On the Pathways of Wind-Driven Coastal Upwelling: Nonlinear Momentum Flux and Baroclinic Instability

Wind-driven upwelling of cold, nutrient-rich water is a key feature near the eastern boundaries of major ocean basins, with significant implications for the local physical environment and marine ecosystems. Despite the traditional two-dimensional description of upwelling as a passive response to surface offshore Ekman transport, recent observations have revealed spatial variability in the circulation structures across different upwelling locations. Yet, a systematic understanding of the factors governing the spatial patterns of coastal upwelling remains elusive. Here, we demonstrate that coastal upwelling pathways are influenced by two pairs of competing factors. The first competition occurs between wind forcing and eddy momentum flux, which shapes the Eulerian-mean circulation; the second competition arises between the Eulerian-mean and eddy-induced circulation. The importance of nonlinear eddy momentum flux over sloping topography can be described by the local slope Burger number, S = αN/f, where α is the topographic slope angle and N and f are the buoyancy and Coriolis frequencies. When S is small, the classic coastal upwelling structure emerges in the residual circulation, where water upwells along the sloping bottom. However, this comes with the added complexity that mesoscale eddies may drive a subduction route back into the ocean interior. As S increases, the upwelling branch is increasingly suppressed, unable to reach the surface and instead directed offshore by the eddy-induced circulation. The sensitivity of upwelling structures to variable wind stress and surface buoyancy forcing is further explored. The diagnostics may help to improve our understanding of coastal upwelling systems and yield a more physical representation of coastal upwelling in coarse-resolution numerical models.