U. Callies et al.: Surface drifters in the inner German Bight 
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Ocean Sci., 13, 799-827, 2017 
windage about 0.043 ms -1 . The resulting relative magnitude 
of 16 % roughly agrees with what Rohrs et al. (2012) found 
for Stokes drift. According to data from an experiment in 
northern Norway, Stokes drift amounted to about 20 % of the 
mean Eulerian currents. 
In Fig. 12, both observations and simulations show regu 
lar intermittent patterns in connection with tidal cycles. Vari 
ations of maximum drift speeds indicate that movements 
along different branches of tidal ellipses have components 
that are alternately oriented in the same or opposite direc 
tion of a superimposed non-tidal drift component. This non- 
tidal drift is possibly but not necessarily related to wind ef 
fects. On days 13 and 14, such non-tidal drift manifests it 
self more in simulations than in observations, while during 
days 15 and 16 alternating drift speed maxima are more pro 
nounced in observations (in particular for drifter no. 6). Ac 
cording to Fig. 6, BSHcmod + W underestimates residual 
drift speeds for all four drifters tracked at that time. A fast 
displacement of drifter no. 6 to the north-west can be dis 
cerned from Fig. 4c. All models fail to reproduce this move 
ment (see Fig. A2c, for instance). Considering the small val 
ues of windage and the even smaller of Stokes drift (wind di 
rections allow for only small fetches over the open sea), tun 
ing these effects cannot substantially improve simulations. 
Remember that Stokes drift and windage were calculated 
offline and added to the Eulerian currents after the model 
had been integrated and the fields stored, hacking success of 
this approach is not to say that deficiencies of drifter simula 
tions are not related to wind conditions. The problem around 
days 15-16, for instance, occurs under non-stationary wind 
directions that affect also the orientation of the residual cur 
rent regime (Fig. 3). Changes of wave-induced forcing of 
the ocean, including sea-state-dependent momentum flux and 
Stokes drift (Staneva et al., 2017), affect water level, high and 
low water times and therefore also ocean currents. 
Rohrs et al. (2012) warn that implementing Stokes drift as 
a simple additive component of drift velocity, parameterized 
in terms of wind forcing, can be inconsistent (i.e. violate con 
servation of both momentum and energy) if Eulerian currents 
were simulated without taking into account the reservoir of 
wave momentum and energy. In the present study, the ex 
changeability of Stokes drift and wind drag indicates that the 
role of waves as a reservoir of momentum was not relevant at 
least during the period considered. One reason for this could 
be that due to limited fetches the North Sea is less swell dom 
inated than other Nordic Seas (Semedo et al., 2015). 
Two crucial and outstanding questions are (a) whether the 
drifters’ behaviours are representative of surface currents and 
(b) if it justifiably can be assumed that all drifters maintained 
their ideal drift properties over the whole period they were 
tracked. Drifter trajectories may reflect a specific exposure to 
winds and waves, well illustrated by the experiment reported 
by Rohrs et al. (2012). Edwards et al. (2006) suggested cor 
rections to improve trajectory simulations when wind er 
rors and characteristics of the specific drifters deployed are 
known. However, for the present study, a tentative positive 
answer to the first question could be given based on the rea 
sonable correspondence between the magnitudes of observed 
tracer displacements and their counterparts simulated based 
on just TRIM Eulerian surface currents (see Fig. 10a). On the 
other hand, Poulain et al. (2009) estimated a higher down 
wind slippage of about 1 % of the wind speed for undrogued 
SVP (Surface Velocity Program) drifters. In the context of an 
oil-drift study, Price et al. (2006) deployed CODE (Coastal 
Ocean Dynamics Experiment)-type drifters drogued in such 
a way that they were supposed to capture the upper 1 m layer 
velocities. Referring to a report by Niiler et al. (1997); Price 
et al. (2006) estimated for these drifters slip velocities of the 
order of 0.03 ms -1 . In BSHcmod + W, such velocity would 
match the parametrized wind drag at a wind speed of 5 m s _1 . 
Fike contributions from wind drag, the estimated downwind 
slippage of drifters is supposedly much smaller than short 
term drift velocities in a tidally dominated regime but may 
nevertheless have considerable impacts on drifter displace 
ments in the long run. Fully disentangling effects of wind 
drag on water masses and drifters, respectively, seems hardly 
possible. 
Answering the second question is again difficult. The joint 
analysis of drifter positions and displacements in this study 
gave at least some indications for possible non-ideal drifter 
behaviour. A period of extreme velocities far beyond what 
models predict occurs for drifter no. 9 at the end of its journey 
(days 22-26; Figs. 6d and 9a and e). These high velocities 
result in a clear separation of drifter no. 9 from the formerly 
concentrated group of drifters. Probably more central for the 
present study is the behaviour of drifter no. 8. From day 34 
onward, drifter no. 8 showed a tendency to move faster than 
the neighbouring drifter nos. 5 and 6 (e.g. days 34-35, day 37 
or days 39-42; Fig. 6). Strikingly, in these cases, drifter no. 8 
tended to move into directions that are more parallel to pre 
vailing winds (see SMI). This latter observation also applies 
to the aforementioned behaviour of drifter no. 9. 
Possible reasons for the deviant behaviours of drifter nos. 
8 and 9 can only be speculated. The simplest explanation 
would be that the different types of the two drifters (and 
of drifter no. 7, which also showed a very fast movement 
at the end of the time period it was tracked) distinguishes 
them from other drifters deployed (Table 1). However, this 
explanation is not in accord with the fact that problems did 
not persist throughout the whole observational period. The 
special behaviour of drifter no. 9 after about day 22 coin 
cided with its entering a more southern region of the German 
Bight (Fig. 9a and e). For this region, Port et al. (2011) iden 
tified a higher variability of surface currents, less correlated 
with wind conditions, which would imply that introducing 
either Stokes drift or an additional wind drag could probably 
be a less promising approach for model improvement. How 
ever, still the most probable explanation for the mismatch of 
observations and corresponding simulations is that the drifter 
experienced problems with its drogue. Unfortunately, drifters