Non-normal dynamics in an idealized ocean GCM

An idealized configuration of the MIT-General Circulation Model (MITgcm) is used to investigate the non-normal dynamics of the Atlantic ocean circulation. In a double-hemisphere ocean basin, we find that appropriate initial conditions of sea surface height, temperature, salinity, zonal and meridional velocities can lead to an amplification by two orders of magnitude of the total integrated perturbation-energy (potential, internal and kinetic) after 5 years. The model is in a stable regime, and therefore this perturbation eventually decays. The transient growth found is evidence for a significant non-normality of the stable linearized dynamical operator. The optimal initial conditions leading to the transient amplification of the total perturbation-energy of the ocean are obtained by solving a generalized eigenvalue problem. The evaluation of the optimals is achieved by using the tangent linear and adjoint models of MITgcm as well the ARPACK software aimed to solve large scale eigenvalue problems. Our results suggest that transient amplification of ocean thermohaline and wind driven circulation perturbations due to stochastic forcing may be an efficient way to create large scale ocean variability. The impact of such perturbations on the model heat and volume transport via transient growth is analyzed.