Combined Effects of Wind-Driven Upwelling and Internal Tide on the Continental Shelf

Abstract

Internal tides 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 setup are performed utilizing the hydrostatic Regional Ocean Modeling System (ROMS) with conditions corresponding, as closely 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 0.15 m s?1, corresponding to observed levels. The internal tide affects the subinertial circulation, mostly through the changes in the bottom boundary layer variability, resulting in a larger bottom stress and a weaker depth-averaged alongshore current in the TW case compared to WO. The spatial variability of the cross-shore and vertical volume transport is also affected. Divergence in the Reynolds stress associated with the baroclinic tidal flow contributes to the tidally averaged cross-shore momentum balance. Internal waves cause high-frequency variability in the turbulent kinetic energy in both the bottom and surface boundary layers, causing periodic restratification of the inner shelf in the area of the upwelling front. 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. Variability in stratification associated with upwelling can affect not only the propagation of the internal tide on the shelf, but also the barotropic-to-baroclinic energy conversion on the continental slope, in this case 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.