High-resolution temperature observations of a trapped nonlinear diurnal tide influencing cold-water corals on the Logachev mounds

Abstract

A high-resolution thermistor string mooring of 120m length was used to measure turbulence processes in the water layer above the foot of a densely populated cold-water coral mound at 919m depth at the southeast slope of Rockall Bank in the Logachev area, North-East Atlantic Ocean. As expected from previous current observations, the temperature data reveal a dominant diurnal (tidal) periodicity associated with topography-trapped, weakly bottom-intensified waves. These baroclinic diurnal waves are driven to (near-) resonance around the mound and have vertical amplitudes exceeding the mooring line. Their horizontal excursion length matches the size of the mound, which causes a residual current around and flux up the mound. As their particle velocities also match the phase speed of the wave traveling around the mound, these waves can become highly nonlinear and show largest turbulence due to wave-breaking at the transition from warming downslope to cooling upslope tidal phase. Averaged over the entire 9-day time series and the 120m vertical range, mean turbulent kinetic energy dissipation amounts to 2.2±1.1×10−7Wkg−1 (and mean vertical eddy diffusivity to 9±5×10−3m2s−1) with short-term variations over four orders of magnitude. Such large turbulence, more than 100 times larger than open-ocean values and comparable with that observed in tidally energetic shelf break-shallow sea areas, will affect the nutrient replenishment of the cold-water corals.