Coupled processes and the tropical climatology : part III : instabilities of the fully coupled climatology

Abstract

Coupled processes between the equatorial ocean and atmosphere control the spatial structure of the annual mean 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 phe- nomenon.In this paper,we consider the connection between both effects of coupling 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 simul- taneous 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 ap- pears 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 qual- itatively similar in this fully coupled problem compared to the ?ux-corrected problem,but the sensitivity of the ENSO mode to parameters and external variations is larger due to feedbacks in the climatology.