El Nino Southern Oscillation (ENSO) is a global coupled ocean-atmosphere phenomenon, associated with floods, droughts, and other disturbances in a range of locations around the world. ENSO is the most prominent known source of inter-annual variability in weather and climate around the world (about 3 to 8 years). In spite of its importance and a long history of studies, the understanding of its nature and mechanism is still lacking, and a careful fundamental level examination of the problem is crucial.

We present in this talk a new mechanism of the ENSO, as a self-organizing and self-excitation system, with two highly coupled processes. The first is the oscillation between the two metastable warm (El Nino phase) and cold events (La Nina phase), and the second is the spatiotemporal oscillation of the sea surface temperature (SST) field.
The interplay between these two processes gives rises the climate variability associated with the ENSO, leads to both the random and deterministic features of the ENSO, and defines a new natural feedback mechanism, which drives the sporadic oscillation of the ENSO. The new mechanism is rigorously derived using a dynamic transition theory developed recently by the authors, which has also been successfully applied to a wide range of problems in nonlinear sciences. This is joint work with Tian Ma.