Structure and Generation of Turbulence at Interfaces Strained by Internal Solitary Waves Propagating Shoreward over the Continental Shelf

Abstract

Detailed observations of the structure within internal solitary waves propagating shoreward over Oregon's continental shelf reveal the evolving nature of interfaces as they become unstable and break, creating turbulent flow. A persistent feature is high acoustic backscatter beginning in the vicinity of the wave trough and continuing through its trailing edge and wake. This is demonstrated to be due to enhanced density microstructure. Increased small-scale strain ahead of the wave trough compresses select density interfaces, thereby locally increasing stratification. This is followed by a sequence of overturning, high-density microstructure, and turbulence at the interface, which is coincident with the high acoustic backscatter. The Richardson number estimated from observations is larger than 1/4, indicating that the interface is stable. However, density profiles reveal these preturbulent interfaces to be O(10 cm) thick, much thinner than can be resolved with shipboard velocity measurements. By assuming that streamlines parallel isopycnals ahead of the wave trough, a velocity profile is inferred in which the shear is sufficiently high to create explosively growing, small wavelength shear instabilities. It is argued that this is the generation mechanism for the observed turbulence and hence the persistent structure of high acoustic backscatter in these internal solitary waves.

Convectively induced shear instability in large amplitude internal solitary waves

“…4 The background stratification in Moum et al 2 is similar to that considered here but not identical. In addition the background flow in the laboratory is quiescent whereas in the field it is not.…”

“…In the two layer counterpart above however (h 3 = 0.29 m, h 2 + h 1 = 0.075 m), the flow is both shearand convectively -unstable at a non dimensional amplitude of a * = 1.59. Figure 4 (in particular) shows a compression of the isopycnals in the front half of the wave, similar to that documented in Dalziel et al 17 and Moum et al 2 The fundamental difference between the two cases is the presence of convective overturning in the two layer regime (it can be shown theoretically that convective instability is not possible in the three layer regime with a fixed upper boundary, see Fructus & Grue 8 ). It is conjectured that convective mixing in the top of the water column enhances isopycnal compression in the two layer regime.…”

“…The undisturbed background stratification in Moum et al 2 can be approximated by a stably stratified two layer fluid consisting of a homogeneous dense layer below a linearlystratified top layer (see Fig. 8 of their work).…”

“…Comparison with the field observations of Moum et al 2 is given in section V, and finally some conclusions are drawn in section VI.…”

Internal Waves and Surface Mixing Observed by CTD and Echo Sounder in the mid-eastern Yellow Sea

“…음향탐지기는 해저면에서 강하게 반사되는 음향으로부터 수심 측정에 주로 이용되지만 수층 내 밀도성층이나 부유물질 혹은 생 물들이 밀집된 층으로부터 반사/산란되는 음향도 감지할 수 있다 (Knauss, 1997). 내부파와 같이 물성구조의 시공간적 변동규모가 황해 중동부해역에서 CTD와 음향탐지기로 관측한 내부파와 표층 혼합 3 작지만 공간적으로 넓게 전파되는 현상인 경우 조사선박에 장착 되어 있는 음향탐지기를 활용하여 많은 연구결과들을 발표해 왔 다 (Haury, 1979;Farmer and Smith, 1980;Sandstrom and Elliott, 1984;Pingree and Mardell, 1985;Farmer and Armi, 1999;Moum et al, 2003) (Shin et al, 2002). 단면 B는 연구해역의 대표적인 물성구조를 보여 준다 (Fig.…”