Optimal surface salinity perturbations of the meridional overturning and heat transport in a global ocean general circulation model

Florian Sévellec, Thierry Huck, Mahdi Ben Jelloul, Nicolas Grima, Jérôme Vialard and Anthony Weaver

Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the last decades. Optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model (OPA); optimality is defined successively with respect to the meridional overturning circulation intensity and the meridional heat transport maximum. Although the system is asymptotically stable, the nonnormality of the dynamics is able to produce a transient growth through an initial stimulation. Optimal perturbations are calculated subject to three constraints: the perturbation applies to surface salinity; the perturbation conserves the global salt content; and the perturbation is normalized, to remove the degeneracy in the linear maximization problem. Maximization using Lagrange multipliers leads to explicit solutions (rather than eigenvalue problems), involving the integration of the model adjoint for each value to maximize.
The most efficient transient growth for the intensity of the meridional overturning circulation appears for a delay of 10.5 yr after the perturbation by the optimal surface salinity anomaly. This optimal growth is induced by an initial anomaly located north of 50°N. In the same way, the most efficient transient growth for the intensity of the meridional heat transport appears for a shorter delay of 2.2 yr after the perturbation by the optimal surface salinity anomaly. This initial optimal perturbation corresponds to a zonal salinity gradient around 24°N. The optimal surface salinity perturbations studied herein yield upper bounds on the intensity of the response in meridional overturning circulation and meridional heat transport. Using typical amplitudes of the Great Salinity Anomalies, the upper bounds for the associated variability are respectively 0.8 Sv (11% of the mean circulation) and 0.03 PW (5% of the mean circulation).