Physical processes governing water circulation in the southeastern limit of the Bay of Biscay

Abstract

The Basque coast and continental shelf, southeastern Bay of Biscay, can be characterised as being more influenced by land climate, than typical 'open sea' areas. The influence of coastal processes and the presence of irregular topography, appear to complicate greatly the water circulation patterns. The surface water circulation is governed by wind forcing, with tidal and density currents being weak. However, there is a lack of knowledge on the marine currents and on the main time-scales involved in circulation, within the lower water column. The main purposes of this investigation are to identify major physical processes, controlling the spatial and temporal variability in the circulation; and to investigate difference in the controlling processes, throughout the water column. The meteorological conditions differ between the coastal and continental shelf waters. The prevailing wind pattern over the Basque coast is representative of the complex (coastal) wind variability. In contrast, over the Basque continental shelf, the wind regime is influenced less by land climate, responding to the seasonal synoptic regime over the mid- latitudes of the eastern North Atlantic Ocean. The breezes are more pronounced at the coastal waters, than over the continental shelf. Indeed, they introduce harmonics of the diurnal wind over the Basque coastal area. Their influence decreases with increasing distance offshore, but they can be detected even at a distance of 200 km from the coast. This investigation confirms that the main contribution to the wind-induced flow comes from synoptic variability, in the lower water circulation. However, the air-sea interaction patterns are more complex within the surface coastal waters, with wind-driven flows being affected over a broad set of meteorological frequencies: breezes; inertial motions; and synoptic, intra-seasonal and seasonal variability. The meteorological forcing frequencies cannot be determined for the surface continental shelf waters, in relation to an absence of appropriate current measurements. The rest of the physical processes, controlling water circulation, can be summarised as follows. Tidal currents contribute minimally to the overall flow, being dominated by semidiurnal tides. However, their contribution is relatively important for the surface coastal waters, with the tidal flows being dominated by diurnal tide. This indicates that the diurnal tide is intensified by the diurnal land-sea breeze. The high-frequency flow is related to inertial and semidiurnal motions. The inertial oscillations are inhibited by the coastal boundaries; their magnitude increases away from these boundaries, propagating deeper into the water column. The semidiurnal motions may be associated with internal tides, as they increase with depth and under strong stratification conditions. The low-frequency flow is due to fluctuations of several days and non-linear interaction frequencies, over the continental shelf. These interactions, explained as a coupling between inertial-tidal and inertial period with itself, decrease with depth and onshore, as do inertial oscillations. Overall, this investigation has revealed that surface coastal water circulation is governed by wind forcing over a wide range of frequencies; whilst the high- or low-frequency currents control circulation in the lower part of the water column. The main physical processes forcing the lower water circulation, over the continental shelf, are related to synoptic wind variability or low-frequency currents.

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