Boundary Mixing

Munk (1966) noted that intense mixing near boundaries, along with an effective communication to the ocean interior, could be vital to the observed stratification. He envisaged internal wave breaking as a important source of mechanical energy for near-boundary mixing.

Armi (1978) observed $50-100 m$ thick well-mixed layers near sloping boundaries, attributed to turbulence in the bottom boundary layer. These layers can then intrude along constant density surfaces and spread into the interior ocean. The combined effect of these two processes can have an equivalent effect to the vertical eddy diffusivity in the open ocean. As noted by Munk and Wunsch (1998), the "average" mixing in the ocean could be accounted for by energetic mixing at sloping boundaries, followed by spreading of the mixed fluid (but not the turbulence) into the quieter ocean interior.

Such intrusions were later observed as an Intermediate Nepheloid Layer (INL) (McPhee-Shaw et al., 2004) in which high concentrations of suspended sediments are advected laterally into deep water from nearby continental shelves and slopes. INLs have also been observed at semidiurnal periods in regions of near-critical slope (Moum et al., 2002), suggesting that the critical reflection or generation of internal $M_2$ tides provides mechanical energy for boundary mixing. In a laboratory experiment, McPhee and Kunze (2002) observe intrusions from a sloping bottom where internal wave reflection provides mechanical energy for the generation of turbulence. They relate the intrusion's growth to the incidence angle of the internal wave, relative to the slope angle, and observe the strongest intrusions during critical reflection conditions.

To illustrate this process, we reproduced the laboratory experiment of McPhee and Kunze (2002) using a rectangular tank filled with salt-stratified water, a sloping boundary and an oscillating rough metal plate along this boundary to create turbulence (Figure 1.1). Fluorescine dye was slowly injected at the boundary to observe the intrusions into the tank interior. We first conducted the experiment with two layers of different density. Almost immediately after the turbulence started, an intrusion was formed that spread laterally at the interface between the two layers (Figure 1.2). We then extended the experiment to a more realistic ocean scenario, with a continuously stratified tank. After a few minutes, the same mechanism produced several evenly spaced intrusions spreading into the tank interior (Figure 1.1). The importance of mixing along sloping boundaries, as opposed to over a flat bottom, is that the mixed water can be exported into the ocean interior along constant density surfaces, thus enhancing mixing efficiency (Munk and Wunsch, 1998).