Coastal water circulation response to radiational and gravitational tides within the southeastern Bay of Biscay

The Basque coastal area, in the southeastern Bay of Biscay, can be characterised as being strongly influenced by land climate. This, together with the presence of irregular and steep topography, complicates considerably the coastal water circulation patterns. Time-series of hourly currents, surface atmospheric pressure and wind data were analysed for two coastal stations. This research quantified the contribution of gravitational and radiational tides (breezes and solar atmospheric tides) to the coastal water circulation. Moreover, the spatial and seasonal differences in such processes were determined. This investigation has revealed that radiational tides induce diurnal and semidiurnal water motions. These exist together with higher frequency diurnal harmonics caused by anharmonic radiational wind forcing. The radiational forcing contributed similarly to water motion at the stations. The radiational currents varied seasonally, accounting for 20% and 8% of the total kinetic energy in summer and winter, respectively. In comparison, the gravitational currents were more pronounced in the station affected by estuarine dynamics.