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Abstract. This study presents Eulerian data from Winyah Bay, SC, USA collected during the passage of a tidal plume. The data captured the evolution and structure of the plume and include high-resolution velocity and temperature time series, supplemented by temperature – salinity profiles from a MicroCTD profiler. The observations identified a pre-existing plume extending to 4 m, with a water density of 1023.6 kg m−3, laying above denser ambient waters. Upon arrival, the newly discharged tidal plume introduced a fresher layer (1020.7 kg m−3) extending to 2.6 m, gradually thinning due to radial spreading. The plume's frontal propagation measured at 0.36 m s−1 with a calculated Froude number of 1.32, indicating gravity current dynamics. In the across-front direction a return flow developed under the plume that extended throughout the water column, resembling an estuarine-like circulation pattern. Mixing processes were examined using the available overturn potential energy (AOPE) in the water column as described in Smith (2020). The analysis showed that near the bed, bottom boundary layer turbulence is the main mixing mechanism both before and after the passage of the front. In the surface layer, before the arrival of the front, mixing is driven by wind-induced shear and overturning. Within the gravity current, despite the high turbulent kinetic energy dissipation rates in certain regions, shear-induced mixing was minimal. These findings were reflected in the density diffusivity estimates near the surface that varied from 10−6 m2 s−1 prior to the arrival of the front, increasing to 10−5 m2 s−1 very near the front and diminishing to 10−10 m2 s−1 within the plume despite the high velocity shear observed there. The limited mixing within the plume despite the high shear observed can be explained using the concept of a layer-interface structure, where layers of high turbulence and low stratification alternate with regions of low turbulence and higher stratification. Although not resolved by these observations, this structure has been observed in numerical simulations of mixing. The development of the counter-flow under the plume suggests that tide and/or wind induced straining may play an important role in enhancing stratification in plumes like the one studied here. 

Abstract River plumes play an essential role in the transport of terrestrially derived materials (like nutrients, sediments, pollutants, etc.) into the coastal ocean. Quantifying the cross‐shore transport in river plumes can help to better understand the contribution of river‐borne substances to marine biogeochemical cycles and to parameterize these processes in global ocean models which are usually too coarse to resolve individual rivers. It is known that besides external factors (like runoff, latitude, wind, and tides) also internal estuarine processes like salt mixing affect the exchange flow between an estuary and the coastal ocean. A theoretical framework to separate the plume and the estuary mixing in isohaline coordinates is presented. An idealized coastal ocean model setup resolving the whole plume‐estuary continuum is used to validate the theoretical relation and to study the link between the estuarine pre‐conditioning and the cross‐shore export of river water under different forcing scenarios. It is found that the most effective cross‐shore transport of river water happens under moderately upwelling favorable wind conditions and weak tidal forcing. This scenario is characterized by relatively weak estuarine mixing, strong stratification, and little interaction between the surface and bottom boundary layers such that a thin layer of buoyant river water can extend far into the ocean. We conclude that reduced estuarine mixing is indicative of an enhanced accumulation of fresh water near the shore, but is not directly related to the cross‐shore transport in river plumes. 

Abstract Convergent estuaries have been shortened by dam‐like structures worldwide. Here, we evaluate 31 long‐term water level stations and use a semi‐analytical tide model to investigate how landward‐funneling and a dam influence tidal and storm surge propagation in the greater Charleston Harbor region, South Carolina, where three rivers meet: the Ashley, Cooper, and Wando. Results show that the phase speed and amplification of the principal tidal harmonic (M2) is larger than other long waves such as storm surge (∼1–4 days) and setup‐setdown (∼4–10 days). Further landward, all waves attenuate, but, as they approach the dam on the Cooper River, a frequency dependent response in amplitude and phase progression occurs. A semi‐analytical tidal model shows that funneling and the presence of a dam amplify tidal waves through wave interference from partial and full reflection, respectively. The different phase progressions of the reflected waves interact with the incident wave to increase or decrease the summed overall wave amplitude. Using a friction‐convergence parameter space, we demonstrate that dominant tides in 23 estuaries and the tidal, storm surge, and setup‐setdown waves in the Cooper River can be delineated into three regimes that describe landward amplification or attenuation associated with funneling, a dam, or both. The regime of each tidal constituent is consistent but can change with the duration and height of each storm surge event; dam associated wave interference can attenuate long‐duration events, while the most intense events (short duration, high water) are amplified by dams more than funneling and greatly increase flood exposure. 

In this study, we investigate the transition of semidiurnal Kelvin waves into Hybrid Kelvin-Edge (HKE) waves and associated generation of internal tides at widening shelves using theory, a realistic global baroclinic ocean model simulation, and quasi-realistic regional barotropic model simulations. Using the global model simulation, we identify several areas where a tidal HKE wave transition co-exists with internal wave generation. Of all areas considered, the Celtic Sea/Bay of Biscay shelf has the widest shelf and the strongest internal tide generation. We find that the global simulation agrees better with the theoretical Kelvin modes on the narrow than with the hybrid edge modes on the wide shelves. To help us understand the effect of complex, realistic bathymetry on the HKE wave transition, we perform quasi-realistic 1/25◦ barotropic simulations of the Celtic Sea/Bay of Biscay shelf areas. In these simulations, we gradually change the realistic bathymetry to a more idealized bathymetry. The idealized simulations show that the complex bathymetry steers the barotropic energy flux and causes standing wave patterns, which mask the HKE wave transition. Based on this analysis, we conclude that the HKE wave transition in the Celtic Sea/Bay of Biscay and other shelf areas in the global ocean is most likely masked by the effects of complex bathymetry and that offshelf baroclinic fluxes cannot be exclusively attributed to the HKE wave transition.