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Abstract Diapycnal upwelling along sloping topography has been shown to be an important component of the abyssal overturning circulation. Theoretical studies of mixing-driven upwelling have mostly relied on a time-averaged description of mixing acting on a mean stratification which ignores the intermittency of mixing. Typically, these studies prescribed a time-invariant turbulent diffusivity profile motivated by scenarios where tidal currents encounter gentle topography with small-scale corrugations, leading to subsequent propagation and breaking of internal waves. Here, a different scenario is considered where a tidal current interacts with smooth but steep topography, a case often encountered near continental margins and troughs. The performed nonhydrostatic simulations resolve both the strong oscillatory shear that develops along the steep critical topography and the associated mixing events. Strong diapycnal mixing is observed during the upslope phase of the tidal flow when both the near-boundary stratification and shear are enhanced. During the downslope phase, strong overturning events do develop, but they are associated with weak stratification and less efficient diapycnal mixing. These results highlight that the temporal evolution of both shear and stratification play a key role in setting when diapycnal mixing and water mass transformation occur along steep topography. In contrast, over gentle topography, tidal shears do not become sufficiently large to generate strong local mixing for typical oceanographic parameters. 

Abstract Coastal upwelling systems play a key role in sustaining productive coastal ecosystems in the global ocean by transporting nutrients to surface waters. However, the fundamental mechanisms and pathways responsible for nutrient upwelling are not fully understood, largely due to the historically employed two-dimensional frameworks in which coastal upwelling systems have long been studied. Using both observations and idealized numerical simulations, we identify and quantify two primary routes of nutrient upwelling: the residual circulation, resulting from a significant cancellation between Eulerian-mean and eddy-induced circulations, and along-isopycnal eddy stirring. Our analysis demonstrates that their relative contributions depend on two distinct parameters: 1) the slope Burger number S, defined here as S=αN/f, whereαis the topographic slope angle and Nandfare the buoyancy and Coriolis frequencies, and 2) the surface nutrient uptake rate by biological activities. Specifically, we propose that wind forcing induces isopycnal tilting and surface outcropping, which creates favorable conditions for along-isopycnal nutrient gradients to develop in regions of strong biological activity at the surface. The magnitude of these gradients depends on both the slope Burger number S, which influences the strength of the residual circulation bringing nutrients from depths, and the surface biological uptake rate, which consumes nutrients. Our diagnostics provide insights into the intricate pathways for nutrient upwelling and underscore the significance of eddy stirring in coastal upwelling systems.