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

Abstract

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 abovementioned 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 .

The Crossover Time as an Evaluation of Ocean Models Against Persistence

“…The model forecast fails to ever reach a crossover time for the entire span of the available prediction. During the study period the average decorrelation times of the drifter velocities was found to be 1.2 days (not shown) in approximate agreement with Poulain et al (1996), LaCasce (2008), Döös et al (2013), and Lumpkin and Johnson (2013). This time scale is very similar to the e-folding time scale of persistence (black line in Figure 2a).…”

Marine litter ensemble transport simulations in the southern North Sea

“…Residence times of litter at the sea surface would not necessarily be an appropriate choice, considering high uncertainty of particle trajectories integrated over very long periods. Kjellsson et al (2013) exemplified this uncertainty for real drifters in the Baltic Sea. We feel that for our application in the North Sea, 90 day integration time was a reasonable choice.…”

“…This is due to the fact that oceanic advection shows chaotic behavior: Small changes in the initial conditions (in time and space) may lead to different system behavior. Kjellsson et al (2013) demonstrated this by analyzing observed drifter trajectories in the Baltic Sea. Within the first 20 days of drift, two drifters which were started close to each other separated by up to 40 km.…”

Time Shift of Wind Influence on the Movement of Surface Water Masses in the Szczecin Lagoon

“…Research on hydrodynamic models of reservoir surface waters is conducted in many research centers. Findings on the distribution of surface drifters in the Baltic Sea in 2010 and 2011 are described in (Kjellsson and Döös, 2011). The authors compare drifter trajectories with trajectories generated by a numerical model using fields, i.e., wind forcing with parameterization of gusty winds, from a regional ocean model.…”