Observations of Tidal Straining Within Two Different Ocean Environments in the East China Sea: Stratification and Near‐Bottom Turbulence

Abstract

Tidal straining describes the straining effect induced by the vertical shear of oscillatory tidal currents that act on horizontal density gradients. It tends to create tidal periodic stratification and modulate the turbulence in the bottom boundary layer (BBL). Here, we present observations of current, hydrology and turbulence obtained at two mooring stations that are characterized by two typical hydrological environments in the East China Sea (ECS). One is located adjacent to the Changjiang River's mouth, and the other is located over a sloping shelf which is far from the freshwater sources. Tidal straining induces a semidiurnal switching between stable and unstable stratification at both stations. Near‐bottom high‐frequency velocity measurements further reveal that the dissipation rate of turbulent kinetic energy (TKE) is highly elevated during periods when unstable stratification occurs. A comparison between the TKE dissipation rate ( ɛ) and the shear production ( P) further reveals that the near‐bottom mixing is locally shear‐induced most of the time except during the unstable stratification period. Within this period, the magnitude of dissipation exceeds the expected value based on the law of the wall by an order of magnitude. The buoyancy flux that calculated by the balance method is too small to compensate for the existing discrepancy between the dissipation and shear production. Another plausible candidate is the advection of TKE, which may play an important role in the TKE budget during the unstable stratification period.

Turbulence Structure and Burst Events Observed in a Tidally Induced Bottom Boundary Layer

“…An imbalance between ε and P has been repeatedly observed in field studies and a variety of explanations have been presented. These included vertical dissipation of turbulent kinetic energy (Scully et al., 2011; Talke et al., 2013), TKE horizontal convective transport (Davis & Monismith, 2011; Tu et al., 2019; Walter et al., 2014), and stratified mixing levels in the near‐bottom water column (Rippeth et al., 2001; Simpson et al., 1990; Trowbridge et al., 1999; Yang et al., 2017). In addition, an inherent noise floor in the dissipation estimates could also be a reason for the imbalance, however, this is not the case in this study.…”

“…The balance of turbulent kinetic energy (TKE) production and dissipation processes within the bottom boundary layer is controlled by horizontal TKE advection, buoyancy flux, vertical divergence, stratification, and other factors. Experimental verification of this balance has been the subject of numerous investigations (Scully et al., 2011; Simpson et al., 1990; Trowbridge et al., 1999; Tu et al., 2019; Yang et al., 2017).…”

Turbulence, Sediment‐Induced Stratification, and Mixing Under Macrotidal Estuarine Conditions (Qiantang Estuary, China)

“…In coastal and estuarine boundary layer studies, a simplified turbulent energy balance between production, dissipation, and buoyancy flux is often assumed (e.g., Sanford & Lien, 1999;Trowbridge et al, 1999). In some experimental studies, this balance is not achieved and this has been attributed to various additional terms including vertical divergence in turbulent kinetic energy (TKE) flux (e.g., Scully et al, 2011), advection of TKE (e.g., Walter et al, 2011), vertical transport by coherent flow structures (e.g., Talke et al, 2013), shear instabilities (e.g., Peters & Bokhorst, 2000), or tidal straining (e.g., Yang et al, 2017).…”

A Subtidal Box Model Based on the Longitudinal Anomaly of Potential Energy for Narrow Estuaries. An Application to the Guadalquivir River Estuary (SW Spain)

“…This is mathematically expressed as follows: where g is the gravity constant, t is the time, x defines the horizontal position, z represents the vertical coordinate, ρ ( x , z , t ) is the water density, and is the reference density in complete mixing conditions, both of which include the suspended sediment density; d is the total depth that is the sum of η ( x , t ), the elevation over mean sea level and h ( x ), the mean depth. From the time when it was first proposed in the mid 1970s (Simpson & Hunter, ), the potential energy anomaly has been widely used to identify physical processes that produce water exchanges in shelf seas (Hofmeister et al, ; Simpson et al, ; Yang et al, ), regions influenced by freshwater (De Boer et al, ; Simpson, ), estuaries (Garvine & Whitney, ; Rice et al, ; Sun et al, ), and lakes (Zhao et al, ).…”