Drifters released at the time of the Prestige oil spill, within the upper part of the water column (approx. 20 cm), have shown the complexity of surface water movements in the Bay of Biscay.
The trajectory shown in Fig. 1 indicates the changing drift direction of the buoy located initially at 45°10′N and 6°36′W (close to the surface oil slicks originated by the Prestige) in response to changing wind directions.
Based upon the buoy trajectories, the derived current velocities reached up to 95 cm/s, with most of them being below 75 cm/s; the average was around 25 cm/s (González et al., 2006).
For comparison, over this period of time, altimeterderived geostrophic velocities for the winter slope current, or Navidad flow, originating from the west, were represented by values ranging from 5 cm/s (6th November 2002) to 35 cm/s (27th November 2002), with a mean of 17 cm/s (García-Soto, 2004). This comparison reveals that wind was the most important mechanism affecting the oil dispersion within the surface layers; it accounted for more than 95% of the drift speeds and
directions (González et al., 2006).
The data provided by the buoy trajectory shown in Fig. 1 have been used: (1) to analyse the accuracy of numerical models in their current and trajectory predictions; and
(2) to establish the fundamental mechanism of oil dispersion, at the sea surface. This buoy transmitted data from the 29th December 2002 until the 26th May 2003; its position was relayed, via satellite, with a mean frequency of 3 to 4 times/day.
Moreover, the information derived from the wind data of the Gascony buoy has been used to analyse the accuracy of the wind fields of the NCEP reanalysis data. These are the input data to estimate the current velocity fields for the Bay of Biscay.
The Gascony buoy is a fixed oceanic and atmospheric station located at 45°12′N and 5°00′W, owned and maintained by UK Met Office in cooperation with Meteo France Buoy.
With respect to the NCEP reanalysis data, these are provided by the National Centers for Environmental Prediction, which is a National Weather Service of NOAA.
These data are obtained using a state-of-theart analysis/forecast system (historical observations and model simulations) to perform data assimilation, including past data from 1948 to the present. This reanalysissystem provides a reconstruction of the climate.
In this case, the six-hourly data for the years 2002 and 2003 have been used for the hydrodynamic modelling.