Turbulence in the wintertime northern Adriatic Sea under strong atmospheric forcing

摘要

In February 2003, we observed the response of the 40 to 50 m deep northern Adriatic Sea to strong surface forcing by 20 m s?1 winds and 600 W m?2 net upward heat flux resulting from cold bora winds blowing onto a relatively warm sea through gaps in the Croatian mountains. Ocean turbulence throughout the water column was observed with a microstructure profiler and a bottom-mounted, upward-looking, 5-beam, acoustic Doppler current profiler (ADCP). Microstructure-based dissipation rates (ε) were close to similarity scaling of the surface wind stress. The surface buoyancy flux, related to the oceanic heat loss, contributed little energy to the turbulence, but led to sustained unstable stratification. The energy-containing range of the turbulence together with the upper end of the inertial subrange, with horizontal scales between a few hundred meters and about 10 m, contained coherent, anisotropic overturning motions aligned with the low-frequency, barotropic ocean currents which carried stress and showed an asymmetry between rare, narrow, faster downdrafts and diffuse, weak updrafts. These motions bear no similarity with Langmuir cells. The turbulence measurements were embedded in surveys of the mesoscale ocean variability. Part of the observations were set in a front a few hundred meters wide with little density contrast. As the bora wind relaxed, the front began to develop a highly stratified “foot” undergoing intense mixing. The paper addresses problems of beam spreading and instrumental noise in ADCPs.