Small Mountainous Rivers Generate High-Frequency Internal Waves in Coastal Ocean

High-frequency internal waves propagating offshore in small river plumes are regularly observed at satellite imagery in many world regions. In this work we describe a mechanism of generation of these internal waves by discharges of small and rapid rivers inflowing to coastal sea. Friction between river runoff at high velocity and the subjacent sea of one order of magnitude lower velocity causes abrupt deceleration of a freshened flow and increase of its depth, i.e., a hydraulic jump is formed. Transition from supercritical to subcritical flow conditions induces generation of high-frequency internal waves that propagate off a river mouth at a stratified layer between a buoyant river plume and subjacent ambient sea and influence turbulence and mixing at this layer. Basing on in situ and satellite data we estimated wavelengths, phase speeds, and frequencies of internal waves generated in small river plumes located off the northeastern coast of the Black Sea. This process is typical for many other world mountainous regions where numerous and closely spaced small and rapid rivers inflow to sea during high discharge periods and can strongly influence, first, structure and dynamics of river plumes and, second, physical, biological, and geochemical processes in adjacent coastal areas.