Effects of Adding Forced Near-Inertial Motion to a Wind-Driven Channel Flow

Abstract

An eddy-resolving primitive equation ocean model is used to examine energy transfers between frequency bands. Steady wind forcing is used to drive a geostrophic channel to which high-frequency winds are added. This excites near-inertial motion, which exerts a Reynolds stress on the slowly varying flow and acts to transfer kinetic energy between low and high frequencies. These transfers extract balanced energy primarily from the mesoscale. A frequency analysis of the transfers shows the bulk of the energy to be extracted from an intermediate range of frequencies that are large relative to the dominant kinetic energy–containing frequencies and small relative to the spectral gap separating high- and low-frequency bands. This phenomenon is robust and is found in systems spanning two orders of magnitude of kinetic energy. Direct calculation of potential energy transfers proved more difficult but nonetheless shows a similar low-to-high frequency transfer. For the parameter range considered, the ratio of potential-to-kinetic energy transfers is slightly larger than unity, and as such is consistent with balanced energy being extracted from horizontal scales that are somewhat larger than the relevant deformation radius.