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 
tidal cycles, relative contributions from wind forcing increase 
(Fig. 6). However, even small systematic errors in the simula 
tion of oscillating tides might possibly give rise to erroneous 
residual current components similar in size to the contribu 
tions from windage. A finding that needs further analysis is 
whether near-shore residual currents underestimated in sim 
ulations indicate such inaccuracies in regions where tides in 
crease with decreasing water depth. 
This study did not substantiate benefits from including 
Stokes drift simulated offline. Directions of winds and waves 
coincided most of the time and effects of Stokes drift on sur 
face currents could successfully be mimicked in terms of ad 
ditional windage. In TRIM, such effects seemed already suf 
ficiently parametrized as part of momentum transfer from the 
atmosphere to marine currents. When winds quickly abate, 
increase or turn, waves adjust with a time lag, needing a cer 
tain fetch to fully develop. Although in these cases the dif 
ferent roles of winds and waves could be more marked, in 
the present study errors in atmospheric or marine circulation 
modelling seemed predominant. Nevertheless, fully coupled 
modelling of currents and waves (Staneva et ah, 2017) could 
probably improve simulated surface currents, given that the 
vertical resolution is fine enough. It must also be kept in mind 
that the present study did not include any extreme events. 
The incident of two drifters converging quickly and sepa 
rating about 10 days later provided evidence that at least in 
some situations an unavoidable increase in prediction uncer 
tainty would be of the order of 3-5 km day -1 , regardless of 
however sophisticated a model used might be. Further stud 
ies would be needed to substantiate this finding in terms of 
its representativity and possible dependence on specific lo 
cations or atmospheric conditions. The observed separation 
rate happened to roughly agree with the average magnitude 
of simulation errors we identified. More experiments would 
help identify the way to go for further model improvements. 
Data availability. The raw data of observed drifter locations are 
freely available from Carrasco and Horstmann (2017). Results of 
the PCA of residual currents are freely accessible at Callies (2016).