Vertical structure of tidal currents over the Camarinal Sill at the Straitof Gibraltar

An acoustic Doppler current profiler (ADCP) is used to estimate the vertica l structure of currents at the Camarinal Sill at the Strait of Gibraltar. T he most important tidal components, are examined. The semidiurnal tides are the most namely, M-2, S-2, O-1, K-1, M-4, M-3, M-sf and M-m, energetic, wi th along-strait speeds of up to 120 cm s(-1) for M-2 and 48 cm s(-1) for S- 2. The diurnal tides have along-strait speeds of 15-35 cm s(-1). M-4 increa ses from 4 cm s(-1) in the upper layer to similar to 15 cm s(-1) below 220 m. The fortnightly component M-sf reaches speeds in excess of 30 cm s(-1) a t the upper 120 m. The vertical structure of the tidal signal is complicate d, differs between semidiurnal and diurnal tides, and varies between spring and neap tides. Plausible assumptions about the density profile and the bo ttom friction yield tidal components similar to those observed. The effects of friction are described by the use of an analytical constant eddy viscos ity model and the effects of stratification by estimating the internal mode s at each tidal frequency. The major part of the tidal signal is barotropic , with the internal tide modes and the frictional effects contributing <20 cm s(-1) for the semidiurnal tides. Nevertheless, individual modes reach 50 cm s(-1). The internal tides for the diurnal components are <5 cm s(-1), w ith individual modes reaching 8 cm s(-1). The vertical profile of M-sf can be explained by the use of internal modes reaching amplitudes of 40 cm s(-1 ). The fitting of the internal wave modes to the vertical profiles also exp lains the observed phase change with depth of the semimajor axis. The compa rison with pressure difference across the strait is consistent with previou s studies. This report is the first on the ADCP time series on the Camarina l Sill.