses a mixed boundary condition (i.e. restoring surface temperature and prescribed freshwater flux), whereas the second uses flux boundary condition 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, the optimal perturbations being closely related to its biorthogonal. The optimal initial perturbation induces a transient growth 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 response of the meridional overturning circulation to freshwater flux perturbation: for typical amplitude of Great Salinity Anomalies, initial perturbations can alter the circulation by 2.25 Sv at most; stochastic perturbations with amplitude typical of the inter-annual variability of the freshwater flux in mid-latitudes induce a circulation variability with a standard deviation of 1 Sv at most.