Unstable coupled atmosphere-ocean basin modes in the presence of a spatially varying basic state

It is shown that the unstable low-frequency eigenfunction consists primarily of a Kelvin mode and a gravest equatorial Rossby mode, and the oscillation can be understood in particularly simple terms. A growing unstable positive wind anomaly in the central Pacific produces a growing eastward-propagating downwelling Kelvin mode and a growing westward-propagating upwelling equatorial Rossby mode. The down-welling Kelvin mode propagates eastward and enhances the growing warm phase of the ENSO. On the other hand, the upwelling Rossby mode propagates westward and produces an upwelling Kelvin mode via reflection at the western boundary. This growing Kelvin mode propagates to the central and eastern Pacific where it then grows without propagation, cools the warm anomaly, eventually changes the phase of the warm event to cold, and therefore switches the sign of the air-sea coupled instability in the eastern Pacific. -from Authors