Near-Inertial Energy Propagation from the Mixed Layer: Theoretical Considerations

Abstract

Wind-generated inertial currents can radiate from the mixed layer as horizontally and vertically propagating new-inertial internal gravity waves. To study the timescale of the decay of mixed layer energy and the magnitude of the energy transfer to the ocean below, the authors developed a numerical, linear model on a β plane, using baroclinic modes to describe the velocity field. The model is unforced-wave propagation is initiated by specifying the mixed layer currents that would he generated by a moving atmospheric front. The numerical results are interpreted using concepts of modal interference and modal departure that can be evaluated analytically, thereby permitting predictions Of some features of wave field evolution without the need to run the numerical model. The energy exchange with the pycnocline and deep ocean is explored as a function of the propagation speed and direction of the front, the horizontal extent of the storm, and the background stratification. The timescale of energy transfer from the mixed layer to the pycocline due to modal interference is greatly affected by the β effect, causing much faster energy transfer for currents generated by southward propagating fronts. The timescale is typically not a strong function of mixed layer depth; however. the magnitude of the energy transfer is. Besides modal interference, vertical energy propagation occurs when low modes leave the area- a possibility for storms of finite horizontal extent. The deep stratification and f also affect the timescale; climatological examples indicate faster wave evolution at low latitudes.