The occasional development of coastal upwelling off southwestern Sumatra plays a key role in the dynamics of the Indian Ocean Dipole. This region has experienced the development of full upwelling only in the years of 1994, 1997 and 2004, which are classified as positive events of the Dipole Model Index. Reasons for the upwelling variability were largely unknown. Here we present a simple new theory of classical wind-driven (Ekman) upwelling along a coast of finite extent. This theory explains why the Sumatran upwelling developed only in certain years. To this end, we can demonstrate that wind effects along the northwestern coast Sumatra are critical as a preconditioning mechanism to support the establishment of full upwelling in the south. Satellite data also indicate that, once this upwelling center has evolved, the associated upwelling jet further enhances negative SST anomalies in the region by bringing in colder surface water from the south. A manuscript detailing these findings is currently under review at Deep-Sea Research (Kämpf and Kavi, 2018, under revision).