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

• Published in: Journal of Physical Oceanography
• Main Authors: Sévellec, Florian, Huck, Thierry, Ben Jelloul, Mahdi, Grima, Nicolas, Vialard, Jérôme, Weaver, Anthony T.
• Other Authors: Laboratoire de physique des océans (LPO), Institut de Recherche pour le Développement (IRD)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), Laboratoire d'Océanographie et du Climat : Expérimentations et Approches Numériques (LOCEAN), Institut de Recherche pour le Développement (IRD)-Centre National de la Recherche Scientifique (CNRS)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Pierre et Marie Curie - Paris 6 (UPMC)-Muséum national d'Histoire naturelle (MNHN)-Institut Pierre-Simon-Laplace (IPSL (FR_636)), École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS)-École normale supérieure - Paris (ENS Paris), Université Paris sciences et lettres (PSL)-Université Paris sciences et lettres (PSL)-Université de Versailles Saint-Quentin-en-Yvelines (UVSQ)-Commissariat à l'énergie atomique et aux énergies alternatives (CEA)-Université Paris Diderot - Paris 7 (UPD7)-École polytechnique (X)-Centre National d'Études Spatiales Toulouse (CNES)-Sorbonne Université (SU)-Centre National de la Recherche Scientifique (CNRS), Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique (CERFACS), CERFACS
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2008
• Subjects: [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; North Atlantic
• Online Access: https://hal.archives-ouvertes.fr/hal-00400418; https://hal.archives-ouvertes.fr/hal-00400418/document; https://hal.archives-ouvertes.fr/hal-00400418/file/%5B15200485%20-%20Journal%20of%20Physical%20Oceanography%5D%20Optimal%20Surface%20Salinity%20Perturbations%20of%20the%20Meridional%20Overturning%20and%20Heat%20Transport%20in%20a%20Global%20Ocean%20General%20Circulation%20Model.pdf; https://doi.org/10.1175/2008JPO3875.1

International audience Recent observations and modeling studies have stressed the influence of surface salinity perturbations on the North Atlantic circulation over the past few decades. As a step toward the estimation of the sensitivity of the thermohaline circulation to salinity anomalies, optimal initial surface salinity perturbations are computed and described for a realistic mean state of a global ocean general circulation model [Océan Parallélisé (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 Lagrangian 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 ...