Diapycnal Mixing and Internal Waves

Diapycnal mixing is an integral factor in the meridional overturning circulation of the ocean. The slow upwelling in the ocean interior across density surfaces requires counteracting diapycnal diffusion. Diapycnal diffusion in a stably stratified fluid requires mechanical energy and the availability of this energy might well be the controlling factor in setting the strength of the overturning circulation as, e.g., discussed by Munk and Wunsch (1998). The conventional wisdom is that Diapycnal mixing in the ocean interior is driven by intermittent patches of small scale turbulence. The turbulent patches have a vertical extent of up to a few meters and are caused by breaking internal gravity waves. Internal waves break by either shear or convective insta-bilities that are caused by chance superpositions or encounters with critical layers. Confirming this connection between oceanic internal gravity waves and diapycnal mixing motivates and leads to funding for a large part of internal wave research. By understanding the internal wave field we may understand diapycnal mixing. Diapycnal mixing can also be caused by double diffusion, but this process will not be considered here. What is the state of our understanding of the oceanic internal wave field? This was the task put before the participants of the eleventh 'Aha Huliko'a Hawaiian Winter Workshop, held from January 18 to 22, 1999, in Hono-lulu, Hawaii. The meeting came at an opportune time. experiment). The interaction of internal waves with topography had extensively been studied and the significance of internal wave induced boundary mixing had been explored. It was hoped that formerly incongruent pieces could be synthesized into a coherent and consistent picture of internal wave dynamics and internal wave induced mixing. However, the pieces still do not fit together. There was a feeling at the workshop that perhaps a break with basic underlying assumptions, a change of paradigm, is necessary. Below is the more detailed story. Much of internal wave research has been organized around the Garrett and Munk spectrum. More than twenty five years ago Garrett and Munk (1972) synthesized existing observations into a kinematic model spectrum that describes the observed distribution of internal wave energy in wavenumber and frequency space. Most of the detailed features of this spectrum were confirmed by the tri-moored internal wave experiment IWEX (Müller et al., 1978). The GM spectrum, as it is known now, describes a wave field that is horizontally isotropic (waves coming in from all horizontal directions equally) …