Tidal currents at the sills of the Northern Gulf of California

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 (M-2, S-2, and N-2) at all of the four sills. M-2 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 similar to 20 degrees-30 degrees phase reduction in the lower 100 m of the water column (depths similar to 400 m) of the M-2 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 M-4. 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 M-2 barotropic tidal currents reach amplitudes up to 0.1 m/s. This modulation produces a 0.2 m/s and up to a 15 degrees annual range of the barotropic M-2 tidal current amplitudes and phases, respectively, with maxima in August.