[A-OS18] Physical Oceanography (General)

[AOS18-01] Water column observation of Kelvin-Helmholtz instability in an offshore and coastal area, and comparisons with three-dimensional numerical simulations

*Ryoichi Horinouchi ^1,2, Shinya Okino³, John Hughes Clarke², Ryosuke Nagasawa¹ (1.Japan Coast Guard, 2.University of New Hampshire, 3.Kyoto University)

Keywords:Kelvin-Helmholtz instability, Multibeam echo sounder, Water column data, Three-dimensional numerical simulation

Kelvin-Helmholtz instability is one of instability phenomena caused by shear and difference in density in a fluid, characterized by a series of vortices at the fluid interface. Underwater KH instability can induce significant water depth errors through ray tracing process of bathymetric data, but also significantly contribute to vertical mixing in the water. Therefore, investigating behaviors of underwater KH instability in detail is very beneficial in a wide range of fields, including hydrography and oceanography, etc. Regarding previous studies, in the coastal areas of shallow seas with a water depth of less than 100 m, KH instability caused by shear and differences in density with river inflow freshwater has sometimes been observed (Tu et al., 2020, etc.), while only a few previous studies using single beam echo sounders are known (Chang et al., 2016, etc.) in the offshore areas of mid-shallow seas with a water depth of several hundred meters, because of the limitation of observational method. In this presentation, we introduce two cases of KH-instability-like acoustic anomalies in the water column data (WCD) of a multibeam echo sounder (MBES) in a coastal and offshore area. In particular, we report on the theoretical implications and comparisons with three-dimensional numerical simulations for the offshore area around Japan. The recent data in the coastal area of the United States will be presented as a bulletin.
The observational data in the offshore area around Japan was obtained in 2017, in an area about 30 km eastward from Fukushima prefecture, using MBES (EM710S) aboard “Takuyo”, which is a survey vessel of Hydrographic and Oceanographic Department in Japan Coast Guard. Also, water temperature profiles and current velocity profiles were obtained using expendable bathythermograph (T-6) and acoustic doppler current profiler (Ocean Surveyor 150 kHz). As for a theoretical consideration, we calculated the critical wavelength and phase speed with linear stability analysis for Rayleigh-Taylor-Kelvin-Helmholtz instability, which is a stratified system with a shear, and referring to the observational water temperature and current velocity data. We compared these to the values inferred with the pattern of acoustic anomalies observed in WCD of MBES. Furthermore, we conducted simplified three-dimensional numerical simulations of shear instability in a stratified system, referring to Smyth (1999). The observational results were visualized as three-dimensional acoustic images, across-track pi-charts, along-track vertical sections and along-track horizontal sections (Nagasawa and Horinouchi, 2023). In these data, mutually intertwining double wave patterns were seen at a depth of around 20 m above the seafloor, and we could see the wave fronts of these anomalies are almost parallel to the across-track. These patterns are not vessel-motion-induced noises, but could be due to a seawater density structure. As a theoretical implication, we calculated the critical wavelength and phase speed of KH instability from linear stability theory and estimation with WCD. From these comparisons, the acoustic anomalies observed in WCD are consistent with the linear stability theory of KH instability. In addition, KH instability was reproduced in the results of three-dimensional simulations with reference to the observational water temperature and current velocity data. The observed and calculated results are very similar in the three transections. That indicates acoustic observations using WCD of MBES are helpful in grasping three-dimensionally overviews of oceanographic phenomena. In this presentation, we will also discuss the appropriate observation settings and limitations of acoustic observations utilizing WCD of MBES, with the aim of visualizing oceanographic phenomena. Additionally, we will introduce data from the coastal area of the shallow sea near the border between the states of New Hampshire and Maine in the US, as for a recent example.