The variability of strongly non-linear solitary internal waves observed during an upwelling season on the Portuguese shelf

Abstract

Observations of non-linear internal waves propagating onto the Portuguese shelf were made for 1 month during the summer upwelling season, using a thermistor chain mooring. The strongly non-linear waves had a typical amplitude of about 35 m and length of order 150 m, significantly longer than weakly non-linear KdV internal solitons. They appeared each semi-diurnal tidal cycle within the troughs of the internal tide and their phase speed (0.57 m s −1) was close to both that of the linear internal tide and that suggested by non-linear internal wave theory. The arrival time of the waves relative to local high water varied by over 5 h during the month and was closely correlated with wind events, with stronger upwelling favourable winds apparently causing the waves to arrive later. Analysis of the observations indicated that this modulation was primarily linked to wind driven currents, rather than fluctuations in the internal wave phase speed due to changes in the stratification. The intensification of the Ekman drift in an offshore upwelling filament may have played an important role in this variability. There appeared to be little correlation between the size of the waves and variations in tidal range. In fact the wave amplitude was close to the theoretical strongly non-linear maximum, even on neap tides. Considerable doubt remains about the origin of the waves; although the observations suggest that they were generated at the local shelf break by the semi-diurnal tide, their amplitude is too large to be explained in terms of the cross shelf tidal currents. It is possible that three-dimensional aspects of the topography and tidal regime are involved.