Stratified-turbulence observations in the deep Mediterranean

Abstract. A nearly half-cubic hectometer of deep Mediterranean-Sea waters is yearlong sampled with about 3000 high-resolution temperature sensors to study different sources of turbulent waterflows, which are vital for life. Although temperature differences are never larger than 0.01 °C, daily, weekly, and seasonal variations are observed. About half the time, relatively warm stratified waters are moved from 100's of meters higher levels to near the seafloor. These internal-wave and sub-mesoscale eddy-induced motions are half an order of magnitude more turbulent than those induced via general geothermal heating from below, and about one order of magnitude more turbulent than those from open-ocean processes. A rough estimate shows that eddy-induced stratified turbulence is likely more important for deep-sea life than rare, not observed, deep dense-water formation at the abyssal-plain mooring site. With a delay of about a week, the stratified turbulence tracks atmospheric disturbances, which are found 35 % more energetic in winter than in summer. From comparison of turbulence-calculation methods, of band-pass filtering with vertical-displacement reordering, for data over one-four days, a generalization is proposed for the filter cut-offs under weakly stratified and near-homogeneous conditions in the deep Mediterranean.