Interdecadal variability of northward-propagating ISOs from July to September is examined by dividing the 32 years into 1979 - 1993 and 1994 - 2010. The enhanced easterly anomalies in 1994 - 2010 could increase the steering flow for the submonthly wave pattern and TCs. Therefore, the propagation routes for TCs and submonthly wave patterns were pushed farther westward toward eastern China in 1994 - 2010 instead of slowly migrating northward to Japan in 1979 - 1993. By separating the background flow into the climatological mean and ISO components, a sensitivity study of the barotropic interaction is conducted when switching the background flow of the other epoch. The results show that the enhancement of the subtropical anticyclone in the later epoch changes the distribution of the background easterly flow and thus the barotropic conversion maximum between the submonthly perturbations and climatological circulation was displaced westward. The strengthening of the ISO background fields in the later epoch enhances the barotropic interaction between the ISO and submonthly perturbations. Thus in the westerly phase of the later epoch, the enhanced ISO cyclonic anomaly favors the barotropic conversion to intensify. The positive areas of interdecadal changes for the barotropic conversion between the submonthly perturbations and the climatological mean fields are co-located with the positive changes due to the ISO background fields where the wave pattern and TCs move further westward in the later epoch. Therefore, the interdecadal impacts by the climatological mean fields and ISO on the energy conversion jointly favor the development of the wave pattern and TCs.