Beschreibung: Records of temperature fluctuations in a thermocline show an energy peak close to the local Brunt-Väisälä frequency. This supports recent observations that short internal waves are common phenomena within the thermocline. The high rate of energy of these waves permits the conclusion that they may break and form patches of turbulence. It is assumed that these short internal waves and turbulence patches contribute to the acoustic backscattering observed from these layers.

Beschreibung: A great number of observations, mainly on temperature and current fluctuations in the Arkona Basin, have been made during the years 1962-1968 (figs. 2 and 4). Corresponding to the wind conditions over the Baltic Sea (mean value about zero, fluctuations of about 5 m sec-1), currents and stratification are extremely variable. The mean currents in fig. 7 are towards NE for the entire eastern part of the Arkona Basin, but the actual flow can have any direction (fig. 32). The current fluctuations are non-Gaussian (fig. 10). The mean stratification in the Arkona Basin underlies also great variations. Temperature fluctuations of 8°C and salinity fluctuations of 3 ‰ occur within 2 days (fig. 5). In the mean thermocline level we find also in the central part of the Arkona Basin a current towards NE (fig. 9). The current spectra of the Belt Sea and the Arkona Basin are principally of the same shape, but inertial waves do not occur in the Belt Sea (fig. 13). The mode decomposition reveals the strongest baroclinity in the period range from approximately 10h - 40h; at the inertial frequency the major baroclinic mode contains 10 times the energy of the barotropic one (fig. 28). The spectrum of the barotropic mode corresponds to the spectrum of the windstress whereas the spectra of the internal modes correspond to divergence and curl of the stress field (fig. 28). The strongest signal are the inertial currents. In general their wave length seems to be short; they consist of at least 4 internal modes and the barotropic one (fig. 37). The Reynolds stress clue to inertial waves leads to negative viscosity (fig. 40). The seiches currents are very weak compared to the inertial currents. The oscillations in the Arkona Basin seem to be short-crested and strongly influenced by Bornholm and the Rönne Bank. Towed thermistor cable records lead to the conclusion that several features of the temperature structure are of quasi-permanent character.