The early nonlinear transition process initiated by a small-amplitude pair of oblique waves is studied using both temporal numerical simulation and theoretical considerations. This investigation is performed under the flow conditions of the experiments by Corke et al. (AIAA J., vol. 40, 2002, pp. 1015–1018) who investigated a sharp $7^{\circ }$ cone in the NASA Mach 3.5 Quiet Tunnel. In particular, both the linear and the nonlinear mechanisms prior to transition onset are investigated in great detail as the physics of this regime predetermine the flow topology of the nonlinear transition zone. The objective of this study is (i) to advance the understanding of the underlying physical mechanisms relevant for the early nonlinear transition regime of oblique breakdown and (ii) to make the connection to oblique transition, the incompressible scenario for bypass transition investigated by Schmid & Henningson (Phys. Fluids A, vol. 4, 1992, pp. 1986–1989). The dominance of the longitudinal vortex mode in oblique breakdown is shown to be a consequence of a constantly forced transient growth instability. In particular, the primary pair of oblique waves serves as an ‘actuator’ that is permanently introducing disturbances into the longitudinal mode where these disturbances exhibit transient growth. The effect of the transient growth instability on the longitudinal mode is to raise its amplitude rather than change the growth rate of this mode.

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