U. Callies et al.: Surface drifters in the inner German Bight 
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www.ocean-sci.net/13/799/2017/ 
Ocean Sci., 13, 799-827, 2017 
Schedule of drifter tracks and residual current variability 
May June July 
Winds Heligoland (25 h means): 
Figure 3. Time bars indicate for each drifter the period for which corresponding simulations were performed (drifter nos. 2, 3 and 4 were 
disregarded in this study). A colour code defined in Table Si (the Supplement) was used for time segmentation. Symbols at the top represent 
the classification of daily surface residual currents based on BSHcmod. In addition, the time series of the leading principal component (PC i) 
of 25 h mean currents simulated with TRIM-2D is shown (see Sect. 2.3). PCj values were normalized with their standard deviation during 
the years 1958-2015. Positive PCj values represent a strengthening of the cyclonic regime; negative values represent its weakening or even 
reversal (see Fig. 2). The 25 h mean wind vectors (10 m height) used in the two model systems (both extracted for the location at 55° N and 
7° E) are contrasted with observations on the island of Heligoland (54.10° N,7.53° E). 
Figure 4 shows six observed drifter trajectories, disregard 
ing the tracks of drifter nos. 2, 3 and 4 that were recorded for 
just a few days. A feature shared by at least four drifters (nos. 
5, 6, 7 and 8) is a general displacement towards the north 
east. Concerning drifter nos. 6 and 8, an interesting special 
situation occurs during 7-16 June (days 11-20). Figure 5 
shows the distance between the two drifters as a function of 
time. At the deployment of drifter no. 8 (30 May, day 3), 
drifter no. 6 had already travelled for nearly 3 days and was 
located at a distance of about 20 km from drifter no. 8. Dur 
ing the next 4 days, the two drifters further separated. On 
4 June (day 8), however, they suddenly started converging 
quickly. From 8 June (day 12) onward, drifter nos. 6 and 8 
stayed at a distance of less than 2 km for nearly 10 days. Just 
after the distance had reached its minimum (about 800 m), 
the drifters started to separate again. Other short periods of 
fast convergence occurred later but never again did the two 
drifters come that close. During the last 8 days of their joint 
journey (starting at around day 35), the distance between the 
two drifters showed particularly large oscillations (Fig. 5). 
Figure 6 provides magnitudes of velocities for drifter nos. 
5, 6, 8 and 9, calculated from velocity vectors smoothed us 
ing a 25 h moving average of hourly data. Drifter movements 
are particularly fast in the beginning (days 5-6), brought 
about by persistent south-westerly winds and a correspond 
ing cyclonic circulation at that time (Fig. 3). Other periods 
with particularly fast movements occur around day 35 and 
days 42-43. In the former case, strong winds from the south 
east trigger a very fast separation of drifter nos. 6 and 8 (see 
Fig. 5). In the latter case, north-westerly winds give rise to 
extreme drift speeds in the south-east direction. Drifter nos. 
5 and 8 are already in near-shore areas at that time (Fig. 4). 
In their central parts, drifter trajectory nos. 5, 6 and 8 ex 
hibit variable drift directions but mostly moderate drift ve-