Monday, 5 April 1999: 9:15 AM
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.
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