Analysis of surface current properties in the Gulf of Finland using data from surface drifters

The accurate prediction of currents in the ocean surface layer is of importance for many applications, such as environmental monitoring, offshore commercial operations, and safety of shipping. Numerical models can be used to obtain such predictions, but in most sea areas the availability of current observations remains scarce. We report results of field experiments involving passive surface drifters in the Gulf of Finland, with the purpose to characterize the mesoscale and sub-mesoscale flow dynamics and spreading rate. A total of 51 deployments of surface drifters were made in 2011 and 2013, with duration of drift lasting from 1 to 35 days. The individual tracks produced a velocity distribution with a mean value close to 0.1 m/s, with close resemblance to the Rayleigh distribution. A Lagrangian integral time scale was calculated based on the autocorrelation of the drifter velocity, using three different methods of calculation and splitting the drifter into segments of different duration. The persistency of motion was 7-12 hours on average, with individual trajectories showing persistent motion up to over 20 hours. When inertial oscillations were filtered out from the drifter positions, the average persistency increased to 14-20 hours. Analysis was also made for the relative dispersion of drifter clusters. At small separation scales the speed of drifter separation appears to follow the Richardson´s Law, where the relative diffusivity increases as the separation distance to the 1/3 power. However, a transition takes place with separation distances close to 5 km, after which the relative diffusivity decreases with increasing separation distance. These results point to the complexity of the underlying surface current fields, and indicate what scales must be resolved in numerical models in order to obtain reliable predictions for surface currents in the Gulf of Finland.