Evaluation and Tuning of Model Trajectories and Spreading Rates in the Baltic Sea Using Surface Drifter Observations

Results from experiments with surface drifters in the Baltic Sea in 2010–2011 are presented and discussed. In a first experiment, 12 SVP-B (Surface Velocity Program, with Barometer) drifters with a drogue at 12–18 m depth were deployed in the Baltic Sea. In a second experiment, shallow drifters extending to a depth of 1.5 m were deployed in the Gulf of Finland. Results from the SVP-B drifter experiment are compared to results from a regional ocean model and a trajectory code. Differences between the observed SVP-B drifters and simulated drifters are found for absolute dispersion (i.e., squared displacement from initial position) and relative dispersion (i.e., squared distance between two initially paired drifters). The former is somewhat underestimated since the simulated currents are neither as fast nor as variable as those observed. The latter is underestimated both due to the above-mentioned reasons and due to the resolution of the ocean model. For the shallower drifters, spreading in the upper 1–2 m of the Gulf of Finland is investigated. The spreading rate is about 200 m/day for separations <0.5 km, 500 m/day for separations below 1 km and in the range of 0.5–3 km/day for separations in the range of 1–4 km. The spreading rate does not follow Richardson’s law. The initial spreading, up to a distance of about d=100–150 m, is governed by the power law d∼t 0.27 whereas for larger separations the distance increases as d∼t2.5.BalticWa