Mean and Turbulent Characteristics of a Bottom Mixing‐Layer Forced by a Strong Surface Tide and Large Amplitude Internal Waves

We present 15 days of both mean and turbulent field observations bottom mixing‐layer at a gently sloping 250 m deep continental shelf site, energized by tides and nonlinear internal waves (NLIWs). The tidal frequency forcing was due to the combined effects of the barotropic tide and a mode‐1 internal tide (IT), while the NLIWs were predominantly mode‐1 waves of depression. The bottom mixing‐layer thickness varied at both semidiurnal and sub‐tidal ∼O(10)d frequencies, with an average thickness of around 10 m. Compression and expansion of the mixing‐layer by both the IT and NLIWs affected the mean velocity profiles in the mixing‐layer, while the phasing between the barotropic and baroclinic flows led to an asymmetry in mean velocity profiles between periods of rising and falling isotherms. With the exception of periods of flow reversal, the turbulent kinetic energy balance and turbulent stress observations were consistent with the existence of an inertial‐sublayer with thickness of approximately 10%–15% of the mixing‐layer thickness ( ∼1 m), even beneath NLIWs. In the outer portion of the mixing‐layer—that is, above the inertial‐sublayer—NLIWs modulated the local turbulence spectra. We discuss the observations in the context of a predictive model for mixing‐layer thickness. The analysis suggests that the high‐frequency variability in mixing‐layer thickness was dominated by internal wave pumping, though strength of the ambient stratification and the frequency of the forcing were important controls on the time‐averaged (sub‐tidal) variation. Plain Language Summary The bottom boundary layer (BBL) is the portion of an ocean flow that “feels” the presence of the seabed. Oceanic bottom boundary layer dynamics are globally significant as they control many important physical, biological, and chemical transport processes in the ocean, and furthermore, most man‐made sub‐sea structures are located within this relatively thin layer. We present 15 days of detailed observations of a near well‐mixed BBL at a 250 m deep continental shelf site, where the ocean flow was characterized by strong tides and large internal waves. The boundary layer thickness varied at tidal and sub‐tidal frequencies, and was ∼10 m thick on average. In the lowermost 1 m of this layer, the flow conformed to conventional boundary layer dynamics as observed in the lowest layers of the atmosphere, for example. More than 1 m above the bed, however, the overlying internal waves modified both the nature o