Abstract The Kelvin‐Helmholtz instability (KHI) mediates the viscous‐like solar‐terrestrial interaction by generating magnetopause surface waves that quickly become non‐linear. Basic theory predicts the locally most‐unstable linear wave dominates. However, Kelvin‐Helmholtz is a broad, convective instability that also amplifies waves originating upstream. We address this conundrum by applying dynamic mode decomposition to a Gorgon global magnetohydrodynamic simulation of the KHI. While distinct modes quickly grow at points along the magnetopause, signaling local generation, their energy continues to slowly grow downtail. Thus, a superposition is present along the magnetopause, where the dominant mode is not always the locally fastest‐growing. Each mode’s wavelength elongates downtail, correlating with the boundary layer flow speed due to the accelerating advective flow around the magnetosphere Doppler shifting the fixed‐frequency waves. This may explain why longer wavelengths are observed in the tail than theory predicts and motivates further exploration of tangential inhomogeneities in basic Kelvin‐Helmholtz theory.