Results and discussion 63 Tidal and wind-induced circulation within the Southeastern limit of the Bay of Biscay: Pasaia Bay, Basque Coast A. Fontán 1 , M. González 1 , N. Wells 2 , M. Collins 1,2 , J. Mader 1 , L. Ferrer 1 , G. Esnaola 1 and Ad. Uriarte 1 [1] AZTI-Tecnalia, Marine Research Division. Herrera Kaia, Portualdea, z/g, 20110 Pasaia, Spain. [2] National Oceanography Centre, Southampton European Way, Empress Dock, Southampton SO14 3ZH, UK. Abstract The Basque coastal area, in the southeastern Bay of Biscay, can be characterised as being more influenced by land climate and inputs, than other typically ‘open sea’ areas. The influence of coastal processes, together with the presence of irregular and steep topography, complicate greatly the water circulation patterns. Water movement along the Basque coastal area is not well understood; observations are scarce and long- term current records are lacking. The knowledge available is confined to the surface currents: the surface water circulation is controlled mainly by wind forcing, with tidal and density currents being weak. However, there is a lack of knowledge available on currents within the lower levels of the water column; likewise, on the main time-scales involved in the water circulation. This study quantifies the contribution of the tidal and wind-induced currents, to the overall water circulation; it identifies the main time-scales involved within the tidal and wind-induced flows, investigating difference in such currents, throughout the water column, within Pasaia Bay (Basque coast). Within this context, extensive oceanographic and meteorological data have been obtained, in order to describe the circulation. The present investigation reveals that the circulation, within the surface and the sub-surface waters, is controlled mainly by wind forcing fluctuations, over a wide range of meteorological frequencies: third-diurnal, semidiurnal and diurnal land-sea breezes; synoptic variability; frequencies, near fortnightly periods; and seasonal. At the lower levels of the water column, the main contribution to the water circulation arises from residual currents, followed by wind- induced currents on synoptic time-scales. In contrast, tidal currents contribute minimally to the overall circulation throughout the water column. Keywords: Wind-induced currents, land-sea breezes, synoptic variability, Bay of Biscay, Basque coast. Introduction The Basque Country is located in the southeastern Bay of Biscay between the west- east-oriented Spanish coast and the north- south-oriented French coast (Figure 19). The coastal waters here can be characterised as being more influenced by land climate and inputs than other typically ‘open sea’ areas (Valencia et al., 2004). In terms of climatology, the Basque Country is located in the mid-latitudes of the eastern North Atlantic Ocean. Atmospheric circulation in the mid-latitudes of the North Atlantic is governed by two main centres of activity: an anticyclonic zone south of 40ºN, centred near the Azores (the ‘Azores High’); and a low pressure area centred around 60ºN, near Iceland (the ‘Iceland Low’). Between these two areas, the prevailing winds are from the west to south-west; they are stronger in winter and lighter and less regular in summer. The locations of the two centres of activity are not fixed, but vary in space and time. In winter, the anticyclone moves towards the southeast and the wind direction rotates from southwest to northwest, with sudden violent squalls. In summer, the ‘Azores High’ moves towards the northwest and gives rise to generally equatorward coastal winds (OSPAR, 2000). The climate is rainy, temperate and oceanic, with moderate winters and warm summers. The climate is classified as marine, west -coast and mild (Usabiaga et al., 2004). The winds at San Sebastian (Figure 19) have an average speed of 18 km h -1 , throughout the year. The winds blow predominantly from the northwest and south, with 27.5% of calms. However, the distribution of winds is very different, from season to season. The dominant winds are from the south in winter and autumn, whereas northerly winds represent the most frequent