The upper mixed layer (UMI) depth obtained from temperature is very close to that from density: the maximum is about 15m. This indicdtes that temperature is a good indicator of mixed layer during measurements When the surface heat flux is balaceed by a cross-shore beat flux, the surface mixed layer depth obtained from the WM model (Weatherly and Martin, 1978), hPRT, is roughly the same as obscrred. The mixed layer depth calculated from the PWP model (Price, Weller and Pinkel, 1986) is close to the depth obtained from thermistor chain temperature data. The results show that both the WM model and PWP model can provide a good estimnate of stratification in the study area during the cruise. The value of log (h\u3) is about 9.5 in the study area, which shows that the study arca is strongly stratified in summer. Observtions on the northern Portugal shelf reveal high variability in stability, giving rise to semi-diurnal, scmi-nonthly and diurnal oscillations, and long term variations. The fortnightly oscillations are highlighted by post-springs and post-neaps. The stirring of spring tide is reinforced by strong wind mixing which brings about completc vertical homogencity everywhere. The semi diurnal periodic stratification is very pronounced because the major axis of the tidal cllipse is orientated acrossshore, even though the tidal current is wear in this area, the maximum stratification is obscrved around the middle of ebb, and, the water at this time is much warmer. The diurnal oscillation results from the upper ocean responsc to henting and wind mixing when solar heating warms and stabilizes the upper ocean There is a clcar relationship between upper mixed layer depth and wind stress magnitude at subtidal frequencies, Stronger winds result in a deeper surface mixed layer. Typically, the surface mixed layer depth lags the wind stress by h 12h.