Geochemistry, Geophysics, Geosystems

Variability of internal frontal bore breaking above Opouawe Bank methane seep area (New Zealand)



[1] Large internal wave breaking is observed exceeding a vertical array of 61 high-resolution temperature sensors at 1 m intervals between 7 and 67 m above the bottom. The array was moored for 5 days at 969 m of Opouawe Bank, New Zealand, a known methane seep area. As breaking internal waves dominate sediment resuspension above sloping topography in other ocean areas, they are expected to also influence methane transport. Despite being visible in single beam echosounder data, indications for turbulence due to rising gas bubbles are not found in the present 1 Hz sampled temperature records. Likely, the mooring was too far away from the very localized bubble release spot. Instead, the temperature sensors show detailed internal wave-turbulence transitions. Every tidal cycle, a solibore (a frontal turbulent bore with a train of trailing solitary waves) changes shape and intensity. These solibores are highly turbulent and they restratify the bottom boundary layer, thereby maintaining efficient mixing. Details of different turbulent bore developments are discussed. Averaged over a few tidal cycles and over the sensors range, mean vertical eddy diffusivity amounts to 3 ± 1 × 10−3 m2 s−1 and mean turbulent kinetic energy dissipation to 1.6 ± 0.7 × 10−7 W kg−1, with variations over 4 orders of magnitude. Such turbulence will affect the distribution of dissolved methane and other geochemical species in the lower 100–150 m above the bottom and their release from the bottom. The above mean values are remarkably similar to those found at various other sites in the NE Atlantic Ocean.