Overlapping turbulent boundary layers in an energetic coastal sea

Abstract. Turbulent mixing properties were directly observed to understand the interactions and overlapping events of wind- and tidally-forced boundary layers in a deep, weakly-stratified coastal sea. Te-Moana-o-Raukawa/Cook Strait of Aotearoa New Zealand is an 𝒪(200 m)-deep, energetic strait, known to experience both strong tidal currents and high wind speeds. More than 𝒪(40,000) quality-controlled turbulence observations were obtained from an ocean glider equipped with a microstructure profiler and a current speed through water sensor. Tidal flows of 𝒪(1 m s^-1) and wind speeds of 𝒪(10 m s^-1) independently enhanced turbulent dissipation to ε = 𝒪(10^-5 W kg^-1) in bottom and surface mixed layers. Over a four-day period, boundary-generated turbulence was evident in the interior water column on ten occasions, enhancing interior diapycnal diffusivity levels by 5-35-fold, reaching K_z = 𝒪(0.1–1 m² s^-1). On three instances, the top and bottom mixed layers overlapped. These overlapping boundary layers were present in water depths five-times deeper than previously observed, which has implications for the vertical extent of material fluxes from the surface or seafloor. Interior stratification was transient, emerging from far-field advection of low density surface waters and supported by vertical buoyancy fluxes, episodically fully eroded by boundary-generated turbulence. Combining observations with one-dimensional General Ocean Turbulence Model (GOTM) outputs, turbulence interactions in the interior were found to be modulated by wind, tides and transient stratification fields, in turn influencing the vertical structure of sinks and sources of turbulent kinetic energy. Enhanced vertical transport toward the interior of the near-boundary shear-produced turbulence was found to erode interior stratification.