Observations of D region electron density and radio wave absorption are anomalous during winter in two important respects. Absorption measured at a constant zenith angle is greater in winter than in summer (the ''smooth anomaly''), and large absorption enhancements occur in winter on time scales of the order of days (the ''sporadic anomaly''). Proposed explanations for the winter anomaly include temperature changes, water vapor variability, energetic particle precipitation, and transport of nitric oxide. In this study we examine the smooth and sporadic anomalies using a coupled model, which includes neutral and ion photochemistry, as well as transport by the mean meridional circulation and planetary waves. We find that both smooth and sporadic absorption enhancements can be understood in terms of auroral production of nitric oxide in polar night and its subsequent transport and ionization. In particular, our results indicate that horizontal wave transport can accout for many of the observed features of the sporadic anomaly. ¿ American Geophysical Union 1987