  Unstable shear layers in environmental and industrial flows roll up into a
series of vortices, which often form complex nonlinear merging patterns like
pairs and triplets. These patterns crucially determine the subsequent
turbulence, mixing and scalar transport. We show that the late-time, highly
nonlinear merging patterns are predictable from the linearized initial state.
The initial asymmetry between consecutive wavelengths of the vertical velocity
field provides an effective measure of the strength and pattern of vortex
merging. The predictions of this measure are substantiated using direct
numerical simulations. We also show that this measure has significant
implications in determining the route to turbulence and the ensuing turbulence
characteristics.
