On Tidal Modulation of the Evolution of Internal Solitary-Like Waves Passing Through a Critical Point

Abstract

Internal solitary-like waves (ISWs) evolve considerably when passing through a critical point separating the deep water where ISWs are waves of depression and shallower water where they are waves of elevation. The location of the critical point is determined by the background current and stratification. In this study, we investigate the influence of tidal currents on the cross-shelf movement of the critical point and elucidate the underlying processes via fully nonlinear numerical simulations. Our simulations reveal phase-locked tidal variations of the critical point, which are mainly attributed to stratification fluctuations that are modulated by the combined effects of cross-shelf barotropic tidal currents and locally generated baroclinic tides. The barotropic tidal currents drive isopycnal displacements as they flow over the slope, and as this occurs baroclinic tides are generated, modulating the stratification and inducing sheared currents. This results in a cross-shelf movement of the critical point, which moves onshore (offshore) when the pycnocline is elevated (depressed) by the flood (ebb) tide. Our idealized numerical simulations for the study region in the South China Sea suggest that the cross-shelf movement of the critical point reaches to O(10) km within a tidal cycle. This distance depends on the strength of tidal currents, stratification, and bathymetry. Because of tidal currents, ISWs of depression may undergo a complex evolution even in a stratification with a shallow pycnocline. For the stratification with a deep pycnocline, the critical point may be at a location deep enough so that its tidal movement becomes insignificant.

Intermittent Generation of Internal Solitary‐Like Waves on the Northern Shelf of the South China Sea

“…Li et al, 2016;Xie et al, 2015). The arrival time of ISWs on the shelf can also be modified by barotropic tidal flows and internal tides generated over shelf break (Bai, Lamb, Hu, & Liu, 2021) or by submarine canyon groups (Q. Li et al, 2023).…”

“…Background currents, such as the Kuroshio intrusion (Q. Li, 2014; Park & Farmer, 2013), basin‐scale geostrophic currents and mesoscale eddies (Huang et al., 2017; Q. Li et al., 2016; Xie et al., 2021; Zhang et al., 2018), can affect the generation and propagation of ISWs from the LS, as well as their amplitude, direction and arrival times on the continental shelf (Q. Li et al., 2016; Xie et al., 2015). The arrival time of ISWs on the shelf can also be modified by barotropic tidal flows and internal tides generated over shelf break (Bai, Lamb, Hu, & Liu, 2021) or by submarine canyon groups (Q. Li et al., 2023). Even if ISWs fission into trains of higher‐frequency internal waves (Bai et al., 2019), they remain appearing as organized wave packets.…”

Estimate of energy loss from internal solitary waves breaking on slopes

“…It is important to understand the mechanisms of the transformation of ISWs at continental slopes and identify "hot spots" of wave energy dissipation. Two shoaling mechanisms can be important: (1) the conversion of ISWs of depression into elevation waves in a two-layer stratification when the thickness of the upper mixed layer is greater than one-half of the total water depth (Helfrich and Melville, 1986;Cheng et al, 2011;Bai et al, 2021); and (ii) ISW breaking on the slope that occurs when fluid velocities in the wave exceed the wave phase speed, which leads to the overturning of the rear face of the wave, shear instability, and intensive mixing. Different types of breaking are commonly distinguished by slope inclination, water column stratification, and wave characteristics.…”