From Atmospheric Circulation to Global Change,
Edited by the Institute of Atmospheric Physics, Chinese Academy of Sciences,
China Meteorological Press,
A Theory for El Nino Cycle
Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii
A theoretical model for the coupled tropical ocean and atmosphere is derived from
physical principles. The model characterizes the temporal evolution of the El Nino cycle as a second
order nonlinear dynamic system. With an annual mean basic state, the
model has, for a rather restricted range of
air-sea coupling strength, a unique limit cycle solution which is a stable
attractor representing a regular interannual oscillation of the coupled ocean and atmosphere.
The oscillation is characterized by a delicate phase lead of thermocline
displacement to SST, which agrees well with observation.
This phase lead results from a nonlinear interaction between
SST and thermocline variations and plays a critical role in providing a
negative feedback to turn a warming to a
cooling or vice versa, sustaining the oscillation.
When the basic state varies annually, the limit cycle develops a strange attractor and the interannual
oscillations exhibit deterministic chaos. We show that the basic state is most unstable (stable)
in northern spring(fall). The season-dependent development of
the coupled model causes a phase "lock"
of the ENSO cycle with annual cycle - the transition phases of
the oscillation tend to occur during most unstable season -
which, in turn, causes the irregularities in the oscillation period and amplitude.
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