Lagrangian and Eulerian measurements of horizontal mixing in the Baltic

Six dye diffusion experiments and simultaneous moored current-meter measurements with vector-averaging current meters (VACM) were carried out in the Baltic surface-mixed layer on four days with different wind and surface wave conditions. These measurements are used to test the Hay-Pasquill (1959) method of calculating Lagrangian diffusion coefficients from Eulerian fluctuation measurements which has been frequently applied to meteorological but not yet to oceanographic measurements.

For the fairly wide range of our experimental conditions it is found that the Lagrangian dye diffusion coefficients and the product u2E. TE of variances and integral over the autocorrelation function of the lateral Eulerian current fluctuations are significantly correlated. Due to the special circumstances of our moored current measurements the factor relating them is not exactly the ? of Hay-Pasquill. This factor, for the definition of the commonly used apparent diffusion coefficient, was determined as 1.4 ± 0.4, but for diffusion coefficients more specifically related to the diffusion velocity model used in our analysis it would be about similar to those determined in earlier meteorological work.

The calculation was also done for the diffusion experiment which Kullenberg (1977) carried out in the pycnocline at about 45 m depth and there the factor was only one-tenth of that in the surface-mixed layer. This small value is due to a larger correlation time scale in the stratified part of the water column which was most likely caused by internal wave effects.