Neap-spring variability of tidal dynamics in the Northern Arabian Gulf

Small-scale (periods < 24 h and horizontal length < 10 km) hydrodynamics are significantly influenced by tides in the Northern Arabian Gulf (NAG). Here we characterize, for the first time, the neap-spring variability of tidal currents and mixing in the NAG from field observations. During the experiment between 14–28 July 2017, measurements in the near bottom layer (~1.5–6.5 m above bottom) show: (1) semi-diurnal horizontal velocities induced by tides, with a maximum horizontal speed of ~0.5 m/s; (2) quarter-diurnal cycles in turbulent kinetic energy (TKE) dissipation rates, with inequalities between the daily two floods and two ebbs; (3) well-correlated relations between turbulent diffusivity and gradient Richardson number. Both current velocities and mixing intensity near the bottom increased from neap to spring tides: the mean horizontal speeds increased from 0.18 to 0.26 m/s; the mean TKE dissipation rates increased from 2.2 × 10^-7 to 5.5 × 10^-7 W/kg and the mean stratification decreased from 2.1 × 10^-5 to 5.5 × 10^-6 s^-2. Thus, the enhanced near bottom turbulent mixing is likely to contribute more significantly to the vertical transports of nutrients and sediments during spring tides in this region. The mean tidal power input (2.6 × 10^-2 W/m²), was found to be one order of magnitude larger than the mean wind power input (1.1 × 10^-3 W/m²) with ~39% of the tidal energy being dissipated locally by turbulent mixing near the bottom.