This paper describes a coupled tropical ocean-atmosphere model
which fills the gap between anomaly coupled models and fully
coupled general circulation models. Both the atmosphere and ocean
models are two and half layer primitive equation models which
accentuate physical processes in oceanic mixed layer and
atmospheric boundary layer, combining with essential dynamics for
the thermocline and free atmosphere. The two media are coupled throguh both
momentum and heat flux exchange without explicit flux correction. The model is capable of
reproducing realistic features of both the climatological annual cycle and interannual variations
(ENSO) in the tropical Pacific. The model allows interaction between the mean, annual cycle
and ENSO.
In the presence of annual mean shortwave radiation forcing, the model exhibits an
intrinsic mode of interannual oscillation (ENSO). The oscillation period depends on the
distribution of the mean shortwave radiation forcing. The annual cycle of the solar forcing is
shown to fundamentally alter the behavior of ENSO cycles through establishing a coupled annual
cycle. As a result of the interaction between the annual cycle and ENSO mode, the single
spectral peak of the intrinsic ENSO mode (with a period of 2.8 years) becomes a double peak
with a quasi-bienniel (2-years) and a low-frequency (4-5 years) component; the evolution of
ENSO becomes phase-locked to the annual cycle; and the amplitude and frequency of ENSO
cycles exhibit interdecadal modulation. The interdecadal variation in ENSO cycles results from
the interaction between the mean state and the two components of the ENSO cycles. It is shown
that the western Pacific monsoon (the annual forcing in the western Pacific) is primarily
responsible for the generation of multi-time scales (the bienniel and low-frequency components).
The annual march of the eastern Pacific ITCZ plays a role in locking the ENSO phase to the
annual cycle. The model's deficiencies, limitations, and future works
are also discussed.