Dissipation in the Baltic proper during winter stratification

Abstract

[1] Profiles of dissipation rates and stratification between 10 and 120 m depth were measured with a loosely tethered profiler over a 9-day winter period in the Gotland Basin of the Baltic Sea. Supplementary measurements of current profiles were made with moored ADCPs. Temporal and spatial patterns of the stratification were observed by means of towed CTD. Shallow freshwater lenses in the surface mixed layer, mesoscale eddies, inertial oscillations, and inertial waves as part of the internal wave spectrum provided the marine physical environment for the small-scale turbulence. Two wellseparated turbulence regimes were detected. The turbulence in the surface mixed layer was well correlated with the wind. The majority of the energy flux from the wind to the turbulent kinetic energy was dissipated within the surface mixed layer. A minor part of this flux was consumed by changes of the potential energy of the fresh water lenses. The penetration depth H pen of the wind-driven turbulence into the weakly stratified surface mixed layer depended on the local wind speed (W 10 ) as H pen = cW 10 3/2 Active erosion of the Baltic halocline by wind-driven turbulence is expected for wind speeds greater than 14 m/s. The turbulence in the strongly stratified interior of the water column was quite independent of the meteorological forcing at the sea surface. The integrated production of turbulent kinetic energy exceeded the energy loss of inertial oscillations in the surface layer suggesting additional energy sources which might have been provided by inertial wave radiation during geostrophic adjustment of coastal jets and mesoscale eddies. The averaged dissipation rate profile in the stratified part of the water column, best fitted by e / EN, was different from the scaling of the dissipation in the thermocline of the ocean [Henyey et al., 1986]. The diapycnical mixing coefficient (K v ) was best fit by K v = a 0 /N according to Stigebrandt [1987] with a 0 % 0.87 Â 10 À7 m 2 /s 2 . The diapycnal diffusivity estimated from the dissipation rate was lower than those estimated by the bulk method.

Denitrification in the water column of the central Baltic Sea

“…Except for one very low estimate of 0.1 Â 10 À5 m 2 s À1 the values ranged between 1.2 Â 10 À5 and 4.8 Â 10 À5 m 2 s À1 with a median of 1.5 Â 10 À5 m 2 s À1 (Table 2). This agrees very well with the values from 0.2 Â 10 À5 to 4.2 Â 10 À5 m 2 s À1 , with medians of 0.8 Â 10 À5 and 2.4 Â 10 À5 m 2 s À1 for summer and fall, respectively, found at 150 m depth in the Gotland Deep over a 30 year period (Axell, 1998) and the winter value of 0.8 Â 10 À5 m 2 s À1 below the halocline in the Gotland Basin (Lass et al, 2003). Our estimates of denitrification using short term bottle incubations thus appear to match the rates that may be expected from the water chemistry and hydrography.…”

“…Coexistence of NO À x and sulfide, therefore, requires mixing of waters across the oxic-anoxic interface. Such mixing may occur via turbulent motions induced by internal waves and mesoscale eddies (Lass et al, 2003). We argue that this mixing transports NO À x from above and sulfide from below into the denitrification zone.…”

Aerobic methanotrophy within the pelagic redox-zone of the Gotland Deep (central Baltic Sea)

“…2) and reflects a water column stratification that limits the vertical mixing and water renewal in the deep strata (Reissmann et al, 2009). Oceanographic investigations, carried out at the redox-zone of the Gotland Deep, show that this depth is periodically perturbed by intrusions, internal waves or eddies which can shift the amplitudes of isoclines up to 10 m within time spans less than an hour (shown for temperature and salinity in Lass et al, 2003;Dellwig et al, 2012). During sampling, the specific water column structure led to oxygen deficiency below a water depth of about 80 m. Further downward, the oxygen concentrations decreased below 0.8 mL L −1 , characterizing the redox-zone between the oxic surface and anoxic deep waters.…”

Nutrient transports in the Baltic Sea – results from a 30-year physical–biogeochemical reanalysis

“…A Hiblertype dynamic-thermodynamic sea ice model (Hibler, 1979) with elastic-viscous-plastic rheology (Hunke and Dukowicz, 1997) and a two-equation turbulence closure scheme of the k − ε type with flux boundary conditions (Meier, 2001) have been embedded into RCO. The deep-water mixing is assumed to be inversely proportional to the Brunt-Väisälä frequency, with the proportionality factor based on dissipation measurements in the eastern Gotland Basin (Lass et al, 2003). In its present version, RCO is used with a horizontal resolution of 2 nautical miles (3.7 km) and 83 vertical levels, with layer thicknesses of 3 m. RCO allows direct communication between bottom boxes of the step-like topography (Beckmann and Döscher, 1997).…”