DIAMIX — An experimental study of diapycnal deepwater mixing in the virtually tideless Baltic Sea

The DIAMIX (DIApycnal MIXing) project aims at investigating wind-driven vertical mixing below the pycnocline. Since tides are exceedingly weak in the Baltic an area east of Gotland was chosen as the study area. Two pilot surveys and two main experiments were conducted between June 1997 and September 2000. The joint effort is motivated to cover as many scales as possible of the distribution of kinetic and potential energy. This paper describes the main features found in the field measurements. Inertial currents, near-inertial waves and internal seiches are the bulk features in the mooring data. Acting against topography these currents may generate strong clockwise and anti-clockwise rotating eddies as well as up- and down-welling which has been observed mainly from ship-borne measurements. From turbulent dissipation measurements the stabilizing effect of the stratification can clearly be seen. Dissipation measurements at the slope indicate a much higher dissipation rate there as compared to the deeper station. All these effects have to be taken into account to add up the power needed to explain the turbulent dissipation and work against buoyancy forces connected to diapycnal mixing in the deepwater as estimated from long-term modeling.