Internal wave turbulence in the Llobregat prodelta (NW Mediterranean) under stratified conditions: A mechanism for sediment waves generation?

Abstract

An array of 76 high-resolution temperature sensors at 0.5m intervals between 5 and 42.5m off the bottom was moored near the Barcelona harbor buoy in 81m water depth, between October 2013 and April 2014. The mooring was located just seaward of an extensive sediment wave area developed in the Llobregat River prodelta, with 1m high crests parallel to the coast and 50–100m wavelengths. In the NW-Mediterranean, the thermal stratification reaches its maximum penetration through the water column in autumn until it is broken by winter convection. Such a deep stratification affects large-scale sub-inertial slope currents, which are mostly confined to the upper half of the water column, by the hampered vertical exchange of frictional turbulence, and supports near-bottom internal waves between the inertial and buoyancy frequencies. Observed onshelf propagating frontal bores most likely interact with the sediment waves and contribute to their generation, as they are trailed by considerable shear-induced turbulence and high-frequency internal waves close to the buoyancy frequency that have wavelengths matching those of the sediment waves. The bores are either driven by near-inertial or 3–7day periodic sub-inertial motions just following a brief period of large convective instability at the end of the offshelf flow phase.