Estimation of the seasonal sea level variations in the Gulf of Cadiz (SW Iberian Peninsula) from in-situ measurements, satellite altimetry and numerical models I. Laiz* a , J. Gómez-Enri a , B. Tejedor a , A. Aboitiz a , P. Villares a a Dept. of Applied Physics, Univ. of Cadiz, Campus Rio San Pedro, 11510; Puerto Real, Cadiz, Spain 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. Keywords: Gulf of Cadiz, sea level, altimetry, tide gauges , seasonal cycle, steric effect, river runoff 1. INTRODUCTION The Gulf of Cadiz (GoC hereinafter) is located between the SW Iberian Peninsula and North Africa (Figure 1) and conforms the basin that connects the North Atlantic Ocean and the Mediterranean Sea. Its continental shelf is approximately delimited by the 100 m isobath where the shelf slopes down into the continental slope. It is divided by Cape Santa Maria (CSM, see Figure 1) in two basins with different circulation characteristics 8 . East of CSM, where this study is focused, the shelf is wide (around 50 km) and is subject to discharges from important rivers like the Guadalquivir River (GqR, Figure 1), that imprint signatures on the surrounding waters in terms of sea surface temperature and several biogeochemical variables. Moreover, the Guadalquivir river dynamics can affect the surface circulation over the eastern continental shelf 9 . Although the average discharge rate of this river is about 105 m 3 /s, the normal rate is about 25 m 3 /s through the year, and can reach values as high as 1000 m 3 /s in spring and autumn 4 . The seasonal cycle of sea level in the North-Atlantic and in the Mediterranean Sea is dominated by the seasonal signal and is mainly driven by atmospheric pressure and the thermosteric effect associated with the gain and loss of heat by the upper layer of the ocean. Other forcing factors such as wind or the net evaporative cycle also contribute to the seasonal sea level signal in the ocean. Moreover, river runoff can also affect the sea level fluctuations in regions located near river outflows 20 , although its contribution has not been quantified yet for the GoC. It is also expected that sea level is strongly influenced by the North Atlantic Oscillation (NAO) 23 through changes in atmospheric pressure and freshwater flux (evaporation, precipitation and river runoff) 21 . *irene.laiz@uca.es; phone: +34 956 956010000 ext (4) 6985; fax: +34 956 016079 NOTE: the edited version of this paper can be downloaded from: http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1269654 Citation: Proc. SPIE 8175, Remote Sensing of the Ocean, Sea Ice, Coastal Waters, and Large Water Regions 2011, 81750K (October 07, 2011); doi:10.1117/12.898132