In order to study the influence of slope on the depression-type internal solitary wave (ISW), experiments on flow field and waveform of ISW evolving over the gentle slope are conducted by using conductivity probe array and PIV technique. Experimental results show that the flow velocity distribution induced by the shoaling ISW is consistent with its waveform under the condition without turning point, meanwhile a newborn depression ISW will be generated. For the case with turning point, the back of ISW is raised and the vertical velocity is greatly enhanced, resulting in the flow field of elevation ISW.
The internal solitary wave (ISW) is a kind of special fluctuation with the large amplitude, strong induced velocity and long propagation distance in the ocean (Leon and Marek. 2019; Jackson, 2007). During the process of propagation, the ISW can contain tremendous energy and aggravate the energy cascades in the ocean. As the ISWs pass over the topography, their waveforms, amplitudes and induced flow field will have non-regular and non-steady changes, leading to ISW structure changes, sudden strong currents and instability such as waveform distortion, inversion, breaking, and energy flux (Sarkar, et al., 2017; Lamb, 2014; Cai, et al., 2012).
Numerous remote sensing and in-situ observations demonstrate that the ISW type will be change through the turning point where the depth of the upper is equal with the bottom layer one in a density stratified water column. In other words, while shoaling across a sloping shelf from deep water to shallow part, the depression-type ISW may commence inversion to elevation-type ISW. Shroyer et al. (2009) studied the ISWs polarity reversal in New Jersey coast and founded that the symmetric waves propagating into shallow water develop an asymmetric shape. The wave front face was to be broaden while the trailing face remained steep because the leading edge accelerates, and this trend continued to develop until the front face cannot be recognized and the elevation-type wave evolved from the tail of the depression-type wave. This polarity reversal of ISW could be judged by the positive and negative vorticity field, which was consistent with the sign change of nonlinear term in KdV equation. Some oceanographers studied the polarity conversion of ISW waveforms in the South China Sea after 2002. (Orr, et al. 2003, Ramp, et al. 2004, Yang YJ, et al. 2004). Moum et al. (2003, 2007) studied the ISWs propagation characteristics in the Oregon continental shelf and showed that the wavelength would be shorter during the propagation from the deep water to the shallow water, moreover, the wave instability and breaking were analyzed and pointed out that the shear instability was the generation mechanism for turbulence. Zachariah, et al. (2005) calculated the kinetic energy, potential energy and energy flux of internal waves in the New Jersey coast by using flow field data. The research showed that the kinetic energy was equal with the potential energy approximately. Alford et al. (2010) studied the ISWs propagating westward along the continental shelf from Luzon Strait, and analyzed the wave speed, wavelength, amplitude, and energy. Martini, et al. (2013) observed the shoaling process of the internal tidal waves along the slope in the Oregon continental shelf, which included steepening, crushing and turbulent mixing. Silva et al. (2015) studied the generation and propagation of internal waves on the upstream side of a large sill of the Mascarene Ridge and pointed out the relationship between the background flow field and the generation of internal waves. Xu, et al. (2016) studied the generation and propagation of ISWs in northern South China by using the numerical simulation and satellite images, and analyzed the influence of coastline on the generation and propagation of waves. Due to the high cost of field observation and the complexity of background elements, the understanding of the influence of topography on both evolution and propagation of ISW was yet very limited.