Combined effects of wind-driven upwelling and internal tide on the continental shelf

Internal tide on the continental shelf can be intermittent as a result of changing hydrographic conditions associated with wind-driven upwelling. In turn, the internal tide can affect transports associated with upwelling. To study these processes, simulations in an idealized, alongshore uniform set-up are performed utilizing the hydrostatic Regional Ocean Modeling System (ROMS) with conditions corresponding, as close as possible, to the central Oregon shelf. "Wind-only" (WO), "tide-only" (TO) and "tide-and-wind" (TW) solutions are compared, utilizing cases with constant upwelling favorable wind stress as well as with time-variable observed stress. The tide is forced by applying cross-shore barotropic flow at the offshore boundary with intensity sufficient to generate an internal tide with horizontal velocity amplitudes near observed values of 0.15 m/s. Variability in stratification associated with upwelling can affect barotropic-to-baroclinic energy conversion on the continental slope by changing the classification of the slope from nearly critical to supercritical, such that less barotropic tidal energy is converted to baroclinic and a larger fraction of the baroclinic energy is radiated into the open ocean. The internal tide affects the subinertial circulation, mostly through the changes in the bottom boundary layer variability, resulting in a larger bottom stress and weaker depth-averaged alongshore current in case TW compared to WO. The spatial variability of cross-shore volume transport is also affected. Increased vertical shear in the horizontal velocity resulting from the superposition of the upwelling jet and the internal tide results in intermittent patches of intensified turbulence in the mid-water column. Internal waves cause high-frequency variability in the turbulent kinetic energy in both the bottom and surface boundary layers, causing periodic restratification in the inner-shelf zone.