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

Abstract

An acoustic Doppler current profiler (ADCP) is used to estimate the vertical structure of currents at the Camarinal Sill at the Strait of Gibraltar. The most important tidal components, namely, M2, S2, O1, K1, M4, M3, Msf and Mm are examined. The semidiurnal tides are the most energetic, with along‐strait speeds of up to 120 cm s−1 for M2 and 48 cm s−1 for S2. The diurnal tides have along‐strait speeds of 15–35 cm s−1. M4 increases from 4 cm s−1 in the upper layer to ∼15 cm s−1 below 220 m. The fortnightly component Msf reaches speeds in excess of 30 cm s−1 at the upper 120 m. The vertical structure of the tidal signal is complicated, differs between semidiurnal and diurnal tides, and varies between spring and neap tides. Plausible assumptions about the density profile and the bottom friction yield tidal components similar to those observed. The effects of friction are described by the use of an analytical constant eddy viscosity model and the effects of stratification by estimating the internal modes 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, with individual modes reaching 8 cm s−1. The vertical profile of Msf 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 explains the observed phase change with depth of the semimajor axis. The comparison with pressure difference across the strait is consistent with previous studies. This report is the first on the ADCP time series on the Camarinal Sill.