Observations of internal waves and associated mixing phenomena in the Portimao Canyon area

Internal wave activity and induced mixing phenomena in the Portimao Canyon area (southern Portuguese coast) are analysed. Observations of current velocities and CTD measurements were taken during a 24-h period at two fixed stations located along the main axis of the canyon. The station nearest shore was located close to the canyon head and the other station was 5 km seaward. These basic measurements are complemented with vertical sections of temperature, acquired by XBT casts, along the main axis of the canyon, and also with time series of temperatures recorded by a thermistor chain moored on the continental shelf, near the canyon head. The time sequences of velocities and density profiles recorded at the fixed stations were analysed using a procedure based on empirical orthogonal function (EOF) analysis and dynamical modes decomposition (DMD) techniques. By this method, two different sources for internal wave activity are identified: a clear internal tide signal on the one hand, and a shorter than tidal period internal waves which exhibit greater current velocities and density oscillation amplitudes than internal tide, on the other. It is suggested that these shorter period internal waves are responsible for the vertical mixing affecting the water column over the slope and shelf waters in this region. Finally, it is also suggested that the evacuation of these mixed water masses formed around the continental shelf break may be related to a core of relatively cold water flowing over the continental slope toward west.