CoAuthors

Paolo Cipollini (National Oceanography Centre, United Kingdom); Marcello Passaro (Deutsches Geodätisches Forschungsinstitut der Technischen Universität München, Germany); Stefano Vignudelli (Institute of Biophysics (CNR), Italy); Josep Coca (University of Cadiz, Spain)

In this work we analyzed the availability and accuracy of coastal altimetry sea level products in the Strait of Gibraltar, the choke point between Europe and Africa connecting the Atlantic Ocean and the Mediterranean Sea. All possible repeats of two segments of 30 km along two sections of the Envisat and AltiKa ground-tracks were used in the eastern and western portions of the Strait. For Envisat, along-track SLA at 18-Hz posting rate were computed using the retracked 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 retracked 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.