Tidal effect on the dense water discharge, Part 1: Analytical model

Abstract

Abstract It is hypothesized that tidally induced shear dispersion may significantly enhance the spread and descent of the dense water. Here, we present an analytical model to elucidate the basic physics, to be followed (in Part 2) by a numerical study using a primitive-equation model. While tides enhance the vertical mixing and thicken the Ekman layer, the tidal dispersion is seen to produce a disparate benthic layer several times the Ekman depths. With its top boundary lying above the frictional shear, the diapycnal mixing with the overlying water would be curtailed on account of its Richardson-number dependence. Over the shelf, this reduced erosion of the dense water and the generation of the Ekman flow by the thermal wind within the layer would act in concert to propel the dense water much beyond the deformation radius, which thus may reach the shelf break with less densification by the air-sea fluxes. Over the slope, even with the strong Ekman flow induced by the sloping interface, the dense water remains confined to the Ekman layer in the absence of tides, hence subjected to strong diapycnal mixing that would curb its descent. With the tidal augmentation of the benthic layer beyond the Ekman depth, on the other hand, the much-reduced diapycnal mixing would allow the Ekman advection to operate more effectively, which may propel the dense water to the lower slope, as observed in the western Ross Sea.