Observations of Turbulence Caused by a Combination of Tides and Mean Baroclinic Flow over a Fjord Sill

Abstract

This study investigates the dissipation rates and flow conditions at the Drøbak Sill in the Oslofjord. The area was transected 13 times with a free-falling microstructure shear probe during 4 days in June 2011. At the same time, an ADCP was deployed inside the sill. During most tidal cycles, internal hydraulic jumps with high dissipation rates were found on the downstream side of the sill. However, the internal response varied strongly between different tidal cycles with similar barotropic forcing. In the beginning of the observational period, ebb tides had no hydraulic jumps, and in the end one of the flood tides did not have a hydraulic jump. During the same period, the mean baroclinic exchange flow changed from inflow to outflow in the bottom layer. The authors conclude that the conditions at the sill are on the edge of forming hydraulic jumps and that the mean baroclinic exchange may push the flow above or below the limit of a hydraulic jump depending on the situation. This conclusion is supported by two-layer hydraulic theory. The volume-integrated dissipation rates within 500 m from the sill crest compare well with estimates of energy loss in the lower layer calculated from the Bernoulli drop under the assumption of no energy loss in the upper layer. Finally, the mean dissipation rate at the sill was compared with the radiation of internal tidal energy away from the sill, and it was found that about 60%-90% of the total energy loss was dissipated locally.

Turbulent Mixing in a Deep Fracture Zone on the Mid-Atlantic Ridge

“…Such a conversion is partially represented by breaking internal waves, which can redistribute some energy of the barotropic tides and as such potentially affect the overflow, despite the absence of fortnightly modulation in the subinertial circulation. Over different shallower sills, the local energy dissipation within the interior and at the bottom boundary can compare with the Bernoulli drop (Nash and Moum 2001) and, along with the internal wave energy radiated, can significantly (Arneborg and Liljebladh 2009;Staalstrøm et al 2015) or moderately (Klymak and Gregg 2004) contribute to the energy loss of barotropic tide. Despite the unknown barotropic energy loss within the fracture zone in deep water, we establish both the energy loss at the sill (with a focus on the lower layer) and the energy radiated by internal tides.…”

“…Overflow processes known to be associated with high levels of turbulence include shear instabilities along the interface between the fast-flowing part of the overflow and the overlying water column as well as hydraulic jumps (Wesson and Gregg 1994). Additionally over shallower sills, the tidal modulation of internal hydraulic control was recognized in fjords (Farmer and Smith 1980;Staalstrøm et al 2015) and in interbasin passages (Wesson and Gregg 1994); some of the barotropic tidal energy can radiate away as internal waves (Klymak and Gregg 2004) and affect the far-field turbulence in the ocean. Internal wave trapping (Gordon and Marshall 1976) can further intensify canyon mixing (Kunze et al 2002;Carter and Gregg 2002) in the BBTRE canyon, where near-inertial waves are known to modulate the shear variance .…”

Internal hydraulic control in the Little Belt, Denmark – observations of flow configurations and water mass formation

“…This is possibly due to the relatively long, narrow part of the Little Belt in which the flow may stay supercritical for a long distance. In comparison, most of the other examples of observations of hydraulic control known to us are concerned with a steep, relatively short sill with a dense, supercritical overflow spilling down the lee side of the sill (Farmer and Smith, 1980;Farmer and Denton, 1985;Farmer and Armi, 1999;Nash and Moum, 2001;Inall et al, 2004;Klymak and Gregg, 2004;Staalstrøm et al, 2015). In these examples mixing is confined to a relatively limited area downstream of the sill.…”

“…Under conditions that are not fully ideal the location of the point of control has been shown to be influenced by, for example, bottom friction and entrainment (Pratt, 1986;Gerdes et al, 2002;Nielsen et al, 2004). Existing observations of hydraulic control in a twolayered system (or with respect to the first internal mode) in a strait include Farmer and Smith (1980), Farmer and Denton (1985), Farmer and Armi (1999), Nielsen (2001), Inall et al (2004), Klymak and Gregg (2004), and Staalstrøm et al (2015). Other observations of control with respect to the first internal mode were made by Nash and Moum (2001), who studied the stratified flow over an isolated bank on the continental shelf.…”

“…An important aspect of the Little Belt is that the topography is subject to relatively smooth variations in terms of both breadth and depth, in contrast to most of the other situations in nature in which internal hydraulic control has been observed directly (Farmer and Smith, 1980;Farmer and Denton, 1985;Farmer and Armi, 1999;Nash and Moum, 2001;Inall et al, 2004;Klymak and Gregg, 2004;Staalstrøm et al, 2015). This implies that the Little Belt may be compared with and understood in terms of some simplified, existing theoretical and laboratory studies, such as Armi (1986) and Lawrence (1993).…”