Optimal surface salinity perturbations influencing the thermohaline circulation

Florian Sévellec, Mahdi Ben Jelloul and Thierry Huck

Optimal surface salinity perturbations influencing the meridional overturning circulation maximum are exhibited and interpreted on a stable steady state of a 2D latitude-depth ocean thermohaline circulation model. In spite of the stability of the steady state the nonnormality of the dynamics is able to create some transient growth and variability through stimulation by optimal perturbations. Two different measures are compared to obtain the optimum, one associated with the departure from steady state in terms of density, and the other with the overturning circulation intensity. It is found that such optimal analysis is measure-dependent, hence the latter measure is chosen for studying the following physical mechanisms. The response to the optimal initial sea surface salinity perturbation involves a transient growth mechanism leading to a maximum modification of the circulation intensity after 67 yr: the amplification is linked to the most weakly damped linear eigenmode oscillating on a 150 yr period. Optimal constant surface salinity flux perturbations are also obtained, and confirm that a decrease in the freshwater flux amplitude enhances the circulation intensity. At last, looking for the optimal stochastic surface salinity flux perturbation, it is established that the variance of the circulation intensity is controlled by the weakly damped 150 yr oscillation. Two approaches are tested to consider extending such studies in more realistic 3D models: explicit solutions (vs. eigenvalue problems) are found for the overturning circulation measure (except for the stochastic optimal); a truncation method on a few leading eigenmodes usually provides the optimal perturbations for analyses on long time scales.