The sound level within woodwinds under playing conditions is sufficiently high so that nonlinear losses are significant, yet time-domain models of sound production have assumed that the air column responds linearly. Using a linear air-column model, time-domain simulations of saxophone tones give results qualitatively similar to performed tones for low-register tones, but dissimilar results for second-register tones and multiphonics, both of which employ an open register hole. A time-domain analysis of a cylindrical tube terminated by an open orifice characterized by a nonlinear resistance shows that the reflection function amplitude is reduced as the Strouhal number tends toward unity, but the time delay is not affected. This nonlinear reduction is most important for small-diameter orifices, such as the register hole. Calculations of the saxophone input impedance for second register notes demonstrate that the nonlinear resistance of the register hole reduces the level of the fundamental resonance by 8 dB and lowers its frequency, thereby reducing the degree of harmonicity with higher frequency resonances and enabling the simulated performer to produce a tone in the upper register. Other resonances are not affected. Similar behavior is predicted for multiphonics.