Vertical structure and bottom-intensification of tidal currents off Northwestern Spain

Abstract

Four-month current and temperature observations collected in two different positions and at four different depths in the Golfo Artabro, off the NW corner of the Iberian Peninsula, have been analysed in order to investigate some aspects of tides in this area. Tidal currents in the deepest stations are much greater than expected for a purely barotropic tide, a result that is even more outstanding if the bottom decay of the interior currents is allowed for. The near-bottom tidal enhancement is confirmed by the large tidal fluctuations of temperature at M2 frequency, equivalent to internal oscillations of some tens of metres of amplitude. The phase of these oscillations suggests propagation of the internal tide parallel to the depth gradient. The slope of the bottom profile is comparable to the slope of the internal rays and favours the generation of internal tides of considerable amplitude. The existence of areas of supercritical slope shoreward of the mooring lines is able to reflect seawards the energy of the incoming internal waves along trajectories nearly parallel to the sea floor, a mechanism that is put forward as responsible for the considerable concentration of tidal energy in the bottom layer.