Indications for the transition of Kelvin-Helmholtz instabilities into propagating internal waves in a high turbid estuary and their effect on the stratification stability
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Internal waves (IWs) are an ubiquitous phenomenon in natural stratified fluids, including oceanic and coastal waters. Turbulence caused by their breaking can trigger vertical mixing of sediments, nutrients, and other dissolved substances. A major generation mechanism of IWs in coastal waters is the interaction of currents with topographic features. Additionally, current shear can lead to the development of Kelvin-Helmholtz instabilities, independent of the estuarine morphology. These instabilities normally form stationary rolled up vortices, which can also be imaged in echograms. In this study, we present new indications that propagating IWs in the Ems estuary arise from Kelvin-Helmholtz instabilities. The Richardson number drops below 0.25 at lutocline at the beginning of IW generation. However, the subsequently appearing lutocline undulations did not roll up, like the typical Kelvin-Helmholtz billows, but evolve into propagating and growing IWs of Holmboe type. Some of these waves were even subject to wave breaking. IW breaking occurred mainly during the second half of the ebb tides and is accompanied by vertical up-mixing of fluid mud. The turbulence and vertical mixing, caused by IW breaking, strongly decreases the local stratification by up to 60%.
The authors would like to thank the captain and crew of “RV Senckenberg” for their excellent job and inexhaustible patience during the acquisition of the datasets for this study. Jens Boczek from our institute is thanked for his support on board. Our student assistants are thanked for analyzing the water samples. Special thanks go also to Sebastian Krastel and Christian Winter for their helpful comments on the manuscript. We also thank Kilian Etter and Rachel Barrett for their language corrections. The Senckenberg Institute kindly provided the ship time.
This study was funded by the Deutsche Forschungsgemeinschaft as part of the Cluster of Excellence “The Future Ocean” and by the Federal Ministry of Education and Research in the frame of the Future Ems project (FO 03FO635A).
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