Observations of the mixing in the Baltic thermo- and halocline layers

The vertical and horizontal mixing of a passive contaminant in the Baltic thermo- and halocline layer has been investigated in a series of experiments during spring and fall. The vertical mixing is very weak. The horizontal spreading is a factor of 2–5 lower than in the surface layer. The integrated vertical transfer across the halocline layer is of the same order of magnitude as the integrated transfer in the coastal boundary zone. It is found that the wind is an essential source of energy for the mixing. The significance of the wind-forcing is demonstrated by the correlation between the fluctuations of the wind and (1) the fluctuations of the oxygen content in the deep water and (2) the fluctuations of the stability across the halocline layer, during the present century. The results support the view that the overall decrease of the oxygen content in the deep waters is due to other factors than an alteration of water exchange characteristics.