Convection and intermittency noise in water temperature near a deep Mediterranean seafloor

Abstract

Turbulent and internal wave motions are important for the exchange of momentum, heat, and suspended matter in the deep sea, which is generally stably stratified in density. Turbulence-generation models involve shear of vertical current differences that deforms stratified waters and convection that is driven by (unstable) buoyancy. In general, shear generation is found more in the interior of the well-stratified ocean, while convection is known to occur near the sea surface, e.g., via nighttime cooling. Far below the surface, the Western Mediterranean Sea is very weakly stratified and offers opportunity to observationally study the deep-sea convection. An opportunistic small set of high-resolution temperature sensors demonstrate not only classic internal-wave-induced turbulence but also convection attributed to geothermal heating and spectral properties that relate to various chaos-theory models, such as 1/σ pink noise (σ denoting frequency), mainly found lying at (0.01 m above) the seafloor, and 1/σ2 Brownian noise, mainly found on a moored line at about 100 m above the seafloor. Near-inertial temperature variations are observed to occur down to the seafloor, thereby disturbing the local convective turbulence regime to shear-dominated one temporarily. The integral turbulence timescale is generally smaller (with dominant higher frequency motions) at the seafloor than about 100 m above it.