Climate Dyn. ,

Roles of shortwave radiation forcing on ENSO: A study with a Coupled Ocean-Atmosphere Model

Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii

Z. Fang
Department of Meteorology, School of Ocean and Earth Science and Technology, University of Hawaii

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.

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