In some recent studies, investigators have postulated that an annual cycle forcing of the coupled atmosphere/ocean system is responsible for the irregularity of ENSO events [ Tziperman et al., 1994; Jin et al., 1994; Chang et al., 1994]. In these studies, a study of the coupled interannual variability is performed by arbitrarily increasing the annual cycle of a favorite parameter or the coupling strength between the atmosphere and ocean. The interannual variability in the models is periodic with no annual cycle forcing, but as the parameter is increased the frequency of the interannual variability increases through a sequence of rational fractions of the annual cycle: the ENSO cycle remains phase-locked to the annual cycle. The transition to higher frequencies is characterized by chaotic variability. In each of these studies, the physics associated with the ENSO mode appears to remain robust (e.g., in Jin et al., the ENSO mode is characterized by the delayed oscillator physics) as the forcing is increased.
Each of the studies mentioned above uses a different atmosphere/ocean
model
to argue for the importance of the seasonal cycle in producing irregularity
of ENSO. We caution that even in the chaotic regime under modest forcing,
the aforementioned models have spectra that are relatively peaked compared
to the spectra from nature. In addition, in none of these studies is the
annual cycle actually forced by the sun: the amplitude of a parameter or
process with an annual cycle is artificially changed in these studies rather
than a change in annual cycle of insolation. Nonetheless, these preliminary
studies may lead to some insight as to how the annual cycle might interact
with the model ENSO.
In contrast to the signal analysis approach taken in the three studies just
discussed, Xie [1995] has suggested a physical mechanism of interaction
between the ENSO mode and the seasonal cycle, based on the very different
physical processes that seem to govern the interannual ENSO mode and the
annual cycle (see, e.g., Koeberle and Philander [1994]). Xie finds a
deepening of the thermocline in the warm phase of ENSO reduces the amplitude
of the annual cycle in SST (and vice versa), leading to a
phase-locking of ENSO to the seasonal cycle.
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