The Importance of Eddy Stirring in Wind-Driven Coastal Upwelling

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 = aN / f , where a is the topographic slope angle and N and f are 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.