On the generation of internal waves by river plumes in subcritical initial conditions

R Mendes,J Pinto,J C B Da Silva,J M Dias,B St-Denis,J M Magalhaes,D Bourgault

doi:10.1038/s41598-021-81464-5

R Mendes, J Pinto + Show 5 more

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https://doi.org/10.1038/s41598-021-81464-5

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Abstract

Internal waves (IWs) in the ocean span across a wide range of time and spatial scales and are now acknowledged as important sources of turbulence and mixing, with the largest observations having 200 m in amplitude and vertical velocities close to 0.5 m s−1. Their origin is mostly tidal, but an increasing number of non-tidal generation mechanisms have also been observed. For instance, river plumes provide horizontally propagating density fronts, which were observed to generate IWs when transitioning from supercritical to subcritical flow. In this study, satellite imagery and autonomous underwater measurements are combined with numerical modeling to investigate IW generation from an initial subcritical density front originating at the Douro River plume (western Iberian coast). These unprecedented results may have important implications in near-shore dynamics since that suggest that rivers of moderate flow may play an important role in IW generation between fresh riverine and coastal waters.

Highlights

Internal waves (IWs) in the ocean span across a wide range of time and spatial scales and are acknowledged as important sources of turbulence and mixing, with the largest observations having 200 m in amplitude and vertical velocities close to 0.5 m s−1
Density gradients and gravity provide the baroclinic pressure gradient forces that drive internal wave motions in the ocean, whose properties comprise a wide spectrum of time and spatial scales—with periods ranging from minutes to hours and wavelengths from a few tens of meters for high-frequency internal waves (IWs) to tens of kilometers for internal tides[1]
The present study was initially motivated by satellite observations of coastal IWs manifestations off the Douro River plume, traveling seaward (Fig. 1)

Summary

Introduction

Internal waves (IWs) in the ocean span across a wide range of time and spatial scales and are acknowledged as important sources of turbulence and mixing, with the largest observations having 200 m in amplitude and vertical velocities close to 0.5 m s−1. Nonlinear IWs are interesting when considering observations in a theoretical or modeling framework since these nonlinear waveforms have the ability to retain their form over an extended time period as they propagate energy and momentum away along the background density stratification at basin s cales[3,4,5] ( referring to internal tidal waves, and valid for internal solitons in deep ocean). This remarkable coherence of form is explained by the balance between nonlinear steepening effects and nonhydrostatic dispersion. These can reveal the coherent crests and troughs of IWs either in the area of direct specular reflection of sunlight from the water surface, or by color signatures originating from different turbidity, chlorophyll concentrations or water constituents integrated in the water column as deep as the satellite effectively “sees” (e.g. da Silva et al.[12] and Kim et al.[13])

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