Tidal currents at the sills of the Northern Gulf of California

Abstract

Current observations at the four main sills in the Northern Gulf of California: San Esteban (SE), San Lorenzo (SL), Delfin (DE) and Ballenas channel (BC), are analyzed to investigate their tidal structure. Tidal currents are mainly semidiurnal (M2, S2, and N2) at all of the four sills. M2 vertically integrated tidal currents at the three most energetic sills (SE, SL and BC) are at least 0.5 m/s, but there are significant vertical variations of the tidal amplitudes and phases of the measured currents. At the SL and DE sills there is a ~20○-30○ phase reduction in the lower 100 m of the water column (depths ~400 m) of the M2 tidal currents, and this is, at least in part, related to near-bottom overflows that are present at those sills. At the SL sill, the phase reduction is also accompanied by a significant reduction in amplitude. Baroclinic tidal currents were estimated using empirical orthogonal functions of the high-passed current vectors, and they typically account for about 10% of the observed variance. At the most energetic sill (SE) the two dominant baroclinic components are the nonlinear MS4 and M4. Complex demodulation of the currents at the semidiurnal band shows that tidal amplitudes and phases decrease with increasing low-frequency (subtidal) currents and with increasing vertical shear of the low-frequency current at the sills with the overflows (SL and DE). We also found that at two of the most energetic sills (SE and SL) which have a strong stratification, the annual satellites of the M2 barotropic tidal currents reach amplitudes up to 0.1 m/s. This modulation produces a 0.2 m/s and up to a 15○ annual range of the barotropic M2 tidal current amplitudes and phases, respectively, with maxima in August.