The non-normal nature of ENSO and intraseasonal variability

Generalised linear stability theory has been applied to an intermediate coupled model to explore the idea that ENSO may be stochastically forced by intraseasonal variability in the tropical Pacific. The patterns of stochastic surface heat flux forcing that are optimal for inducing ENSO variability in the model are found to bear a strong resemblence to those observed in relation to the Madden and Julian Oscillation (MJO). The mature phase of the model ENSO however is very different to the spatial structure of a typical MJO event. Thus an ENSO episode can be initiated by perturbations that have a very different structure to the episode itself. This very important property of the coupled system is due to asymmetries in the dynamical operators that control the linear growth of stochastically-induced perturbations. There are two dynamical factors that control the ability of MJO-like perturbations to excite ENSO episodes in the coupled model: (i) the easterly trade winds of the Walker circulation in the tropical Pacific, and (ii) the different reflection properties of east and west ocean boundaries. Possible dynamical links between ENSO and intraseasonal variability, such as the MJO, will be discussed.