Estimation of the seasonal sea level variations in the Gulf of Cadiz (SW Iberian Peninsula) from in-situ measurements, satellite altimetry and numerical models

Abstract

Time series (1997-2008) of near-shore altimetry data and in-situ tide gauge records have been analyzed to investigate the seasonal variability of sea level along the Gulf of Cadiz. A high level of agreement was obtained between altimeter and in-situ observations, indicating that altimeter data can be a valuable tool to study the sea level seasonal cycle near the coast. Harmonic analysis showed that more than 95% of the average seasonal cycle is explained by the annual and, to a lesser extent, semiannual components. The average seasonal cycle of sea level anomalies is very similar at the four coastal stations, with minimum values during winter and maximum during autumn. Atmospheric pressure accounts for 20-38% of the sea level variability, its effect diminishing toward the Strait of Gibraltar. The steric contribution is notable at the westernmost stations (32-37%) and it also decreases eastward (9-17%). River discharge explains about 15% of the sea level variability, indicating that its effects should be taken into account. The contribution of direct atmospheric forcing for a section of the sea level time series (1997-2001) has also been explored using the output of a barotropic oceanographic model (HIPOCAS project) forced with wind and atmospheric pressure, revealing that the contribution of wind is generally small (6-12%) at seasonal time scales. Small but significant correlations are found between the time series of winter-autumn mean sea level and the winter-autumn North Atlantic Oscillation (NAO) indices. Analysis show that the effect of NAO is mainly reflected on atmospheric pressure, wind and river runoff.