Variations of the seasonal sea level cycle in southern Europe

The temporal and spatial variability of the seasonal signals in the Mediterranean Sea and the Atlantic Iberian coast is explored on the basis of tide gauge data. Mean amplitudes range in the various stations between 3 and 7 cm for the mean annual component and between 1 and 3 cm for the semiannual component. On average, seasonal signals account for approximately 20% of the total variance of monthly records. The seasonal cycle is unsteady in time, with large variations in amplitudes and phases in some stations reaching up to 8 cm and 40 d, respectively. The contribution of the direct atmospheric effects to the seasonal cycle has been estimated by means of the output of a barotropic model forced with wind and atmospheric pressure. This contribution is regionally coherent, albeit variable in time. The annual cycle of the residuals (observations minus atmospherically induced sea level) is larger than that of the observations by about 2-3 cm. The residual semiannual cycle is smaller than that of the observations by about 25%. The temporal variability of the seasonal cycle of the residual series is also explored. Changes in water temperatures at the upper layers is the only parameter that appears connected with the observed variations.