A Hybrid Coupled General Circulation Model for El Niño Studies

A model is developed for tropical air–sea interaction studies, which is intermediate in complexity between the large coupled general circulation models (coupled GCMs) coming into use and the simple two-level models with which pioneering El Niño–Southern Oscillation studies were carried out. The model consists of a stripped-down tropical Pacific ocean GCM, coupled to an atmospheric model which is sufficiently simple that steady state solutions may be found for low level flow and surface stress, given oceanic boundary conditions. This hybrid coupling of an ocean GCM to a steady atmospheric model permits examination of the nature of interannual coupled oscillations in the absence of atmospheric noise. Tests of the atmospheric model against an atmospheric GCM simulation of El Niño anomalies are presented, and the ocean model climatology is examined under several different conditions. Experiments with the coupled model exhibit a variety of behaviors within a realistic parameter range. These indicate a partial bifurcation diagram in which the coupled system undergoes a Hopf bifurcation from a stable climatology, giving rise to sustained El Niño-period oscillations. The amplitude, period and eastward extent of these oscillations increase with the strength of coupling and the El Niño-period oscillation itself becomes unstable to a higher frequency coupled mode which coexists with it and may affect predictability. The difference between these flow regimes may be relevant to results found by other investigators in coupled GCM experiments.