Abstract The rudiments of a self-consistent theory of intraseasonal and interannual variability for the tropical ocean-atmosphere system are presented. In this paper we study some basic properties of low frequency phenomena in the tropical atmosphere and coupled ocean-atmosphere system and attempt to seek a unified dynamical framework in which the mechanisms of intraseasonal oscillations and ENSO can be studied and understood. Specific physical processes are identified and their roles in leading to intraseasonal oscillations and ENSO are elucidated both separately and collectively for a simple shallow-water system. A common thread linking the two phenomena is in the vital role played by moist processes in the atmosphere and how they can be affected by mean sea surface temperature (SST) and interactive SST anomaly. The importance of including a time-dependent, moist atmosphere in studies of the coupled atmosphere-ocean system is emphasized. For intraseasonal oscillations, results show that the slow eastwar...