Interannual variation of the tropopause is investigated by applying principal component analysis (PCA) to the zonal mean and eddy tropopause temperatures obtained from the National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) reanalysis. For the zonal mean tropopause temperatures, we find that the first PC represents the out-of-phase coherent variations between the tropics and the Northern Hemispheric (NH) polar region during NH late winter to early spring and the second PC represents the variations between the tropics and the Southern Hemispheric (SH) polar region during SH spring. The same analysis is applied to the zonal mean tropopause simulated in SUNYA Community Climate Model version 3 (CCM3), and the tropopause variations associated with the two PCs are reproduced, with about 74% of the NCEP/NCAR tropopause variability for the first PC and about 44% of the variability for the second PC. The model thermal budget indicates that the first PC is associated with the variation of dynamical heating in the NH polar region and the second PC is associated with the variation of O3 anomalies in the SH polar region. To investigate how the tropopause variation is modulated by the quasi-biennial oscillation (QBO), which is not simulated in the model, singular value decomposition (SVD) is applied to the NCEP/NCAR tropopause temperatures and zonal winds. The QBO accounts for about 16% of the zonal wind variability associated with the first PC and about 36% of the zonal wind variability associated with the second PC. For the eddy tropopause temperatures, the interannual variation associated with the El Ni¿o--Southern Oscillation (ENSO) is evident. In the extratropics over the Pacific North American (PNA) sector, the anomalies are out of phase with the dumbbell-shaped anomalies located in the tropical eastern Pacific and are related to the anomalies in horizontal heat advection caused by the response of extratropical stationary waves. The model can simulate the ENSO-related tropopause variability close to the corresponding NCEP/NCAR's variability, although the model anomalies in the tropical western Pacific are too small.