We review simple instabilities in linear theories of coupled atmosphere-ocean models in both bounded and unbounded ocean basins and describe the mechanisms for instability in these linear theories. We then review nonlinear coupled atmosphere-ocean simulations of the El Nino Southern Oscillation (enso) phenom enon and relate the instabilities seen in linear theory to the fully nonlinear enso simulations. We present a general discussion of the relation between instability and predictability in

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... enso problem and review some recent work on predictability in coupled models. Finally, we comment on some recent predictions in light of our discussion of predictability.