373 / 2024-09-15 15:55:44

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

Turbulent mixing; Internal waves; Velocity jets; Yangtze River Estuary

ABSTRACT: Off the Yangtze River Estuary (YRE), a submerged canyon stretches southeast, offering a critical slope for the formation of internal tides and waves. However, the dynamics of internal waves and how they affect turbulent mixing in the YRE are intricate and poorly understood. The presence of M₂ internal tides is shown by the mooring observation during the 2016 flood season on the landward slope of the submerged canyon. Isopycnal bulges in the middle water layer enclosed the center of the velocity jets during peak ebb tides, featuring a convex mode-2 internal wave. Beneath the jet core, intense turbulence dissipation with rates of O (10^-3) W kg^-1 was observed. The mode-2 internal waves with amplitude of 2.7 m and buoyance frequency of 10^-2 s^-2 can enhance the velocity by up to 0.5 m s^-1 according to theoretical estimation. This enhanced velocity matches the observed jet velocity, providing further support for the existence of mode-2 internal waves. The jet further promoted shear instability beneath it, leading to intense turbulent dissipation. During ebb tides, the nonlinear lee waves were produced locally by the lee waves generation mechanism, as indicated by the supercritical lee wave Froude number (Fr) and supercritical mode-2 Fr of the bottom layer. Given that the algae bloom and hypoxia frequently occur near the submerged canyon, the mode-2 lee waves-induced turbulent mixing may be critical in the YRE biochemistry processes, as it greatly facilitates vertical transport of nutrient-rich bottom waters to the sunlit surface layers.