This paper analyzes the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar. All possible repeats of two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the Strait. For Envisat, along-track sea level anomalies (SLA) at 18-Hz posting rate were computed using ranges from two sources, the official Sensor Geophysical Data Records (SGDR) and the outputs of a coastal waveform retracker, the Adaptive Leading Edge Subwaveform (ALES) retracker; in addition, SLA at 1 Hz were obtained from the Centre for Topographic studies of the Ocean and Hydrosphere (CTOH). For AltiKa, along-track SLA at 40 Hz was also computed both from SGDR and ALES ranges. The Sea State Bias correction was recomputed for the ALES-retracked Envisat SLA. The quality of these altimeter products was validated using two tide gauges located on the southern coast of Spain. For Envisat, the availability of data close to the coast depends crucially on the strategy followed for data screening. Most of the rejected data were due to the radar instrument operating in a low-precision non-ocean mode. We observed an improvement of about 20% in the accuracy of the Envisat SLAs from ALES compared to the standard (SGDR) and the reprocessed CTOH data sets. AltiKa shows higher accuracy, with no significant differences between SGDR and ALES. The use of products from both missions allows longer times series, leading to a better understanding of the hydrodynamic processes in the study area.

SUMMARY: Detailed topo-bathymetric levellings were performed biannually for four years at Victoria Beach (Cadiz, Spain) after a beach renourishment carried out in Spring 1991. The subsequent time series were analysed using the Empirical Orthogonal Functions (EOF) method. The evolution of some characteristic longshore contour lines, such as the Highest High Water Level and the Lowest Low Water Level, is studied. The mean coastal line is related to the first spatial EOF mode. Furthermore, an objective criterion for distinguishing between a generalised recession and cyclic accretion-erosion processes due to seasonal sea-swell changes is described, and a uniformly clockwise turn of the shoreline to minimise longshore transport is identified.

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Sea level series were recorded from October 2006 to January 2007 at two different sites in a shallow-water estuary of SW Spain. Results showed a very significant temporal variability in daily sea level. Moreover, 85% of the variance was explained by atmospheric pressure variability, obtaining a relation between sea level and atmospheric pressure of around -2 cm hPa -1 . A more detailed study showed the existence of two different types of oscillations in daily sea level: the first, with periods of 10 days, was related to the inverted barometer (IB) effect associated with synoptic-scale atmospheric pressure oscillations (IB factor of around 1.4 ± 0.1 cm hPa -1 ), and the second, with periods of more than 10 days, was related to pressure changes in the planetaryscale atmospheric waves. Even though the high barometric behaviour of the 10-day period was partially related to the local winds, the shallowness of the study area invalidated the assumption of a static IB effect (-1 cm hPa -1 ). Rather, it was the dynamic IB effect that explained the displacement of the IB factor relative to the static assumption.Key words: inverted barometer effect, long-period sea level oscillations, shallow waters. ResumenSe analizaron dos series simultáneas del nivel del mar registradas entre octubre de 2006 y enero de 2007 en una zona estuarina somera al sur de España. Los resultados evidenciaron una variabilidad temporal muy significativa del nivel medio diario del mar. Del análisis de las series se dedujo que 85% de la varianza es explicada por variaciones de la presión atmosférica, con un factor de proporcionalidad entre ambas series en torno a -2 cm hPa -1 . Un estudio más exhaustivo demostró la existencia de dos tipos de oscilación en el nivel medio diario del mar, uno con periodos en torno a los 10 días cuyo comportamiento fue explicado por el efecto de barómetro invertido originado por las oscilaciones de presión atmosférica de escala sinóptica, con valores de factor de barómetro invertido (BI) en torno a -1.4 ± 0.1 cm hPa -1 , y otro con periodos superiores a 10 días, asociado a las variaciones de presión atmosférica ligadas a las ondas atmosféricas de escala planetaria. Aunque el efecto local del viento permitió explicar una pequeña parte del comportamiento supra-barométrico observado, la poca profundidad de la zona de estudio hizo que la suposición de efecto de BI estático (-1 cm hPa -1 ) dejase de tener validez en esta zona, siendo el efecto de barómetro invertido dinámico el que permitió explicar las desviaciones del FBI respecto a la esperada respuesta estática.Palabras clave: aguas someras, efecto de barómetro invertido, oscilaciones del nivel del mar de periodo largo.

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