Observations of Intense Turbulent Mixing by Unsteady Mode-2 Internal Lee Waves off the Yangtze River Estuary

Abstract

Under suitable dynamic conditions, mode-2 internal waves (IWs) may arise when the stratified water of a river plume region interacts with steep topography. However, the dynamics and effect of mode-2 IWs in river plume systems remain largely unexplored. In this study, the presence of mode-2 IWs was confirmed by observations during the 2016 flood season on the landward slope of the submerged canyon off the Yangtze River Estuary (YRE). The isopycnal bulges in the mid-depth layer enclose the center of the velocity jet during peak ebb tides, featuring a convex mode-2 internal wave. Beneath the jet core, intense turbulence dissipation rates on the order of O (10⁻⁴) W kg⁻¹ were observed. Mode-2 IWs with an amplitude of ∼3.0 m and a buoyancy frequency of O (10⁻²) s⁻² can theoretically increase the current velocity by ∼0.5 m s⁻¹. The estimated velocity increase aligns with the observations, confirming the development of mode-2 IWs. The current jet promoted shear instability beneath it, causing intense turbulent dissipation. The interaction between the stratified current and the supercritical slope off the YRE forms internal lee waves. The cross-shore ebb current can be supercritical relative to mode-2 IWs, thereby forming mode-2 internal lee waves. The background shear in the river plume can amplify the velocity jet of mode-2 IWs, indicating its effect on turbulent mixing. Our findings reveal that in river plume systems characterized by supercritical topography and pronounced barotropic tides, mode-2 internal waves may frequently develop and increase turbulent mixing.