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∼t 2.5.
KeywordsRelative Dispersion Spreading Rate Inertial Oscillation Surface Drifter Lagrangian Velocity
This study was performed in the framework of the BalticWay project, which was jointly supported by the funding from the by the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas, Ref. No. 2008–1900), Estonian Science Foundation and the European Commission’s Seventh Framework Programme (FP7 2007–2013) under grant agreement No. 217246 made with the joint Baltic Sea research and development programme BONUS. The research was partially supported by the Estonian Science Foundation (grant No. 9125), targeted financing by the Estonian Ministry of Education and Research (grant SF0140007s11), and by the European Regional Development Fund via support to the Centre of Excellence for Non-linear Studies CENS. The authors wish to thank Tallink Silja shipping company for allowing us to deploy the SVP drifters from the Stockholm–Riga line and in particular we wish to thank Captain Lembit Uustulnd and his crew on M/S Silja Festival for permission and help in deployment of the drifters. We also acknowledge and appreciate the help given by Prof. Peter Lundberg and Dr. Anders Engqvist in deployment. The experiments with the surface drifters in the Gulf of Finland were performed very professionally by Mr Mikk Viidebaum. His cooperation towards deployment and rescue of SVP drifters is also gratefully acknowledged. Finally, we also express our gratitude to Markus Meier and Anders Höglund at the Swedish Meteorological and Hydrological Institute for providing the RCO data as well as help and fruitful discussions about it. All trajectory simulations have been made using supercomputers maintained by NSC at Linköping University, Sweden.
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