Recently, simulations using coupled atmosphere/ocean general circulation
models that include an annually varying solar forcing have been reported by
Nagai et al. [1992] and Robertson et al. [1995]. In both of
these studies, the annual cycle in the tropical Pacific SST, surface fluxes,
and wind are qualitatively consistent with that observed.
And both models also contain coordinated atmosphere and
surface ocean interannual variability in the tropical Pacific, albeit much
weaker and only qualitatively similar in pattern to that which is observed.
Hence, overall, simulating interannual variability in the presence of an
annually varying sun continues to be a difficult problem: there have been
models that reproduce interannual SST variability and models that reproduce
the annual cycle but reproduction of the full spectrum of variability
remains elusive. (It should be recalled that the annual cycle is an average
over all the variability present in the system [i.e., the average of all
Januarys, Februarys, etc.] so that the annual cycle is not independent of
interannual variability.)
The basic problem is that the processes that determine the annual cycle
appear to be different from the processes that determine the interannual
variability. In particular, we now believe that interannual SST variability
in the Pacific depends in an essential way on wind-driven thermocline
variations with heat fluxes at the surface acting mainly to damp the
interannual perturbations [e.g., Barnett et al., 1991]. Annual
variations of SST depend critically on heat flux variations at the surface
and therefore depend in an essential way on radiative and cloud feedbacks
(see, e.g., Koeberle and Philander [1994]). The treatment of low level
stratus clouds seems to have given particular problems. These clouds exhibit
a positive feedback to SST at low tropical SSTs and therefore induce a
special sensitivity. They are generally dealt with poorly in existing
models. Finally, vertical mixing is presently poorly represented in the
current generation OGCMs used for tropical studies (but see Gent and
Cane [1989] and McWilliams and Gent [1994]). This has a deleterious
impact on the simulated SST in the eastern equatorial Pacific, where the
changes in the meridional wind stress play a key role in causing annual SST
variability [ Mitchell and Wallace, 1992; Xie, 1995].