Propagation And Dissipation Of Lee Wave Energy In A Single Column Model

Abstract

When an ocean current flows over uneven topography a specific kind of in-ternal gravity wave called lee wave is emitted. These lee waves are propagatingthrough the water column and can interact with the ocean currents and otherwaves. They are thought to play a role in the global ocean energy cycle andcan also affect the momentum balance in the interior. The waves extract energyfrom the mean flow or eddies near the bottom and then dissipate this energysomewhere in the water column.However, the waves can not be resolved directly in global ocean models and inparticular their vertical propagation is still largely unknown. In order to studythese waves Eden and Olbers proposed a model of the lee wave energy. In this modelthe radiative transfer equation is integrated over the wavenumber space whichyields a prognostic equation of the lee wave energy. This energy equation canthen be added to an ocean model. This model includes a term for the interac-tion with the mean flow and a dissipation term parameterizing the interactionwith the background wave field.In this work an additional term concerning the dissipation due to critical layersis added to the energy equation. The critical layers can occur when the back-ground current shift the wavelength to small scales so that the waves break. Forthis critical layer parameterization the vertical refraction term in the radiativetransfer equation is integrated.The energy equation is then added to the python ocean model (pyOM) andsimulations using a single column are conducted. The key results show that theinteraction with the background wave field typically dominates the other effects.This leads to an exponential decay of the energy away from the ocean bottom.If the waves reach a region with a vertical velocity shear the waves can also ex-tract energy and momentum from the current. The leads to a slight downwardsdeflection of the current and also enables some critical layer dissipation. Thusin these conditions the lee waves also lead to some dissipation and mixing inthe ocean interior far away from the bottom. The maximum dissipation nearthe bottom is found to be larger than 10-8m2/s3, which is in accordance withother simulations and observations.