804 
U. Callies et al.: Surface drifters in the inner German Bight 
Ocean Sci., 13, 799-827, 2017 
www.ocean-sci.net/13/799/2017/ 
BSHcmod currents averaged over a 5 m depth surface layer. 
By contrast, drift simulations based on TRIM output (lm 
deep top layer) were performed without taking into account 
additional wind effects. 
The assumed strengths of either wind forcing or Stokes 
drift resulted from trying to achieve an overall eastward dis 
placement of simulated drifters that roughly agreed with ob 
servations. This approach must not be confused with sound 
model calibration, which seems impossible based on the very 
limited data available. Models perform differently during dif 
ferent periods, and it is hard to distinguish, for instance, be 
tween deficiencies in the hydrodynamic model and implica 
tions of imperfect atmospheric forcing. Also, independent 
data needed for model validation are not available. How 
ever, already the simple approach enables an appraisal of how 
drifter simulations will depend on a distinction between wind 
drag and Stokes drift. 
2.2.4 Analysis of 25 h drifter displacements 
Comparing simulated trajectories with concurrent observa 
tions enables a qualitative assessment of a model’s ability 
to reproduce overall drift patterns. However, accumulation 
of possibly intermittent simulation errors makes it difficult 
to localize the origin of major deviations in either space or 
time. Therefore, a series of short-term (25 h) simulations was 
started once per day (13:00 UTC) from each drifter’s ob 
served location at that time. The short-term simulation errors 
were analysed against the backdrop of prevailing winds and 
residual currents (see Sect. 2.3). 
2.3 Characterization of residual currents on a daily 
basis 
BSH classifies the residual circulation in the German Bight 
(between 53.25 and 55.5° N and between 6.5 and 9.0° E) on 
a daily basis, referring to surface currents from BSHcmod. 
The classification 1 is performed manually based on subjec 
tive assessments of 24 h averages. The small deviation of the 
averaging interval from two tidal periods does not affect the 
analysed frequency distribution of circulation patterns. Most 
frequent are a cyclonic circulation with a pronounced inflow 
at the south-western border and outflow at the northern bor 
der, a reverse anticyclonic circulation and a category with 
variable current patterns. Cyclonic circulations correspond 
with what is observed in the long-term mean (see Fig. 2a). 
Six specific directional types with currents towards the east, 
west, north, south, north-west and south-east play only minor 
roles. They are related to strong local winds and for statistical 
purposes combined into just one class. Due to topographical 
constraints, south-west and north-east patterns do not occur. 
Figure 3 includes results of the BSH classifications for the 
period relevant in this study. 
1 http://www.bsh.de/de/Meeresdaten/Beobachtungen/ 
Zirkulationskalender_Deutsche_Bucht/index.js 
An alternative analysis is based on the 2-D version of 
TRIM. Slightly different from the above approach, Callies 
et al. (2017) defined residual currents as 25 h means (close 
to one lunar day - 24.8 h). A principal component analysis 
(PCA) was performed on these residual currents, focusing on 
the inner German Bight (east of 6.0° E and south of 55.6° N; 
see Fig. 2b) and excluding inshore areas with a bathymetric 
depth of below 10 m. Corresponding data are freely acces 
sible at Callies (2016). Figure 2b displays the leading mode 
of variability (first empirical orthogonal function (EOF); see 
von Storch and Zwiers, 1999). The time series of correspond 
ing principal component PCi is shown in Fig. 3. The struc 
ture of the dominant residual current anomaly pattern (ex 
plaining more than 70 % of variability) roughly agrees with 
that of long-term mean residual currents in the area of the 
white box in Fig. 2a. 
3 Results 
3.1 Observations 
Figure 3 places drifter schedules into the context of vari 
able atmospheric winds and marine residual currents. Time 
bars show travel times of all nine surface drifters. To facili 
tate a synopsis of synchronous drifter movements, the time 
coordinate was segmented, subjectively assigning different 
colours to periods with different drift behaviour. In this con 
text, a continuous daily index was introduced, counting days 
since when the first 25 h simulation for drifter no. 5 was 
started on 27 May at 13:00UTC (see Table SI). To represent 
atmospheric forcing used in BSHcmod and TRIM, respec 
tively, simulated 10 m winds at 55° N and 7° E near the cen 
tre of the study area are shown together with observations on 
the island of Heligoland (54.10° N, 7.53° E). All wind vec 
tors represent 25 h means and are plotted at the centre of 
the respective 25 h interval starting at 13:00 UTC. The winds 
from three different data sources are in reasonable agreement 
with each other. 
Figure 3 also includes the representation of (a) the subjec 
tive classification of daily mean BSHcmod surface currents 
and (b) the first principal component (PCi) of 25 h mean cur 
rents simulated with a 2-D version of TRIM (see Sect. 2.3). 
Positive values of PCi (i.e. amplitudes of the anomaly pat 
tern shown in Fig. 2b) indicate a strengthening of the mean 
cyclonic circulation; negative values refer to its weakening or 
even reversal. Although the two representations of residual 
current variability have different roots (different models, sur 
face layer vs. vertical means, subjective vs. objective, differ 
ent atmospheric forcing), a clear correspondence between the 
two representations is discernible. Cyclonic hydrodynamic 
regimes and positive values of PCi tend to coincide with 
winds from the south-west, while anticyclonic circulations 
and negative PCi values are mainly driven by winds from 
the north-west (Callies et al., 2017).