Circulation patterns at Le Danois Bank, an elongated shelf-adjacent seamount in the Bay of Biscay

Abstract

Le Danois Bank is a seamount-like topographical feature located at the continental shelf in the central Cantabrian Sea (Bay of Biscay), recently declared Marine Protected Area. The bank is partially connected to the shelf by a saddle, topographically bounding an inner valley. Hydrographical cruises and direct current measurements from current meters, vessel-mounted ADCP and subsurface floats are analysed to describe the main circulation patterns in the area. Records are consistent with the presence of anticyclonic flow at the seamount summit. The inner valley seems to support the development of retentive flow structures and the whole system is affected by strong and variable along-slope currents, thus complicating the overall picture. Scaling arguments, based on non-dimensional parameters accounting for the surrounding flow characteristics and the geometry of the seamount, allow an estimate of the dynamical processes that may be active at the bank. Temporary Taylor cap formation by weak impinging flow may occur, although a clear signature of this feature does not emerge from the observed density fields. The strong local amplification of the diurnal tide suggests the presence of resonant seamount trapped waves. The strength of the semidiurnal tide M 2 in the region, in combination with the steepness of the flanks of the bank, points to a significant contribution of tidally rectified flow to the observed residual anticyclonic flow. An observation of an isopycnal vertically repositioned by 50 m within one downcast/upcast period of a CTD profile indicates the importance of high frequency internal wave dynamics.