Spatial variability of internal waves in an open bay with a narrow steep shelf in the Pacific off NW Mexico

Small scale spatial patterns (<10 km) in nearshore internal wave fields are rarely reported on, yet can have a large impact on nearshore mixing and productivity. In this study, the spatial pattern of internal wave characteristics were explored in Todos Santos Bay, Baja California (Mexico), using time series of temperature and currents from moored and towed thermistor chains and acoustic profiling current meters, as well as cross-shore transects with a towed undulating CTD system. Spectra of temperature and currents showed significant spatial variability within the bay, with the northern sector dominated by the internal tidally-forced semidiurnal signal, and the southern sector dominated by wind-forced, subinertial, baroclinic, diurnal fluctuations, which decreased with distance from shore. Semidiurnal internal tidal waves were generated by the barotropic tide at various sites on the continental slope to the west of the bay. They traveled toward the NE and reached the observation site in the northern part of the bay, after bouncing once or twice off the surface and the bottom. Despite the narrowness of the shelf, the semidiurnal internal tides at this site presented a first-mode structure, although not completely formed at times. On average, the semidiurnal internal waves had a ~9 km wavelength, traveled in the form of an arc, and propagated with a phase velocity of ~20 cm/s. When they reached shallow waters near the coast, they disintegrated rapidly into groups of short, nonlinear internal waves, with 15-20 m amplitudes, 5-20 min periods, and 50-200 m wavelengths. The spatial patterns found in this study are most likely due to variability in distance from generation sites, complex bottom topography, and small scale (<10 km) spatial variability in meteorological conditions such as winds.