Frequency-Domain Analysis of Atmospherically Forced versus Intrinsic Ocean Surface Kinetic Energy Variability in GFDL’s CM2-O Model Hierarchy

Abstract

Low-frequency variability at the ocean surface can be excited both by atmospheric forcing, such as in exchanges of heat and momentum, and by the intrinsic nonlinear transfer of energy between mesoscale ocean eddies. Recent studies have shown that nonlinear eddy interactions can excite an energy transfer from high to low frequencies analogous to the transfer of energy from high to low wavenumbers (small to large spatial scales) in quasi-two-dimensional turbulence. As the spatial inverse cascade is driven by oceanic eddies, the process of energy exchange across frequencies may be sensitive to ocean model resolution. Here a cross-spectrum diagnostic is applied to the oceanic component in a hierarchy of fully coupled ocean–atmosphere models to address the transfer of ocean surface kinetic energy between high and low frequencies. The cross-spectral diagnostic allows for a comparison of the relative contributions of coupled atmospheric forcing through wind stress and the intrinsic advection to low-frequency ocean surface kinetic energy. Diagnostics of energy flux and transfer within the frequency domain are compared between three coupled models with ocean model horizontal resolutions of 1°, 1/4°, and 1/10° to address the importance of resolving eddies in the driving of energy to low frequencies in coupled models.