Impacts of shortwave radiation forcing on ENSO: a study with a coupled tropical ocean-atmosphere model

 We describe a coupled tropical ocean-atmosphere model that represents a new class of models that fill the gap between anomaly coupled models and fully coupled general circulation models. Both the atmosphere and ocean are described by two and half layer primitive equation models, which emphasize the physical processes in the oceanic mixed layer and atmospheric boundary layer. Ocean and atmosphere are coupled through both momentum and heat flux exchanges without explicit flux correction. The coupled model, driven by solar radiation, reproduces a realistic annual cycle and El Nino-Southern Oscillation (ENSO). In the presence of annual mean shortwave radiation forcing, the model exhibits an intrinsic mode of ENSO. The oscillation period depends on the mean forcing that determines the coupled mean state. A perpetual April (October) mean forcing prolongs (shortens) the oscillation period through weakening (enhancing) the mean upwelling and mean vertical temperature gradients. The annual cycle of the solar forcing is shown to have fundamental impacts on the behavior of ENSO cycles through establishing a coupled annual cycle that interacts with the ENSO mode. Due to the annual cycle solar forcing, the single spectral peak of the intrinsic ENSO mode becomes a double peak with a quasi-biennial 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 become variable on an interdecadal time scale due to interactions of the mean state and the two ENSO components. The western Pacific monsoon (the annual shortwave radiation forcing in the western Pacific) is primarily responsible for the generation of the two ENSO components. The annual march of the eastern Pacific ITCZ tends to lock ENSO phases to the annual cycle. The model's deficiencies, limitations, and future work are also discussed. Received: 15 June 1999 / Accepted: 11 December 1999