Turbulent Entrainment and Excitation of Internal Gravity Waves at the Base of the Surface Mixed Layer

Lumps and bumps at the base of the mixed layer associated with turbulent eddies are advected by inertial oscillations and disturb the underlying stratified ocean layer, resulting in the generation of internal gravity waves. The amplitudes and horizontal scales of the disturbances in the mixed layer base depend crucially on the source of turbulence. Large eddy simulations are used to investigate different sources of turbulence including shear, convection, Langmuir and sub-mesoscale eddies. Convection and Langmuir show some similarities in this respect. Both distribute momentum over the mixed layer leading to a sharper vertical velocity gradient at the mixed layer base compared to ‘pure’ wind-driven shear. The length scale of the turbulent eddies also determines the frequency of the generated internal gravity waves. Convection cells are usually somewhat larger than Langmuir cells but both leading typically to frequencies near but larger than N. The smaller shear driven eddies often reach the natural limit N which prevents internal gravity wave radiation. Sub-mesoscale eddies in the mixed layer disturb the base on the much larger scale of the local Rossby radius and have the potential to excite internal gravity waves close to the inertial frequency f. Based on our LES model results, we will quantify the effect on entrainment at the based of the mixed layer, and the energy transfered to the internal wave field by the different processes.