Coupled Processes and the Tropical Climatology. Part III: Instabilities of the Fully Coupled Climatology*

Coupled processes between the equatorial ocean and atmosphere control the spatial structure of the annualmean state in the Pacific region, in particular the warm pool–cold tongue structure. At the same time, coupled processes are known to be responsible for the variability about this mean state, in particular the El Niño–Southern Oscillation phenomenon. In this paper, the connection between both effects of coupling is considered by investigating the linear stability of fully coupled climatologies in an intermediate coupled model. The new element here is that when parameters—such as the coupling strength—are changed, the potential amplification of disturbances can be greatly influenced by a simultaneous modification of the mean state. This alters the stability properties of the coupled climatology, relative to the flux-corrected cases that have been previously studied. It appears possible to identify a regime in parameter space where ENSO-like unstable modes coincide with a reasonable warm pool–cold tongue structure. These unstable modes are mixed SST–ocean dynamics modes, that is, they arise through an interaction of oscillatory modes originating from ocean dynamics and oscillatory SST modes. These effects are qualitatively similar in this fully coupled problem compared to the flux-corrected problem, but the sensitivity of the ENSO mode to parameters and external variations is larger due to feedbacks in the climatology.