8 Trajectories and Spreading of Observed and Simulated Drifters in the Baltic Sea
253
1993; Launiainen et al. 1993). These drifter data sets have not been used to evaluate
the accuracy of any ocean circulation model. This lack of observations can, at least
partly, be explained by the small horizontal extent over which the depth of the Baltic
Sea exceeds the depth of the SVP drifter drogue (which sits between 12 and 18 m
depth), and by the heavy ship traffic. The risk of a surface drifter getting caught up
in too shallow waters or colliding with a ship is likely higher in the Baltic Sea than
in the open ocean.
For the Baltic Sea, only a few short-term drifter experiments have been performed in the uppermost 1–2 m thick layer. For instance, shallow drifters have been
used to validate the output of the High Resolution Operational Model of the Baltic
Sea (HIROMB) and to study ice drift in the Gulf of Finland (Kõuts et al. 2010).
Also, experiments targeting at the validation of the Seatrack Web oil spill model
were performed with the same drifters in the middle of the Gulf of Finland, western
Estonian archipelago and in the eastern sector of the Northern Gotland Basin (Verjovkina et al. 2010). Their duration ranged from 8 hours up to 7 days. The longest
distance covered by a drifter during a single experiment was 52 nautical miles.
This chapter presents an overview of results of experiments with SVP surface 2
drifters and shallow (∼1.5 m deep) drifters (called GPS/GSM drifters below) deployed in the Baltic Sea in 2010–2011. The advection of SVP drifters has also been
simulated using a regional ocean model and a trajectory code. The drifter observations and model trajectories were compared using several statistical measures.
Results from the comparison may then be applied to tune the algorithms used to
simulate the trajectories to obtain a better fit to observations. The realism of the
original and tuned model trajectories is discussed, as well as the implications for
Lagrangian modelling in the Baltic Sea. We also present results of a series of experiments with the GPS drifters in the Gulf of Finland that were designed to quantify
the spreading of floats’ pairs in the uppermost 1–1.5 m thick layer. This behaviour
is discussed in the context of predictions from the turbulence theory.
8.2 Surface Drifters in the Baltic Sea
Twelve SVP-B (Surface Velocity Program, with barometer port) drifters (Lumpkin
and Pazos 2007) were deployed in the Baltic Sea in 2010–2011 (Kjellsson and Döös
2012). Their drift was tracked over the time interval of 14 July 2010–19 November
2011. The drifters were manufactured by Marlin–Yug Ltd. in Sevastopol, Ukraine,
and are approved by the Naval Oceanographic Office (NAVOCEANO) to comply
with the World Ocean Circulation Experiment (WOCE) type. SVP-B drifters are
equipped with a surface buoy containing GPS-sensors for measuring the position,
sea surface temperature and atmospheric pressure, and a system to transmit the data
to the Argos or Iridium satellites. Attached to the surface buoy is a holey sock anchored between 12 and 18 m depth. This configuration allows for the drifters to
2 They actually follow subsurface currents at depths 12–18 m.
253
1993; Launiainen et al. 1993). These drifter data sets have not been used to evaluate
the accuracy of any ocean circulation model. This lack of observations can, at least
partly, be explained by the small horizontal extent over which the depth of the Baltic
Sea exceeds the depth of the SVP drifter drogue (which sits between 12 and 18 m
depth), and by the heavy ship traffic. The risk of a surface drifter getting caught up
in too shallow waters or colliding with a ship is likely higher in the Baltic Sea than
in the open ocean.
For the Baltic Sea, only a few short-term drifter experiments have been performed in the uppermost 1–2 m thick layer. For instance, shallow drifters have been
used to validate the output of the High Resolution Operational Model of the Baltic
Sea (HIROMB) and to study ice drift in the Gulf of Finland (Kõuts et al. 2010).
Also, experiments targeting at the validation of the Seatrack Web oil spill model
were performed with the same drifters in the middle of the Gulf of Finland, western
Estonian archipelago and in the eastern sector of the Northern Gotland Basin (Verjovkina et al. 2010). Their duration ranged from 8 hours up to 7 days. The longest
distance covered by a drifter during a single experiment was 52 nautical miles.
This chapter presents an overview of results of experiments with SVP surface 2
drifters and shallow (∼1.5 m deep) drifters (called GPS/GSM drifters below) deployed in the Baltic Sea in 2010–2011. The advection of SVP drifters has also been
simulated using a regional ocean model and a trajectory code. The drifter observations and model trajectories were compared using several statistical measures.
Results from the comparison may then be applied to tune the algorithms used to
simulate the trajectories to obtain a better fit to observations. The realism of the
original and tuned model trajectories is discussed, as well as the implications for
Lagrangian modelling in the Baltic Sea. We also present results of a series of experiments with the GPS drifters in the Gulf of Finland that were designed to quantify
the spreading of floats’ pairs in the uppermost 1–1.5 m thick layer. This behaviour
is discussed in the context of predictions from the turbulence theory.
8.2 Surface Drifters in the Baltic Sea
Twelve SVP-B (Surface Velocity Program, with barometer port) drifters (Lumpkin
and Pazos 2007) were deployed in the Baltic Sea in 2010–2011 (Kjellsson and Döös
2012). Their drift was tracked over the time interval of 14 July 2010–19 November
2011. The drifters were manufactured by Marlin–Yug Ltd. in Sevastopol, Ukraine,
and are approved by the Naval Oceanographic Office (NAVOCEANO) to comply
with the World Ocean Circulation Experiment (WOCE) type. SVP-B drifters are
equipped with a surface buoy containing GPS-sensors for measuring the position,
sea surface temperature and atmospheric pressure, and a system to transmit the data
to the Argos or Iridium satellites. Attached to the surface buoy is a holey sock anchored between 12 and 18 m depth. This configuration allows for the drifters to
2 They actually follow subsurface currents at depths 12–18 m.