Sammanfattning
Turbulent diffusion in the weak halocline in the Himmerfjärd in the fall was studied by a tracer experiment . The tracer, Rhodamine B, was injected instantaneously at a prescribed depth and after that the spreading of the dye patch was followed by a systematic mapping of the concentration distribution.
Vertical exchange coefficients computed from the experiment range from 1.9 · 10-2 to 3.1 · 10- cm2/s . The vertical exchange is rather weak in comparison with results from open waters at the same windspeed and stratification. This is thought to be due to a reduction of the wind induced mixing caused by the physical boundaries of the narrow fiord. Horizontal diffusion characteristics are computed from the time rate of change of the concentration distribution variances. Apparent horizontal diffusivities range up to about 5 · 103 cm2/s when the observed scale of turbulence ranges up to about 800 m
The results are not consistent with a model for generation of horizontal dispersion by vertical current shear and vertical diffusion and it is concluded that horizontal diffusion and also horizontal shear are important factors for generation of dispersion in the Himmerfjärd.
Particle distribution in the Himmerfjärd was investigated by measuring vertical profiles of light attenuation at a number of stations.The results show a layering of the fiord into three distinct layers such that the particle content is relatively high in the upper and the bottom layer and relatively low in the intermediate layer. The intermediate layer is believed to be intruding Baltic water.
The work has been carried out by Dr Erik Buch, Institute of Physical Oceanography , University of Copenhagen . It is a part of the project "Materialtransporter i Himmerfjärden" financed by grants to SMHI from the Research Department of the Swedish Environment Protection Board, contract nr 7-329/79. This project is intended to give hydrographic information for the system ecological study "Himmerfjärdsundersökningen" run by the Askö Laboratory. Project leader at SMHI is Erland Bergstrand. The tracer study was initiated in a cooperation between Dr Wayne Wilmot at the RGASA group, University of Stockholm and Dr Erik Buch.