<p>Spanwise heterogeneity in surface properties, such as surface roughness and temperature, at the scale of boundary-layer height, leads to the formation of large-scale secondary vortices in a turbulent boundary layer. Most prior studies on the topic have focused on the topology of secondary vortices in flows that involve little streamwise variation. By introducing streamwise change at regional scales, this study investigates the evolution of large-scale secondary vortices after the removal of the spanwise heterogeneity that sustains them. A suite of roughness-resolved large-eddy simulations is conducted and the results show both the development and decay processes of the secondary vortices. Through analysis of the Reynolds-averaged vorticity equation, a phenomenological model is proposed. The model predicts a slow, exponential decay of the secondary vortices, which receives support from our data.</p>