Although coastal upwelling has now been the subject of many studies, the only available dynamical upwelling index is derived from a simple relation between upwelling intensity and alongshore winds stress. In this study, we first present a primitive equation model simulation of the New Caledonia island region to show that upwelling intensity can be limited by onshore geostrophic flow. Then, we propose a new analytical model accounting for the effect of onshore geostrophic flow on the structure of upwelling. The analytical model is shown to match remarkably well the solution of the primitive equation model. Used as an upwelling index, it is also a better fit to SST indices over the world's four large upwelling regions than the classical Bakun index. We conclude that alongshore pressure gradients (in addition to shelf topography) are crucial to the upwelling problem and its addition to theoretical models can improve their explanatory value and predictive skills.