Observations of advection and turbulent interfacial mizing in the Saint John River Estuary, New Brunswick, Canada

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Theoretical studies, laboratory experiments, numerical simulations and field observations suggest that turbulent interfacial mixing in stratified environments initially takes the form of internal waves. Regardless of this suggestion, the generation and development of internal waves in the field is still not well observed. The main factor that limits the observations and as a result our understanding of internal waves in the field, is the spatial and temporal resolution of observations acquired. Under highly stratified conditions oceanographic surveys are performed in Long Reach (part of the Saint John River Estuary) for the duration of a tidal cycle. Survey sensors such as an Acoustic Doppler Current Profiler (which measures current magnitude and direction), Conductivity Temperature Depth sensor (from which density is derived) and an echosounder (which indirectly measures density interfaces, turbulence, zooplankton and suspended sediments) are all implemented to map the main oceanographic processes occurring. The results show that turbulent interfacial mixing occurs at a period of maximum velocity shear, within the vicinity of the lateral and vertical constrictions of Long Reach. The survey sensors and survey design used, allow the generation and evolution processes of internal waves to be captured. Different types of internal waves are observed before, during and after the turbulent mixing process. However at the exact moment of turbulent mixing, the pycnocline dips downward into the bottom salty layer. This latter observation, suggests that there may possibly be another mechanism, other than internal waves that contribute to turbulent interfacial mixing in Long Reach.