A dynamically-based filter is used to separate tropical sea surface temperatures (SSTs) into three components: the evolving El Nino signal, the global tropical trend, and the background. The components thus isolated are not indepen- dent. On the contrary, this procedure allows us to see the importance of the interdecadal signal is to the predicta- bility of El Nino. In this talk, we examine the temporal and geographical properties of interdecadal signals that affect the development of El Nino.